Power Quality Enhancement of Wind Energy Systems 3031432428, 9783031432422

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Wessam Arafa Hafez Adel A. Elbaset

Power Quality Enhancement of Wind Energy Systems

Power Quality Enhancement of Wind Energy Systems

Wessam Arafa Hafez • Adel A. Elbaset

Power Quality Enhancement of Wind Energy Systems

Wessam Arafa Hafez Electrical Department Faculty of Technology and Education Sohag University Sohag, Egypt

Adel A. Elbaset Faculty of Engineering, Electrical Engineering Department, Department of Electromechanics Engineering Minia University, Heliopolis University El-Minia, Cairo, Egypt

ISBN 978-3-031-43242-2 ISBN 978-3-031-43243-9 https://doi.org/10.1007/978-3-031-43243-9

(eBook)

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland Paper in this product is recyclable.

Preface

Egypt is experiencing a high increase in power consumption due to rapid population growth. The Egyptian government is cognizant of the need for a sustainable energy mix to both address increasing demand and move to a more environmentally sustainable and diverse electricity sector. The strategy target of the new and renewable energy authority of Egypt’s (NREA), which was confirmed in 2008, is to contribute 20% of the overall power generation to renewable energy sources by 2022, including 12% contribution from wind energy systems. Additionally, they have set targets to achieve 42% of the country’s electricity mix from renewables by 2035, focusing on the rapid deployment of solar and wind technologies. The continuous increase of the wind power penetration level brings a result that wind power generation gradually becomes an important component of power generation in the grid, which makes the study of the wind power quality issues and the interaction between the wind turbines and the grid necessary and imperative. This book studies power quality issues of an electrical network-connected wind energy system. Harmonics are the very important power quality disturbances, and various mitigation techniques were discussed to eliminate the harmonics of wind energy systems. In this book, there is an extensive review of various control techniques for active power filters, and positive and negative sides of each introduced control technique are presented. Egyptian power grid connected to Al-Zafarana wind system was taken as a study system for simulation test. This book introduces a design and simulation of shunt active power filter (SHAPF) using a fuzzy controller to reduce the total harmonic distortion, and thus improve the power quality of the studied system. The simulation results showed that the proposed SHAPF provides efficient cancellation of both load current harmonics, thus making the studied system more efficient by improving the quality of power. Doubly fed induction generator (DFIG)-based wind farm has now gained prominence due to its many advantages, such as variable speed operation and autonomous control of active and reactive power. DFIG stator windings are directly connected to

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Preface

the power grid, when a grid fault occurs, some unwanted high current may be produced in the rotor windings, and a protection system will prevent the rotor side converter from operating. Therefore, voltage stability is a significant factor in maintaining the DFIG-based wind farm in operation during grid faults and disturbances. This book also introduces the application of a static synchronous compensator (STATCOM) for restoring the voltage levels of the studied system. The simulation results show that STATCOM devices improve the effects of the grid faults and disturbances such as a single line to ground fault, a line to line fault, voltage sag, and voltage swell. Also, they improve the stability of the system and help to improve the power system restoration procedures for the existing and future-planned wind farms. The simulation for all the studies is carried out in a MATLAB/Simulink environment using the Simulink power system toolbox. This is the most common approach taken by a large number of researchers because of the flexibility of the toolboxes and the availability of well-tested component models. The details of the models utilized are added in this book. This book also presents an overview of the feasibility of having wind power plants at several windy regions in Egypt, along the Gulf of Suez, both sides of the Nile, Mediterranean Sea, and South Upper Egypt. Electricity cost values are computed based on the levelized cost of energy (LCOE) for the electrical power generation from different wind turbines at three scattered regions. The results would offer objective guidelines for energy policymakers and utility operators to consider energy portfolios that are more economically feasible and help to study the impact of power quality issues on the economic evaluation of electrical wind energy in Egypt.

Contents

1

Introduction and Literature Review . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Wind Power Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Worldwide Annual Growth of Wind Power Generation . . . . . . . . . . 1.4 Wind Power Generation in Egypt . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Wind Farm in Egypt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 Book Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7 Literature Review of Related Researches . . . . . . . . . . . . . . . . . . . . 1.8 Book Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9 Book Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 1 3 4 4 6 6 8 8

2

Wind Energy Conversion System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Types of Wind Farms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Onshore Wind Farms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Near-Shore Wind Farms . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3 Offshore Wind Farms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Airborne Wind Farms . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Wind Turbine Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Configurations of the Wind Energy Conversion System . . . . . . . . . 2.5 Wind Turbine Generators in the Current Market . . . . . . . . . . . . . . . 2.5.1 Squirrel Cage Induction Generator . . . . . . . . . . . . . . . . . . 2.5.2 Wound Rotor Induction Generator . . . . . . . . . . . . . . . . . . 2.5.3 Permanent Magnet Synchronous Generator . . . . . . . . . . . . 2.5.4 Doubly Fed Induction Generator . . . . . . . . . . . . . . . . . . . . 2.6 Comparison of WTG Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7 Main Components of a Wind Turbine . . . . . . . . . . . . . . . . . . . . . . 2.7.1 Aerodynamic Wind Turbine . . . . . . . . . . . . . . . . . . . . . . . 2.7.2 Mechanical Drive Train Model . . . . . . . . . . . . . . . . . . . . . 2.7.3 Pitch System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.4 Control of Variable Speed Wind Turbine . . . . . . . . . . . . . .

11 11 11 13 13 13 14 15 15 17 17 18 18 19 20 21 21 24 24 25 vii

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Contents

2.7.5 2.7.6 2.7.7 2.7.8 3

4

Turbine Speed Control Regions . . . . . . . . . . . . . . . . . . . Operating Modes of Wind Turbine . . . . . . . . . . . . . . . . . Doubly Fed Induction Generator . . . . . . . . . . . . . . . . . . . Power Converters of DFIG . . . . . . . . . . . . . . . . . . . . . . .

. . . .

Power-Quality and Grid Code Issues of Wind Energy Conversion System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Need of Power-Quality Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Voltage Sag and Swell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Voltage Variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Switching Operation of Wind Turbine on the Grid . . . . . . . . . . . . . 3.6 Harmonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.1 Effect of Harmonics on Wind Farms . . . . . . . . . . . . . . . . . 3.6.2 Types of Harmonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.3 Harmonic Phase Sequence . . . . . . . . . . . . . . . . . . . . . . . . 3.6.4 Mathematical Definitions for the System with Harmonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.5 Total Harmonic Distortion . . . . . . . . . . . . . . . . . . . . . . . . 3.6.6 Harmonic Distortion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.7 Total Demand Distortion . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.8 Crest Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.9 Mathematical Analysis of Harmonics . . . . . . . . . . . . . . . . 3.6.10 Harmonic Standard Limits . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Flickers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Reactive Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.9 Location of Wind Turbine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10 Low-Voltage Ride-Through Capability . . . . . . . . . . . . . . . . . . . . . 3.11 IEC Recommendation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.12 Grid Code for Wind Farms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Modeling of an Egyptian Electrical Network–Connected AlZafarana Wind Energy System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 System Under Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Description of the System Under Study . . . . . . . . . . . . . . . 4.2.2 Modeling and Control of DFIG . . . . . . . . . . . . . . . . . . . . . 4.2.3 Wind Turbine Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4 Mechanical Drive Train Model . . . . . . . . . . . . . . . . . . . . . 4.2.5 Rotor-Side Converter Controller . . . . . . . . . . . . . . . . . . . . 4.2.6 MATLAB/Simulink Model of Rotor-Side Converter Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.7 Grid-Side Converter Controller . . . . . . . . . . . . . . . . . . . . . 4.2.8 MATLAB/Simulink Model of Grid-Side Converter Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

26 27 30 42 45 45 46 46 49 49 49 53 53 54 55 57 58 58 58 59 60 61 63 64 65 65 66 69 69 69 69 71 71 71 74 75 78 79

Contents

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5

Harmonic Improvement of an Electrical Network–Connected Wind Energy System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 5.2 Harmonic Mitigation Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . 83 5.2.1 Passive Power Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 5.2.2 Active Power Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 5.2.3 Unified Power-Quality Conditioner . . . . . . . . . . . . . . . . . . 93 5.3 Modeling of the System Under Study with a Shunt Active Power Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 5.3.1 Control Strategies of Proposed Shunt Active Power Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 5.3.2 Simulation Results of the System Under Study with a Shunt Active Power Filter . . . . . . . . . . . . . . . . . . .102

6

Voltage Stability Enhancement of an Egyptian Electrical Network–Based Wind Energy System Using STATCOM . . . . . . . . . . .107 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .107 6.2 Voltage Stability Enhancement of System Under Study . . . . . . . . . .107 6.2.1 Performance of DFIG-Based Wind Turbine with Faults on the System . . . . . . . . . . . . . . . . . . . . . . . . . . . .107 6.2.2 A Static Synchronous Compensator . . . . . . . . . . . . . . . . .109 6.2.3 V-I Characteristic of STATCOM . . . . . . . . . . . . . . . . . . . .112 6.2.4 Modeling of the System Under Study with a Static Synchronous Compensator . . . . . . . . . . . . . . . . . . . . . . . .115

7

Economic Evaluation of Electrical Wind Energy in Egypt . . . . . . . . . 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Levelized Cost of Energy (LCOE) . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Wind Energy Characteristics at Different Regions in Egypt . . . . . . 7.4 Environmental Cost Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8

Conclusions and Suggestions for Future Work . . . . . . . . . . . . . . . . . . .173 8.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .173 8.2 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .174

.161 .161 .161 .164 .165

Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .175 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .179 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .189

List of Symbols and Abbreviations

List of Symbols ωt At CFI CFV F1 HDI HDV I1 Ih Imax Pm THDI THDV V1 Vh θI1 θIh θV1 θVh Cp ISC Il Iqr, Idr Iqs, Ids Lm Lr Ls

Turbine Angular Speed Area Swept Current Crest Factor Voltage Crest Factor Fundamental Frequency Individual Harmonic Distortion of Current Individual Harmonic Distortion of Voltage Current RMS Value of the Fundamental Component Current RMS Value of the hth Order Harmonic Maximum Load Current Mechanical Power Total Harmonic Distortion of Current Total Harmonic Distortion of Voltage Voltage RMS Value of the Fundamental Component Voltage RMS Value of the hth Order Harmonic Current Phase Angle of the Fundamental Component Current Phase Angle of the hth Order Harmonic Voltage Phase Angle of the Fundamental Component Voltage Phase Angle of the hth Order Harmonic Power Coefficient Short Circuit Current at the PCC Maximum Demand Loads Current at the PCC Q and D-Axis Rotor Currents Q and D-Axis Stator Currents Mutual Inductance Rotor Inductance Stator Inductance xi

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Pg Plt Pr Ps Ptotal Qr Qs Qtotal Te Tem Tm Vh VDC Vc Vn Vqr,Vdr Vqs, Vds Vr Vtip Vω r s, r r βref ɸqr, ɸdr ɸqs, ɸds ωe ωg C(ψK) D J Q R S k p β ρ σ

List of Symbols and Abbreviations

Grid Power Outflow Long Term Flicker Active Power Outputs from Rotor Side Active Power Outputs from Stator Side Total Active Power Generated by DFIG Reactive Power Outputs from Rotor Side Reactive Power Outputs from Stator Side Total Reactive Power Generated by DFIG Electromagnetic Torque Generated by the DFIG Generator Torque Turbine Torque Magnitude of Individual Harmonic Component Capacitor Voltage Cut-In Wind Speed Nominal System RMS Voltage Q and D-Axis Rotor Voltages Q and D-Axis Stator Voltages Rated Wind Speed Tip Speed Wind Speed Stator and Rotor Resistances Pitch Reference Q and D-Axis Rotor Fluxes Q and D-Axis Stator Fluxes Angular Velocity Generator Angular Speed Flicker Coefficient Calculated from Rayleigh Distribution of the Wind Speed Distortion Power Inertia of the Rotor Reactive Power Rotor Blade Radius Apparent Power Weibull Shape Parameter = 2 Number of the DFIG Poles Pitch Angle The Air Density Leakage Coefficient

List of Symbols and Abbreviations

List of Abbreviations AWES CF CO2 DFIG EPC FACTS FC FSWT GSC HAWT HD IEC IEEE LVRT MCS MPC MPPT PCC PMSG PWM RSC SCIG SFO SHAPF SMPS SSSC STATCOM SVC SVM TCSC TDD THD UPFC UPQC VAWT VSWT WECS WEGS WRIG

Airborne Wind Energy Systems Crest Factor Carbon Dioxide Doubly Fed Induction Generator Electrical Power Control Flexible Alternating Current Transmission System Friction Coefficient Fixed Speed Wind Turbine Grid Side Converter Horizontal Axis Wind Turbines Harmonic Distortion International Electro Technical Commission Institute of Electrical and Electronics Engineers Low Voltage Ride Through Mechanically Capacitor Switched Mechanical Power Control Maximum Power Point Tracking Point of Common Coupling Permanent Magnet Synchronous Generator Pulse Width Modules Rotor Side Converter Squirrel-Cage Induction Generator Stator-Flux Orientation Shunt Active Power Filter Switch Mode Power Supply Static Synchronous Series Compensator Static Synchronous Compensator Static VAR Compensator Space Vector Modulation Thyristor Controlled Series Capacitor Total Demand Distortion Total Harmonic Distortion Unified Power Flow Controller Unified Power Quality Conditioner Vertical Axis Wind Turbines Variable Speed Wind Turbine Wind Energy Conversion Systems Wind Energy Generation Systems Wound Rotor Induction Generator

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List of Figures

Fig. 1.1 Fig. 1.2

Global cumulative wind generation capacity . . . . . . . . . . . . . . . . . . . . . . Wind farm sites in the map of Egypt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 5

Fig. 2.1 Fig. 2.2 Fig. 2.3 Fig. 2.4 Fig. 2.5 Fig. 2.6

12 13 14 14 15

Fig. 2.20 Fig. 2.21 Fig. 2.22 Fig. 2.23

Grid integration of interconnected system . . . . . . . . . . . . . . . . . . . . . . . . . Onshore wind farms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Near-shore wind farms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Offshore wind farms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Airborne wind farms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of wind turbine: (a) horizontal axis wind turbine (b) vertical axis wind turbine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wind energy conversion system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Squirrel cage induction generator wind turbine . . . . . . . . . . . . . . . . . . . Wound rotor induction generator wind turbine . . . . . . . . . . . . . . . . . . . Permanent magnet generator wind turbine . . . . . . . . . . . . . . . . . . . . . . . . Doubly fed induction generator wind turbine . . . . . . . . . . . . . . . . . . . . . Interaction between the different subsystems . . . . . . . . . . . . . . . . . . . . . Model of pitch angle controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pitch-regulated variable speed wind turbine control scheme . . . . Power curve of a variable speed wind turbine . . . . . . . . . . . . . . . . . . . . Wind turbine characteristic curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wind Turbine mechanical power output vs rotor speed . . . . . . . . . . Power regulation operation with wind speed less than the rated wind speed .. . .. . .. . .. . .. . .. . .. . .. . .. . . .. . .. . .. . .. . .. . .. . .. . Power regulation operation with wind speed more than the rated wind speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sub-synchronous operating mode of DFIG . . . . . . . . . . . . . . . . . . . . . . . Super-synchronous operating mode of DFIG . . . . . . . . . . . . . . . . . . . . . Torque–speed characteristics of induction machine . . . . . . . . . . . . . . The ac/dc/ac bidirectional power converter in DFIG . . . . . . . . . . . . .

Fig. 3.1 Fig. 3.2 Fig. 3.3

Voltage sag waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Voltage swell waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summation of fundamental and harmonic content . . . . . . . . . . . . . . .

Fig. 2.7 Fig. 2.8 Fig. 2.9 Fig. 2.10 Fig. 2.11 Fig. 2.12 Fig. 2.13 Fig. 2.14 Fig. 2.15 Fig. 2.16 Fig. 2.17 Fig. 2.18 Fig. 2.19

16 17 18 18 19 19 22 24 25 26 27 28 29 30 35 36 37 42 47 48 51 xv

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Fig. 3.4 Fig. 3.5 Fig. 3.6 Fig. 3.7 Fig. 3.8 Fig. 3.9 Fig. 4.1 Fig. 4.2 Fig. 4.3

Fig. 4.4 Fig. 4.5 Fig. 4.6 Fig. 4.7 Fig. 4.8 Fig. 4.9 Fig. 5.1 Fig. 5.2 Fig. 5.3 Fig. 5.4 Fig. 5.5 Fig. 5.6 Fig. 5.7 Fig. 5.8 Fig. 5.10 Fig. 5.9 Fig. 5.11 Fig. 5.12 Fig. 5.13

Fig. 5.14 Fig. 5.15 Fig. 5.16

List of Figures

Harmonic waveform from the fundamental to the seventh orders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phase sequence of (a) fundamental, (b) second, and (c) third harmonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Simplified voltage flicker waveform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Influence of frequency on the perceptibility of sinusoidal voltage change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wind turbine generators coupled to MV transmission line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Low-voltage ride-through (LVRT) capability . . . . . . . . . . . . . . . . . . . . . Schematic diagram of Al-Zafarana fifth-stage wind farm . . . . . . . . Simulink-based diagram of Al-Zafarana fifth-stage wind farm (overall wind farm diagram) . . . . . . . . . . . . . . . . . . . . . . . . . . . Single unit diagram of each wind turbine: (a) wind turbine and mechanical drive train, (b) DFIG model, and (c) control system model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MATLAB/Simulink model of the wind turbine . . . . . . . . . . . . . . . . . . Two-mass model of the mechanical drive train . . . . . . . . . . . . . . . . . . . The control scheme of the DFIG rotor-side converter . . . . . . . . . . . . MATLAB/Simulink model of the RSC controller . . . . . . . . . . . . . . . . Control scheme of the DFIG Grid-Side Converter (GSC) . . . . . . . MATLAB/Simulink model of the GSC controller . . . . . . . . . . . . . . . . Active harmonic filter and schematic waveforms . . . . . . . . . . . . . . . . . Various control methods of APFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Control technique of a synchronous reference frame algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Control technique of a (p-q) algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . Control technique of direct voltage error by (a) PI and (b) FLC controller . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . Control technique of self-charging with PI and FLC algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Current hysteresis control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Current controller with PWM signal generation . . . . . . . . . . . . . . . . . . Current controller with SVM technique . . . . . . . . . . . . . . . . . . . . . . . . . . . Space vector hexagon – basic space vectors . . . . . . . . . . . . . . . . . . . . . . Left-series UPQC configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Left-shunt UPQC configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The complete model of Egyptian Electrical Network– connected to Al-Zafarana Egypt wind system with SHAPF . .. . .. .. . .. . .. .. . .. .. . .. .. . .. . .. .. . .. .. . .. .. . .. .. . .. . .. .. . The control techniques of SHAPF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Three-phase inverter of SHAPF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Model of compensation current calculation (p-q theory) . . . . . . . . .

52 55 62 63 64 66 70 70

72 73 73 76 77 80 81 85 86 88 89 90 91 92 92 93 93 94 94

95 96 96 97

List of Figures

Fig. 5.17 Fig. 5.18 Fig. 5.19 Fig. 5.20

Fig. 5.21 Fig. 5.22 Fig. 5.23 Fig. 5.24 Fig. 5.25 Fig. 5.26 Fig. 5.27 Fig. 5.28 Fig. 5.29 Fig. 5.30 Fig. 5.31 Fig. 6.1 Fig. 6.2 Fig. 6.3 Fig. 6.4 Fig. 6.5a Fig. 6.5b Fig. 6.5c Fig. 6.6 Fig. 6.7a Fig. 6.7b Fig. 6.8a Fig. 6.8b Fig. 6.8c

xvii

Sallen–Key of the second-order low-pass filter . . . . . . . . . . . . . . . . . . . Magnitude and phase plots of LPF . .. . . .. . . .. . . . .. . . .. . . .. . . .. . . .. . Model of DC voltage regulation (fuzzy logic controller) . . . . . . . . (a) Input Vdc membership function. (b) Input Vdc_ref membership function. (c) Output Plosses membership function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Switching control (hysteresis band controller) . . . . . . . . . . . . . . . . . . . . Voltage waveforms at low voltage side . . . . . . . . . . . . . . . . . . . . . . . . . . . Load current waveforms before filtering at PCC (B 0.96 kV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Load current waveforms (phase a) before filtering at PCC (B 0.96 kV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Source current waveforms before filtering at PCC (B 0.96 kV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Current waveform (phase a) before filtering at PCC (B 0.96 kV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The compensating current of shunt active filter at PCC (B 0.96 kV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The compensating current of shunt active filter at PCC (B 0.96 kV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Current waveforms after filtering at PCC (B 0.96 kV) . . . . . . . . . . . Frequency spectrum of the current waveform before filtering at PCC (B 0.96 kV) . . .. . . . . .. . . . .. . . . .. . . . . .. . . . .. . Frequency spectrum of the current waveform after filtering at PCC (B 0.96 kV) . . . . .. . . . . . . . . . . . . . . . . . . . . .. . . . . . . LVRT requirement for wind generation facilities per FERC order no. 661 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Single line diagram of the STATCOM connected to power grid .. . . . . . .. . . . . . . .. . . . . . .. . . . . . . .. . . . . . .. . . . . . .. . . . . . . .. . . . Block diagram of STATCOM with PI control system . . . . . . . . . . . Block diagram of STATCOM with fuzzy logic control system . .. . .. .. . .. . .. .. . .. .. . .. . .. .. . .. . .. .. . .. . .. .. . .. .. . .. . .. Input variable error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Input variable Δ error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Output variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . STATCOM V-I characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Single line diagram of the system under study after connecting a STATCOM at PCC1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Single line diagram of the system under study after connecting a STATCOM at PCC2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The complete model before using STATCOM . . . . . . . . . . . . . . . . . . . The complete model after using STATCOM at PCC1 (B0.69 kV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The complete model using STATCOM at PCC2 (B22 kV) . . . . .

98 99 99

100 102 102 103 103 103 103 104 104 104 105 105 108 109 111 113 114 114 114 115 116 116 117 117 118

xviii

Fig. 6.9 Fig. 6.10 Fig. 6.11 Fig. 6.12 Fig. 6.13 Fig. 6.14 Fig. 6.15a Fig. 6.15b Fig. 6.15c Fig. 6.16a Fig. 6.16b Fig. 6.16c Fig. 6.17a Fig. 6.17b Fig. 6.17c Fig. 6.18a Fig. 6.18b Fig. 6.18c Fig. 6.19a Fig. 6.19b Fig. 6.19c Fig. 6.20a Fig. 6.20b Fig. 6.20c Fig. 6.20d Fig. 6.20e Fig. 6.21a Fig. 6.21b Fig. 6.21c

List of Figures

Voltage at PCC1 during normal operation . . . . . . . . . . . . . . . . . . . . . . . . Current at PCC1 during normal operation . . . . . . .. . . . . . .. . . . . . . .. . . The average wind speed in the Al-Zafarana region . . . . . . . . . . . . . . . DFIG active power during normal operation . . . . . . . . . . . . . . . . . . . . . . DFIG reactive power during normal operation . . . . . . . . . . . . . . . . . . . DC-link voltage during normal operation . . . . . . . . . . . . . . . . . . . . . . . . . Voltage at PCC1 during SLGF without STATCOM . . . . . . . . . . . . . Voltage at PCC1 during SLGF with PI STATCOM . . . . . . . . . . . . . Voltage at PCC1 during SLGF with fuzzy STATCOM . . . . .. . . . . Current at PCC1 during SLGF without STATCOM . . . . . . . . . . . . . Current at PCC1 during SLGF with PI STATCOM . . . . . . . . . . . . . . Current at PCC1 during SLGF with fuzzy STATCOM . . . . . . . . . . Active power at PCC1 during SLGF without STATCOM . . . . . . . Active power at PCC1 during SLGF with PI STATCOM . . . . . . . Active power at PCC1 during SLGF with fuzzy STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . Reactive power at PCC1 during SLGF without STATCOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactive power at PCC1 during SLGF with PI STATCOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactive power at PCC1 during SLGF with fuzzy STATCOM .. . .. . .. . .. .. . .. . .. .. . .. . .. .. . .. . .. . .. .. . .. . .. .. . .. . DC-link voltage at PCC1 during SLGF without STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . DC-link voltage at PCC1 during SLGF with PI STATCOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DC-link voltage at PCC1 during SLGF with fuzzy STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . Voltage at PCC1 during SLGF without STATCOM, with PI STATCOM and with fuzzy STATCOM . . . . . . . . . . . . . . . . . Current at PCC1 during SLGF without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Active power at PCC1 during SLGF without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Reactive power at PCC1 during SLGF without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . DC-link voltage at PCC1 during SLGF without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Voltage at PCC2 during SLGF without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Current at PCC2 during SLGF without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Active power at PCC2 during SLGF without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . .

118 119 119 119 120 120 120 121 121 121 122 122 122 123 123 123 124 124 124 125 125 125 126 126 126 127 127 127 128

List of Figures

Fig. 6.21d Fig. 6.22a Fig. 6.22b Fig. 6.22c Fig. 6.23a Fig. 6.23b Fig. 6.23c Fig. 6.24a Fig. 6.24b Fig. 6.24c Fig. 6.25a Fig. 6.25b Fig. 6.25c Fig. 6.26a Fig. 6.26b Fig. 6.26c Fig. 6.27a Fig. 6.27b Fig. 6.27c Fig. 6.27d Fig. 6.27e Fig. 6.28a Fig. 6.28b Fig. 6.28c Fig. 6.28d Fig. 6.29a Fig. 6.29b Fig. 6.29c Fig. 6.30a Fig. 6.30b Fig. 6.30c Fig. 6.31a

xix

Reactive power at PCC2 during SLGF without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Voltage at PCC1 during LLF without STATCOM . . . . . . . . . . . . . . . Voltage at PCC1 during LLF with PI STATCOM . . . . . . . . . . . . . . . Voltage at PCC1 during LLF with fuzzy STATCOM . . . . . . . . . . . Current at PCC1 during LLF without STATCOM . . . . . . . . . . . . . . . Current at PCC1 during LLF with PI STATCOM . . . . . .. . . . . . .. . . Current at PCC1 during LLF with fuzzy STATCOM . . . . . . . . . . . . Active power at PCC1 during LLF without STATCOM . . . . . . . . Active power at PCC1 during LLF with PI STATCOM . . . . . . . . . Active power at PCC1 during LLF with fuzzy STATCOM . . . . . Reactive power at PCC1 during LLF without STATCOM . . . . . . Reactive power at PCC1 during LLF with PI STATCOM . . . . . . Reactive power at PCC1 during LLF with fuzzy STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . DC-link voltage at PCC1 during LLF without STATCOM . . . . . DC-link voltage at PCC1 during LLF with PI STATCOM . . . . . . DC-link voltage at PCC1 during LLF with fuzzy STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . Voltage at PCC1 during LLF without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Current at PCC1 during LLF without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Active power at PCC1 during LLF without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Reactive power at PCC1 during LLF without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . DC-link voltage at PCC1 during LLF without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Voltage at PCC2 during LLF without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Current at PCC2 during LLF without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Active power at PCC2 during LLF without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Reactive power at PCC2 during LLF without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Voltage at PCC1 during voltage sag without STATCOM . . . . . . . Voltage at PCC1 during voltage sag with PI STATCOM . . . . . . . Voltage at PCC1 during voltage sag with fuzzy STATCOM . . . . Current at PCC1 during voltage sag without STATCOM . . . . . . . Current at PCC1 during voltage sag with PI STATCOM . . . . . . . . Current at PCC1 during voltage sag with fuzzy STATCOM . . . . Active power at PCC1 during voltage sag without STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. .

128 131 131 132 132 132 133 133 133 134 134 134 135 135 135 136 136 136 137 137 137 138 138 138 139 141 141 142 142 142 143 143

xx

Fig. 6.31b Fig. 6.31c Fig. 6.32a Fig. 6.32b Fig. 6.32c Fig. 6.33a Fig. 6.33b Fig. 6.33c Fig. 6.34a Fig. 6.34b Fig. 6.34c

Fig. 6.34d

Fig. 6.34e

Fig. 6.35a Fig. 6.35b Fig. 6.35c

Fig. 6.35d

Fig. 6.36a Fig. 6.36b Fig. 6.36c Fig. 6.37a Fig. 6.37b

List of Figures

Active power at PCC1 during voltage sag with PI STATCOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Active power at PCC1 during voltage sag with fuzzy STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . Reactive power at PCC1 during voltage sag without STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . Reactive power at PCC1 during voltage sag with PI STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . Reactive power at PCC1 during voltage sag with fuzzy STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . DC-link voltage at PCC1 during voltage sag without STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . DC-link voltage at PCC1 during voltage sag with PI STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . DC-link voltage at PCC1 during voltage sag with fuzzy STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . Voltage at PCC1 during voltage sag without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Current at PCC1 during voltage sag without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Active power at PCC1 during voltage sag without STATCOM, with PI STATCOM, and with fuzzy STATCOM .. . .. . .. . .. .. . .. . .. .. . .. . .. .. . .. . .. . .. .. . .. . .. .. . .. . Reactive power at PCC1 during voltage sag without STATCOM, with PI STATCOM, and with fuzzy STATCOM .. . .. . .. . .. .. . .. . .. .. . .. . .. .. . .. . .. . .. .. . .. . .. .. . .. . DC-link voltage at PCC1 during voltage sag without STATCOM, with PI STATCOM, and with fuzzy STATCOM .. . .. . .. . .. .. . .. . .. .. . .. . .. .. . .. . .. . .. .. . .. . .. .. . .. . Voltage at PCC2 during voltage sag without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Current at PCC2 during voltage sag without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Active power at PCC2 during voltage sag without STATCOM, with PI STATCOM, and with fuzzy STATCOM .. . .. . .. . .. .. . .. . .. .. . .. . .. .. . .. . .. . .. .. . .. . .. .. . .. . Reactive power at PCC2 during voltage sag without STATCOM, with PI STATCOM, and with fuzzy STATCOM .. . .. . .. . .. .. . .. . .. .. . .. . .. .. . .. . .. . .. .. . .. . .. .. . .. . Voltage at PCC1 during voltage swell without STATCOM . . . . . Voltage at PCC1 during voltage swell with PI STATCOM . . . . . Voltage at PCC1 during voltage swell with fuzzy STATCOM .. . .. . .. . .. .. . .. . .. .. . .. . .. .. . .. . .. . .. .. . .. . .. .. . .. . Current at PCC1 during voltage swell without STATCOM . . . . . Current at PCC1 during voltage swell with PI STATCOM . .. . ..

143 144 144 144 145 145 145 146 146 146

147

147

147 148 148

148

149 151 151 151 152 152

List of Figures

Fig. 6.37c Fig. 6.38a Fig. 6.38b Fig. 6.38c Fig. 6.39a Fig. 6.39b Fig. 6.39c Fig. 6.40a Fig. 6.40b Fig. 6.40c Fig. 6.41a Fig. 6.41b Fig. 6.41c

Fig. 6.41d

Fig. 6.41e

Fig. 6.42a Fig. 6.42b Fig. 6.42c

Fig. 6.42d

xxi

Current at PCC1 during voltage swell with fuzzy STATCOM .. . .. . .. . .. .. . .. . .. .. . .. . .. .. . .. . .. . .. .. . .. . .. .. . .. . Active power at PCC1 during voltage swell without STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . Active power at PCC1 during voltage swell with PI STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . Active power at PCC1 during voltage swell with fuzzy STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . Reactive power at PCC1 during voltage swell without STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . Reactive power at PCC1 during voltage swell with PI STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . Reactive power at PCC1 during voltage swell with fuzzy STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . DC-link voltage at PCC1 during voltage swell without STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . DC-link voltage at PCC1 during voltage swell with PI STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . DC-link voltage at PCC1 during voltage swell with fuzzy STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . Voltage at PCC1 during voltage swell without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Current at PCC1 during voltage swell without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Active power at PCC1 during voltage swell without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . .. . . . . . . .. . . . . . . .. . . . . . .. . . . . . . .. . . . . . . .. . . Reactive power at PCC1 during voltage swell without STATCOM, with PI STATCOM, and with fuzzy STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . DC-link voltage at PCC1 during voltage swell without STATCOM, with PI STATCOM, and with fuzzy STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . Voltage at PCC2 during voltage swell without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Current at PCC2 during voltage swell without STATCOM, with PI STATCOM, and with fuzzy STATCOM . . . . . .. . . . . . . .. . . Active power at PCC2 during voltage swell without STATCOM, with PI STATCOM, and with fuzzy STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . Reactive power at PCC2 during voltage swell without STATCOM, with PI STATCOM, and with fuzzy STATCOM .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. . . .. . .. .

152 153 153 153 154 154 154 155 155 155 156 156

156

157

157 157 158

158

158

xxii

Fig. 7.1 Fig. 7.2 Fig. 7.3 Fig. 7.4 Fig. 7.5 Fig. 7.6 Fig. 7.7 Fig. 7.8 Fig. 7.9 Fig. 7.10 Fig. 7.11 Fig. 7.12 Fig. 7.13 Fig. 7.14

List of Figures

Egyptian wind speed atlas estimated at 50 m above ground level . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . Capital expenditures for the wind reference project . . . . . . . . . . . . . . Monthly average wind speed of Al-Zafarana, Abu Darag, Gulf of Elzayt, and Ras Ghareb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Annual average wind speed of Al-Zafarana, Abu Darag, Gulf of Elzayt, and Ras Ghareb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Monthly average wind speed of Marsa Matruh, Sidi Barrani, El-Suez and Hurghada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Annual average wind speed of Marsa Matruh, Sidi Barrani, El-Suez, and Hurghada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Monthly average wind speed of Alexandria, Port Said, Qena, Luxor, and Aswan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Annual average wind speed of Alexandria, Port Said, Qena, Luxor, and Aswan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recent capital cost estimates for wind energy technology . . . . . . . Operation and maintenance (O&M) cost for wind energy technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Capacity factor curves for different wind speed of the selected wind turbines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Impact of selected input parameters on the wind energy cost for region I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Impact of selected input parameters on the wind energy cost for region II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Impact of selected input parameters on the wind energy cost for region III . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

162 163 165 165 166 166 167 167 168 168 168 170 171 171

List of Tables

Table 1.1

Al-Zafarana wind farm projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5

Table 2.1 Table 2.2

The advantages and disadvantages of WTGs in the current market . . . .. . . .. . . .. . . .. . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . The comparison of the different types of WTGs . . . . . . . . . . . . . . . . . .

20 21

Table 3.1 Table 3.2 Table 3.3

Harmonics and their corresponding sequence components . . . . . . IEEE 519-1992 voltage harmonic limits . . . . . . . . . . . . . . . . . . . . . . . . . . . IEEE 519-1992 current harmonic limits . . . . . . . . . . . . . . . . . . . . . . . . . . .

56 60 61

Table 5.1 Table 5.2

Strengths and weaknesses of control methods used in AHFs . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . . .. . . 87 Fuzzy rule base . . .. . .. .. . .. . .. . .. . .. .. . .. . .. . .. . .. . .. .. . .. . .. . .. . .. .. . 101

Table 6.1 Table 6.2 Table 6.3 Table 6.4 Table 6.5 Table 6.6 Table 6.7 Table 6.8 Table 6.9

Fuzzy rule base . . .. . .. .. . .. . .. . .. . .. .. . .. . .. . .. . .. . .. .. . .. . .. . .. . .. .. . Comparison of the results during SLGF at PCC1 . . . . . . . . . . . . . . . . . Comparison of the results during SLGF at PCC2 . . . . . . . . . . . . . . . . . Comparison of the results during LLF at PCC1 . . . .. . . . . .. . . . . .. . . Comparison of the results during LLF at PCC2 . . . .. . . . . .. . . . . .. . . Comparison of the results during voltage sag at PCC1 . . . . . . . . . . . Comparison of the results during voltage sag at PCC2 . . . . . . . . . . . Comparison of the results during voltage swell at PCC1 .. . .. . . .. Comparison of the results during voltage swell at PCC2 .. . .. . . ..

113 129 130 140 141 149 150 159 159

Table 7.1 Table 7.2

Technical specifications of the selected wind turbines . . . . . . . . . . . The impact of capacity factor on the LCOE for windy regions in Egypt .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . The impact of capital cost on the LCOE for windy regions in Egypt .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . The impact of O&M cost on the LCOE for windy regions in Egypt .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. . .. .

167

Table 7.3 Table 7.4

169 169 170

xxiii

xxiv

Table A.1 Table A.2 Table A.3 Table A.4 Table A.5 Table A.6 Table A.7

List of Tables

DFIG model parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Design parameters of the variable-speed wind turbine . . . . . . . . . . . Parameters of transmission lines . . . . .. . . . .. . . .. . . . .. . . . .. . . . .. . . .. . . Parameters of the mechanical drive train . . . . . . . . . . . . . . . . . . . . . . . . . . Parameters of the rotor-side converter (RSC) controller . . . . . . . . . . Parameters of the GSC controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Parameters of the used shunt active filters . . . . . . . . . . . . . . . . . . . . . . . . .

175 176 176 176 176 177 177

Chapter 1

Introduction and Literature Review

1.1

Introduction

Nowadays, the most critical issue in the entire world is to meet the permanent growth of the energy demand. Some projections indicate that the global energy demand will almost triple by 2050 [1]. Moreover, the rapid depletion of conventional power sources and their adverse impact on the future of the planet have necessitated imperative researches for renewable energy sources as alternative sources of energy. In addition, the use of renewable energy sources is desired to improve energy efficiency, which is essential to sustainable economic development. Furthermore, the use of renewable energy sources also reduces the combustion of fossil fuels and consequent carbon dioxide (CO2) emission, which is the principal cause of greenhouse effect/global warming [2, 3]. Wind energy is the most promising among the several renewable resources from both technical and economic prospects, and thus the wind turbine systems have gained growing attention worldwide [4]. The term “wind energy” or “wind power” describes the process by which the wind is employed to generate mechanical power or electricity. Nowadays, wind power generation can effectively compete with the conventional power generation sources, and it may become the most cost-effective source of electrical power in the near future [5, 6].

1.2

Wind Power Generation

Developments in many other areas of technology were adapted to wind turbines and have helped to hasten their quick emergence. A few of the many areas that have contributed to the new generation of wind turbines include materials science,

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 W. A. Hafez, A. A. Elbaset, Power Quality Enhancement of Wind Energy Systems, https://doi.org/10.1007/978-3-031-43243-9_1

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1 Introduction and Literature Review

aerodynamics, power electronics, computer science, testing, and analytical methods. The main options in wind turbine design and construction include the following [7]: • • • • • • • • • •

Axis of rotation: horizontal or vertical Number of blades (commonly two and three) Rotor orientation: downwind or upwind of tower Blade material, construction method, and profile Hub design: rigid, teetering, or hinged Power control via aerodynamic control (stall control) or variable pitch blades (pitch control) Fixed or variable rotor speed Orientation by self-align action (free yaw) or direct control (active yaw) Synchronous or induction generator Gearbox or direct drive generator

Recently, most of the commercially available utility-scale wind turbines that are employed in wind energy generation systems (WEGS) are based on the “Danish concept” turbine configuration [8]. This configuration has a horizontal axis, threebladed rotor, an upwind orientation, and an active yaw system to keep the blades always oriented in the direction of wind flow. In addition, the mechanical drive train consists of a low-speed shaft connecting the turbine rotor to the gearbox, a 2–3 stage speed-increasing gearbox, and a high-speed shaft connecting gearbox to the generator [9]. Unlike the conventional power generation plants that utilize the synchronous generators with fixed speed, the wind turbines can operate as a fixed-speed wind turbine or a variable-speed wind turbine. In the fixed-speed wind turbines, the stator of the electrical generator is directly connected to the electrical grid [10]. However, in the variable-speed wind turbines, the electrical generator is controlled and interconnected with the electrical grid through a back-to-back converter [11]. There are various merits of employing variable-speed wind turbines such as high efficiency, supplying active power, flexibility of bidirectional rotor power flow, and a wide range of speed control. Most of the major wind turbine manufacturers are developing new larger wind turbines. These wind turbines are all based on variable-speed operation with pitch control. Three main types of variable-speed wind turbine are illustrated as follows: • Wind turbines equipped with squirrel cage induction generator, connected to the grid through a stator converter cascade [12] • Wind turbines equipped with doubly fed induction generator (DFIG), connected to the grid through a rotor converter cascade [13]. • Wind turbines equipped with a synchronous generator and a stator direct current (DC)-link cascade for network connection [14].

1.3

Worldwide Annual Growth of Wind Power Generation

3

Nowadays, the DFIG has become the most commonly employed in the WEGS due to its several advantages such as variable-speed operation, independent control capabilities of active and reactive power, low power losses, partially rated converters, maximized power capture, reduced stresses of mechanical structure, improved power quality, and less acoustical noise.

1.3

Worldwide Annual Growth of Wind Power Generation

Global cumulative wind generation capacity (GW)

The worldwide annual growth of wind power generation systems gives an exponential curve during the period from 2007 to 2020, as illustrated in Fig. 1.1. By the end of June 2012, the global cumulative wind generation capacity reached about 254 GW, out of which 16.546 GW was added in the first 6 months of 2012. In addition, the cumulative wind generation capacity increased by 36.023 MW, 51.675 MW, 63.330 MW, 54.642 MW, 53.21 MW, 51 MW, 60 MW, and 71.3 MW in 2013, 2014, 2015, 2016, 2017, 2018, and 2019, respectively. By the end of 2020, the global cumulative wind generation capacity amounted to 722.3 GW, with an increase of 11% compared to the previous year. Therefore, as shown in Fig. 1.1, the global cumulative wind generation capacity increased from 93.3 GW in 2007 to roughly 722.3 GW in 2020 [15].

700 600 500 400 300 200 100 0 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020

Year Fig. 1.1 Global cumulative wind generation capacity

4

1.4

1

Introduction and Literature Review

Wind Power Generation in Egypt

As one of 38 countries worldwide with a published National Wind Atlas, Egypt enjoys an excellent wind regime, particularly in the Suez Gulf, where average wind speeds reach over 10 m/s. The Egyptian wind energy market increased from just 5 MW in 2001 to 310 MW at the end of 2007, and 80 MW of new capacity was added in 2007 to the Al-Zafarana wind farm. Above 3000 MW is earmarked for wind power developments in the near future on the Gulf of Suez coast. In April 2007, Egypt’s Supreme Council of Energy announced an ambitious plan to generate 20% of the country’s electricity from renewable sources by 2020, including a 12% contribution from wind energy (the equivalent of 7200 MW of grid-connected wind farms) through wind energy by 2022; this goal comes as part of Egypt’s 2030 strategic plan. With the Al-Zafarana project, Egypt has moved on from limited experimental projects to large-scale grid-connected wind farms. Overall, 305 MW has been installed in different stages: 63 MW in 2001, 77 MW in 2003/2004, 85 MW in July 2006, and 80 MW in December 2007. The electricity production from the Al-Zafarana from is over 1000 GWh per year at an average capacity factor of 40.6%. A further 240 MW extension of the wind farm is currently being put into place. In addition to this, an area of 656 km2 has been earmarked to host a 3000 MW wind farm at Gulf of EL-Zayt on the Gulf of Suez coast. Studies are being conducted to assess the site potential to host large-scale connected wind farms of 200 MW capacity in cooperation with Germany, 220 MW (in cooperation with Japan), and 400 MW as (a private-sector project) [16].

1.5

Wind Farm in Egypt

Egypt enjoys high wind speeds along the Gulf of Suez with an average wind speed of 10.5 m/s, and Egypt is just one of 38 countries in the world. Since 2001, a series of large-scale wind farms have been established, with a total capacity of 1.2 GW, in cooperation with Germany (KFW), Denmark (DANIDA), Spain (Siemens Gamesa), and Japan (JICA). Implementation of the Spanish project in Jebel El Ziet took place in 2013. Launched in 2017, the Ras Ghareb wind farm project, near the Gulf of Suez (approximately 30 km northwest of Ras Ghareb), is expected to come online in 2019, producing 262.5 MW [17]. Table 1.1 shows the wind farm projects that were implemented in Egypt with the installed capacity of each project, the number of turbines, and the power of each turbine. Figure 1.2 shows wind farm sites in Egypt. The most important wind turbine in Egyptian wind farms is Gamesa; it is also the newest wind turbine and the only one that contains the advanced technology equipped with a DFIG. Al-Zafarana, the fifth-stage project, has been considered a case study in this work.

1.5

Wind Farm in Egypt

5

Table 1.1 Al-Zafarana wind farm projects

No. of projects Al-Zafarana-1

No. of turbines 50

Total nominal power (MW) 30.00

Wind turbine power (KW) 600

Wind farm type Onshore

Al-Zafarana-2

55

33.00

600

Onshore

Al-Zafarana-3

46

30.36

660

Onshore

Al-Zafarana-4

71

46.86

660

Onshore

Al-Zafarana-5

100

85.00

850

Onshore

Al-Zafarana-6

94

79.90

850

Onshore

Al-Zafarana-7

142

119.85

850

Onshore

Al-Zafarana-8

142

119.85

850

Onshore

Gulf of El-Zayt-1

20

40.00

2000

Onshore

Gulf of El-Zayt-2

100

200

2000

Onshore

Ras Gharib

125

262.5

2100

Onshore

Fig. 1.2 Wind farm sites in the map of Egypt

Turbine type Nordex (N43) Nordex (N43) Vesatas (V47) Vesatas (V47) Gamesa (G52) Gamesa (G52) Gamesa (G52) Gamesa (G52) Gamesa (G80) Gamesa (G80) Gamesa (G97)

Financing country Netherlands Germany Netherlands Germany Spain Germany Japan Netherlands Spain Spain United States

6

1

1.6

Introduction and Literature Review

Book Motivation

The wind power plants are generally integrated into the power grid as a conventional generator through grid codes that allows the use of generators in voltage and frequency regulation; the integration results in technical challenges such as powerquality issues and voltage instability problems [18]. The grid code technical requirements for wind energy connection have already been addressed in a number of papers in the past [19, 21]. Depending on the wind turbine technique, significant power-quality issues related to integration of wind farms in weak grids in Egypt are as follows: • • • • • • •

Harmonics and interharmonics Voltage variation Voltage sags and swells Voltage unbalance Flicker Transient Faults

Power-quality problems and voltage instability problems have emerged as significant challenges and issues confronting electric utilities and customers. Flexible alternating current transmission system (FACTS) devices provide significant advantages in transmitting and regulating the alternating current (AC) while also responding instantaneously to improve power quality and power system stability [22]. FACTSs such as static synchronous compensator (STATCOM), static volt-ampere reactive (VAR) compensator (SVC), thyristor-controlled series capacitor (TCSC), unified power-quality conditioner (UPQC), static synchronous series compensator (SSSC), and unified power flow controller (UPFC) can improve power quality and regulate bus voltages, phase angles, and impedance of transmission lines [23, 24].

1.7

Literature Review of Related Researches

There is a plethora of information available in the public domain dealing with various issues in wind power systems such as system applications, design, control strategy, and policy-related matters. This wealth of knowledge can be accessed and utilized by researchers, engineers, policymakers, and other stakeholders to further advance the understanding and implementation of wind energy technologies. There is a group of researchers looking into topics related to policy, carbon emission, global warming, global politics, energy independence, and renewable energy applications. Wind is the most used and developed non-hydro renewable energy source. In addition to the mentioned topics, researchers are also exploring system integration and related issues, such as voltage stability, protection, and control

1.7

Literature Review of Related Researches

7

[25, 26]. Furthermore, variable-speed turbines have the potential to increase energy production by 10–15% compared to fixed-speed wind turbines, depending on site conditions and design parameters [27–29]. The use of DFIG with large wind turbines that operate at variable speeds in order to enhance the efficiency of energy transfer and optimize the performance of wind turbines has been accepted as the best choice at this time [30–34]. Literature [30, 32] states that DFIG wind turbines can provide decoupled active and reactive power control of the generator, which results in more efficient energy production, improved power quality, and improved dynamic performance. Enhancing the performance of the vector control (decoupled control) approach has been an attractive contribution to extensive research [31–33]. Control of a DFIG is more complicated than the control of a standard induction machine. In order to control the DFIG, the rotor current is controlled by a power electronic converter in the rotor circuit [35, 37]. Recently, utility companies have been asked to fulfill certain criteria (grid codes) for the interconnection of wind turbines to the power grid. The grid codes mainly require that wind turbines have low voltage ride through capability and reactive power capability [38]. The first specification seeks to improve transient stability in a power system with a high penetration of wind energy, while the second specification aims to support steady-state voltage regulation in such a power system. The power-quality problems for grids connected with wind turbines by DFIGs involve variations of voltage, flicker, and harmonics [39]. The measurement procedures and estimation of the major power-quality characteristic parameters for wind turbines are indicated in the international electrotechnical commission (IEC) 6140021 standard [40]. Harmonics are one of the most power-quality problem types; IEC61000-3 and IEEE519 standards determine desired regulations governing the harmonics. Passive filters represent traditional solutions to treat this situation [41]. These filters are not effective for the frequency range of harmonic distortions. In Ref. [42], authors have used active filter for eliminating the harmonics and compensating the reactive power. In Ref. [43], simulations were performed to determine the most suitable location for connecting active harmonic filters (AHFs). In Ref. [44], hybrid AHFs were used in the offshore wind energy system (WES) to compensate harmonic resonances. Hybrid active harmonic filters have benefits in WESs. The passive section of this filter is used to reduce the harmonics at the tuning frequency, while the active section supports filtering action and prevents the appearance of unwanted resonances. In Ref. [45], a literature review introduces power-quality problems. The IECAQ9 61400-21 standard addresses various mitigation techniques for harmonic currents produced in wind energy systems (WES). These techniques include several control methods for active harmonic filters and the modification of wind energy conversion systems (WECS) performance as an active filter. The work presented in [46, 47] was based on the fact that the DFIG has a limited reactive power capability. This limitation in the reactive power capability of the DFIG and the need for an external dynamic reactive power compensation can be observed when it is connected to a remotely weak power grid during a fault period [48, 49]. The goal is to maintain the DFIG-based wind farm in service during the

8

1 Introduction and Literature Review

fault by providing a controlled dynamic reactive power compensation, assuming that during the fault periods, the utilities would immediately disconnect the wind farm when an external reactive compensation source does not exist. With reactive power compensation, the possibility of tripping many of wind turbines in a large wind farm during grid faults would be lower and prevent influencing the overall power system voltage stability [50, 51]. Many researches and studies have been conducted in order to maintain DFIG-based wind farm in service during a grid fault. Dynamic reactive power compensation using FACTS devices has been widely investigated as a significant solution to achieve uninterrupted operation of a wind farm equipped with DFIG during grid faults [52, 54]. STATCOM and SVC are the two options available to provide controlled dynamic reactive power compensation. However, the focus of [55, 57] was to investigate DFIG behavior with the STATCOM for voltage support during grid faults.

1.8

Book Objectives

From the above-mentioned research motivations, the objectives of the book are summarized as follows: • Studying power-quality issues of an electrical network – connected wind energy system • Modelling of Egyptian electrical network connected to Al-Zafarana wind system • Application of shunt active power filter on an Egyptian electrical network – connected to Al-Zafarana wind system in order to reduce the total harmonic distortion • Application of STATCOM on an Egyptian electrical network – connected to Al-Zafarana wind system, Egypt, in order to reduce the voltage fluctuations and maintain DFIG-based wind farm in service during grid fault • Computing the energy cost for electrical power generation to study the impact of power-quality issues on the economic evaluation of electrical wind energy in Egypt.

1.9

Book Organization

To achieve the above objectives, this book is organized in six chapters as follows: Chapter 1 Introduction It gives the background for the research and presents a brief description of the published research works in this area. In addition, it presents the motivation, objectives, and the contents of this book.

1.9

Book Organization

9

Chapter 2 Wind Energy Conversion System This chapter investigates the wind farm types, wind turbine structure, and configurations of the wind energy conversion system. Moreover, this chapter presents the main components of a wind turbine model in detail. Chapter 3 Power-Quality and Grid Code Issues of Wind Energy Conversion System This chapter provides the challenges regarding the integration of wind energy into the power systems. In this chapter, issues linked to power quality that arise due to wind turbine integration are discussed, i.e., voltage sag and swell, voltage variation, harmonics, and flicker. In addition, low-voltage ride-through capability and IEC recommendation for determining the power-quality characteristics of wind turbines are discussed. Chapter 4 Power-Quality Improvement of an Egyptian Electrical Network-Connected Al-Zafarana Wind Energy System This chapter focuses on studying the dynamic performance of the grid-connected wind energy system. It includes the case study of Al-Zafarana fifth-stage wind farm station. Chapter 5 Harmonic Improvement of an Egyptian Electrical Network– Connected Wind Energy System This chapter presents a design and simulation of shunt active power filter (SHAPF) using fuzzy logic controller (FLC) to improve the power quality and also injecting the energy generated by wind energy with a very low total harmonic distortion (THD) of the studied system. Chapter 6 Voltage Stability Enhancement of an Egyptian Electrical Network– based Wind Energy System Using STATCOM This chapter displays design and simulation of a STATCOM using FLC for restoring the voltage levels and improving the effects of the grid faults and disturbances such as a single line to ground fault, a line-to-line fault, voltage sag, and voltage swell of the studied system. Chapter 7 Economic Evaluation of Electrical Wind Energy in Egypt This chapter presents an overview of the feasibility of having wind power plants at several windy regions in Egypt. Electricity cost values are computed based on the levelized cost of energy for the electrical power generation from different wind turbines at three scattered regions. Chapter 8 Conclusions and Suggestions for Future Work It summarizes the main conclusions drawn from this book along with recommendations for future work.

Chapter 2

Wind Energy Conversion System

2.1

Introduction

Urbanization and rising living standards have led to an increased demand for electricity, and thus electric utilities have failed to meet modern power demand. Renewable energy sources have appeared over the last two decades as a complement to traditional energy sources in order to satisfy load demand and overcome power problems [58]. Today’s trends are to connect all sizes of generating units such as wind farms, solar farms, biogas generation, and conventional sources such as coal, hydro, and nuclear power plants to the grid system as shown in Fig. 2.1. Wind energy is the most promising among the several renewable resources from both technical and economic prospects, and thus wind turbine systems have gained growing attention worldwide. The term “wind energy” or “wind generation” describes the process by which wind is utilized to generate mechanical power or electrical power. The wind energy conversion system (WECS) is the overall system that converts wind energy into useful electrical energy. Recently, the doubly fed induction generator (DFIG) has become the most commonly used in the WECS due to its special features such as independent control of active and reactive power, partially rated converters, and the ability to extract maximum power from the wind turbine [59].

2.2

Types of Wind Farms

A wind farm is a group of wind turbines that are connected with each other to produce electrical power. A large wind farm may consist of several hundred individual wind turbines and cover an extended area of hundreds of square miles, but the

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 W. A. Hafez, A. A. Elbaset, Power Quality Enhancement of Wind Energy Systems, https://doi.org/10.1007/978-3-031-43243-9_2

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Wind Energy Conversion System

Nuclear Plant Coal Plant

Hydro-electric Plant

Extra High Voltage

Industrial Power Plant

High Voltage

Medium Sized Power Plant

Transmission Grid Distribution Grid

City Network

Power Plnt

Low Voltage Industrial Customers

Rural Network

Solar Farm

Wind Farm Farm

Fig. 2.1 Grid integration of interconnected system [60]

land between the turbines may be used for agricultural or other purposes. A wind farm can also be located offshore. There are different types of wind farms, which are classified on the basis of the location or the area of installation. These are the establishment of wind farms in areas where wind energy is available to generate the maximum amount of electric energy [61]. Wind farms are divided into four different types: onshore, near shore, offshore, and airborne wind farms.

2.2

Types of Wind Farms

13

Fig. 2.2 Onshore wind farms [62]

2.2.1

Onshore Wind Farms

In this type of wind farm, wind turbine installations in hilly or mountainous regions tend to be on ridge lines, generally 3 km or more inland from the nearest shoreline. This is done to exploit the topographic acceleration as the wind accelerates over a ridge [61] as shown in Fig. 2.2; the additional wind speeds gained in this way can increase energy produced because more wind goes through the turbines. The exact position of each turbine matters, because a difference of 30 m could potentially double the output. Many of the largest operational onshore wind farms are located in Germany, China, and the United States.

2.2.2

Near-Shore Wind Farms

Another type of wind farm is the near shore, as shown in Fig. 2.3. Wind turbine installations are on land within 3 km of shoreline or on water within 10 km of land. These areas are good sites for turbine installation because of the wind produced by convection due to differential heating of land and sea each day. Wind speeds in these zones share the characteristics of both onshore and offshore wind, depending on the prevailing wind direction [63].

2.2.3

Offshore Wind Farms

Offshore wind farms are an exciting new area for the industry, largely due to the fact that there are higher wind speeds available offshore, and economies of scale allow for the installation of larger wind turbines offshore. Offshore wind turbine

14

2

Wind Energy Conversion System

Fig. 2.3 Near-shore wind farms [64]

Fig. 2.4 Offshore wind farms [65]

technology is based on the same principles as onshore technology [63]. Foundations are constructed to hold the superstructure, of which there are a number of designs, but the most common is the driven pile. The top of the foundation is painted a bright color to make it visible to ships and has an access platform to allow maintenance teams to dock, as shown in Fig. 2.4. Subsea cables take the power to a transformer (which can be either offshore or onshore), which converts the electricity to a high voltage (normally between 33 kV and 132 kV) before connecting to the grid at a substation on land.

2.2.4

Airborne Wind Farms

An airborne wind turbine is a design concept for a wind turbine with a rotor supported in the air without a tower. Thus, benefiting from more mechanical and aerodynamic options and the higher velocity and persistence of wind at high

2.4

Configurations of the Wind Energy Conversion System

15

Fig. 2.5 Airborne wind farms [67]

altitudes, an electrical generator may be on the ground or air. Airborne wind turbines may operate at low or high altitudes; they are part of a wider class of airborne wind energy systems (AWES) addressed by high-altitude wind power and crosswind kite power. When the generator is on the ground, then the tethered aircraft does not carry the generator mass or have a conductive tether [66]. When the generator is aloft, then a conductive tether would be used to transmit energy to the ground, as shown in Fig. 2.5.

2.3

Wind Turbine Structure

Wind turbines are commonly classified into horizontal axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs), as shown in Fig. 2.6. The classification is made based on the structure and the rotor shaft orientation of such a configuration. The main rotor shaft, gearbox, and generator of HAWT are placed on top of the tower with the main rotor shaft’s orientation parallel to the wind direction. In VAWT, the main rotor shaft, which is perpendicular to the wind direction, is located on top of the turbine, while the gearbox and generator are located near the turbine foundation [68].

2.4

Configurations of the Wind Energy Conversion System

WECS are designed to convert the energy of wind movement into mechanical power. With wind turbine generator (WTGs), this mechanical energy is converted into electricity. The main components of a wind turbine are the rotor, nacelle, tower, and foundation. The rotor of a wind turbine contains blades and hub and is crucial to

16

2

Wind Energy Conversion System

Fig. 2.6 Types of wind turbine: (a) horizontal axis wind turbine (b) vertical axis wind turbine [69]

the efficiency of power output. The blades capture the wind energy from the air passing through and transform it into rotational energy. The shape, dimensions, and number of blades vary, and they are designed in such a way that they can withstand the forces acting on them and achieve the best possible aerodynamic performance for the required energy efficiency. The nacelle consists of the generator, gearbox, and control and protection system [70]. The foundation supports the entire wind turbine to ensure it is firmly fixed onto the ground or the roof. The generator is used to convert mechanical energy into electrical energy. The gearbox is used to increase the rotor rotational speed toward the required generator rotational speed. The control and protection system act as a safety feature to ensure that the turbine does not operate under stressed conditions, as it includes a braking system triggered by the higher wind speed signal by means of excessive wind gusts, such as when the rotor stops. A wind turbine applies WECS, which consists of a mechanical power control (MPC) side and an electrical power control (EPC) side, as shown in Fig. 2.7. However, a power converter is not required in a fixed-speed wind turbine system as this configuration produces a constant voltage and frequency. WECSs are dependent on wind flow dynamics, which are highly nonlinear, non-deterministic, and chaotic in nature. Besides, the most striking characteristic of wind flow that bothers control engineers is its variability. The EPC side of the system demands maximum

2.5

Wind Turbine Generators in the Current Market

17

Mechanical power side

Wind Power

Electrical power side

Generator Rotor

Gearbox

RSC

DC

GSC

Electrical Grid

Fig. 2.7 Wind energy conversion system

mechanical power from the MPC side despite wind intermittent and seasonal interference [71].

2.5

Wind Turbine Generators in the Current Market

Wind turbine generators (WTGs) can be classified into two types according to its operation speed and the size of the associated converters as follows: • Fixed-speed wind turbine (FSWT) • Variable-speed wind turbine (VSWT) FSWT including squirrel cage induction generator (SCIG) led the market until 2003 when DFIG, which is the main type of VSWT, overtook and has been the leading the WTG concept with 85% of the market [72]. For VSWT, wound rotor induction generator (WRIG) has been the main type, but permanent magnet synchronous generator (PMSG) has been drawing more attention and increasing its market share in recent years due to the benefits of PMSG and the drawbacks of WRIG.

2.5.1

Squirrel Cage Induction Generator

The fixed-speed concept is used in this type of wind turbine. In this configuration, the rotor winding consists of uninsulated conductors in the form of copper and aluminum bars embedded in the semi-closed slots. These solid bars are short circuited at both ends by end rings of the same material. Without the rotor core, the rotor bars and end rings look like the cage of a squirrel. The rotor bars form a uniformly distributed winding in the rotor slots. The SCIG is directly connected to the wind through a transformer, as shown in Fig. 2.8. A capacitor bank is used for reactive power compensation, and a soft starter is used for smooth grid connection [73].

18

2

Wind Energy Conversion System

Soft Starter

Gearbox

Grid Transformer

SCIG

Capacitor Bank

Fig. 2.8 Squirrel cage induction generator wind turbine Variable Resistance

Soft Starter

Gearbox

Grid

Grid Transformer

WRIG Capacitor Bank

Fig. 2.9 Wound rotor induction generator wind turbine

2.5.2

Wound Rotor Induction Generator

In the wound rotor-type synchronous generator, the rotor slots accommodate an insulated distributed winding similar to that used on the stator. The wound rotor type of induction generator costs more and requires increased maintenance [74]. WRIG is directly connected to the grid in this type of turbine, as shown in Fig. 2.9. The variable rotor resistance is used for controlling the slip and power output of the generator. The soft starter is used here to reduce inrush current, and a reactive power compensator is used to eliminate the reactive power demand.

2.5.3

Permanent Magnet Synchronous Generator

PMSGs are synchronous machines with their rotor windings replaced by permanent magnets. The generator is connected to the grid via a full-scale frequency converter [75]. As shown in Fig. 2.10, the frequency converter helps to control both the active and the reactive power delivered by the generator to the grid.

2.5

Wind Turbine Generators in the Current Market

19

Grid

Gearbox

DC Bus

PMSG

Grid Transformer Grid Side Converter

Generator Side Converter

Fig. 2.10 Permanent magnet generator wind turbine Grid P, Q Stator

Transformer

GSC

RSC

AC Filter

P,Q Rotor DC Bus

Rotor Gearbox

Rotor Side Control

Grid Side Control

Stator DFIG

Fig. 2.11 Doubly fed induction generator wind turbine

2.5.4

Doubly Fed Induction Generator

In order to satisfy the modern grid codes, the grid turbine system must have the capability of reactive power support. The DFIG-based wind turbine system has more advantages than others. The DFIG wind turbine delivers power through the stator and rotor of the generator. The reactive power can be provided on two sides, hence the use of the term “doubly.” Reactive power can be supported either through a gridside converter or through a rotor-side converter. The stator part of the turbine is directly connected to the grid, and the rotor is interfaced through a crowbar and a power converter. The stator voltage is applied from the grid, and the rotor voltage is induced by the power converter. The power is delivered from the rotor through the power converter to the grid if the generator operates above the synchronous speed. If the generator is below the synchronous speed, then the power is delivered from the grid through the power converter to the rotor. The power converter, as depicted in Fig. 2.11, utilizes pulse width modulation (PWM) techniques to effectively regulate

20

2

Wind Energy Conversion System

the active and reactive power flow, as well as the DC voltage of the link capacitor between the grid and the doubly fed induction generator (DFIG) wind turbine [76]. A crowbar is implemented between the generator and converter to prevent short circuit in the wind energy system, which may result in high current and high voltage. The RSC converter controls the flux of the DFIG wind turbine, which operates at the slip frequency that depends on the rotor speed of the generator. According to the maximum active and reactive power control capability of converter, the power rating of the RSC is determined.

2.6

Comparison of WTG Types

The advantages and disadvantages of the WTGs discussed in the previous section are summarized in Table 2.1 [77]. Table 2.2 represents a comparison of those WTGs with respect to the five criteria; energy yield, cost, reliability, grid support ability, and technical maturity. PMSG has the largest energy yield, followed by other VSWTs, while SCIG has the lowest energy yield, with a value of 10–15% lower than PMSG due to its fix speed [77]. SCIG has the lowest cost, followed by VSWTs; nevertheless, because of its massive wound machine, WRIG has the highest cost. Table 2.1 The advantages and disadvantages of WTGs in the current market Generator type SCIG (FSWT)

WRIG (VSWT)

PMSG (VSWT)

DFIG (VSWT)

Advantages Easier to design, construct, and control Robust operation Low cost

High energy yield Easier to design, construct, and control Variable speed operation can be achieved by controlling the energy extracted from the WRIG rotor Low mechanical stress Highest energy yield Higher active/reactive power controllability Absence of brush/slip ring Low mechanical stress No copper loss on rotor High energy yield High active/reactive power controllability Lower cost on power electronics converter (PEC) Lower losses by PEC Less mechanical stress Compact size

Disadvantages Low energy yield No active/reactive Power controllability High mechanical stress High losses on gear The speed range is limited Poor control of active reactive power High losses in the variable resistance High cost of PM material Demagnetization of PM Complex construction process Higher cost on PEC Higher losses on PEC Large size Existence of brush/slip ring High losses on gear

2.7

Main Components of a Wind Turbine

21

Table 2.2 The comparison of the different types of WTGs Generator concept SCIG WRIG PMSG

Energy yield Low Medium High

DFIG

Medium– High

Cost Low High Medium– high Medium

Reliability High High High

Grid support ability Low Low High

Technical maturity High Medium Medium–High

Medium

Medium

High

When based on the predicted levels of energy output and cost in Table 2.2, the concept of “energy yield per cost” seems intriguing. The highest value is achieved by neither PMSG nor SCIG; doubly fed induction generator (DFIG) technology is the dominant technology in the growing global market for wind power generation, due to the combination of variable-speed operation and a cost-effective partially rated power converter. Reliability is closely related to the existence of brushes and slip rings, which is the main drawback of DFIG. The main focus of this book is on the DFIG, which is the main type of generator used in the wind power industry.

2.7

Main Components of a Wind Turbine

The wind turbine model is composed of the following systems: • Aerodynamic model evaluates the turbine torque Tm as a function of wind speed Vω and the turbine angular speed ωt. • Mechanical system evaluates the generator and turbine angular speed (ωg and ωt) as a function of turbine torque and generator torque (Tm and Tem). • Pitch system evaluates the pitch angle β dynamics as a function of pitch reference βref. • DFIG and power converters transform the generator torque Tem into a grid current as a function of grid voltage. • Control system evaluates the generator torque Tem, pith angle β, and reactive power references as a function of wind speed and grid voltage as shown in Fig. 2.12.

2.7.1

Aerodynamic Wind Turbine

The main objective of the wind turbine is to convert the kinetic energy of the wind into a rotational mechanical power that is transmitted through the mechanical gearbox to the DFIG. Therefore, the wind turbine can be modeled by an aerodynamic input torque that drives the DFIG.

22

2

Wind Energy Conversion System

Wind Model Vω

ωt Aerodynamic Model

Tm

Mechanical Drive Train model

Tem

Vgrid DFIG

ωg Grid Model

Vgenrator Vgrid Converters

β

Vconverter

Pitch Control system

Torque and Reactive Power Control

βref Tem-ref

Qref

Wind Turbine Control Strategy

Vω

Vgrid

Fig. 2.12 Interaction between the different subsystems

The extracted mechanical power and mechanical torque from the wind turbine can be expressed as follows [78]: 1 C ðλ, βÞρAt V 3ω 2 p

ð2:1Þ

3 Pm 12 Cp ðλ, βÞρAt V ω = ωt ωt

ð2:2Þ

Pm = Tm =

where ρ is the air density (kg/m3), At is the area swept by the rotor blades (m2), Vω is the wind speed (m/s), and Cp is the power coefficient, which is a function of λ and β, where λ is the ratio of the rotor blade tip speed and the wind speed (Vtip/Vω) and β is the blade pitch angle in degrees. The relationship between blade tip speed and generator rotor speed is a constant and is defined as:

2.7

Main Components of a Wind Turbine

cp ðλ, βÞ = 0:5176

23

116 - 21 - 0:4β - 5 e =λi þ 0:0068λ λi

1 1 0:035 = λi λ þ 0:08β β3 þ 1

ð2:3Þ ð2:4Þ

With the tip speed ratio: λ=

Rωt Vω

ð2:5Þ

where R is the rotor blade radius and ωt is the turbine angular speed. Then, the extracted mechanical power Pm can be simplified and normalized for particular values of the air density and the area swept out by turbine blades as follows: Pm = K p Cp V 3ω

ð2:6Þ

Example 2.1 Consider a wind turbine with 5 m diameter rotor. The speed of the rotor at 10 m/s wind velocity is 130 r/min and its power coefficient at this point is 0.35. Calculate the tip speed ratio and torque coefficient of the turbine. What will be the torque available at the rotor shaft? Assume the density of air to be 1.24 kg/m3. Solution The area of the rotor is πD2 π52 = = 19:63 m2 4 4

A=

As the speed of the rotor is 130 r/min, the turbine angular speed is: ωt =

2π  130 = 13:6 rad=s 10

The tip speed ratio at this velocity is: λ=

2:5  13:6 = 3:4 10

The torque coefficient is: CT =

0:35 = 0:103 3:4

24

2

Wind Energy Conversion System

From this, the torque developed can be calculated as: T m = 0:5  1:24  19:63  102  0:103 = 313:39 N:m

2.7.2

Mechanical Drive Train Model

The mechanical drive train is composed of the low-speed shaft that couples to the wind turbine, mechanical gearbox, and the high-speed shaft that connects to the DFIG. The mechanical drive train is employed to transfer the mechanical torque from the wind turbine to the rotor of the DFIG. Moreover, the gearbox is the mechanical interface between the low-speed shaft (turbine rotor) and high-speed shaft (generator rotor). The main objective of the mechanical gearbox is converting the low speed of turbine rotor to a high speed of the DFIG rotor. In addition, the two-mass model neglects the moment of inertia of the shaft and gearbox since they are small as compared to the moment of inertia of the wind turbine and the DFIG [79].

2.7.3

Pitch System

Pitch angle controller is based on the principle which is changing the blades angle at the revolutions over the maximal generator speed as well as protecting the generator before overloading at high wind speeds. The optimal angle for wind speeds below the nominal value is approximately zero, and then it increases as the wind speeds grow. This angle has a significant impact on the performance coefficient and the value of the turbine torque [80]. Regulation of the blade angle is modeled, as shown in Fig. 2.13, by a proportional-integral (PI) controller that generates a reference rate of change of pitch; this reference is limited, and a first-order system gives the dynamic behavior of speed control of pitch variation. The pitch angle itself is then obtained by integrating the variation of the angle.

Fig. 2.13 Model of pitch angle controller

2.7

Main Components of a Wind Turbine

2.7.4

25

Control of Variable Speed Wind Turbine

Control of a variable speed wind turbine is needed to calculate the generator torque and pitch angle references in order to fulfil several requirements: • Extract the maximum energy from the wind. • Keep the turbine in the safe operating mode (power, speed, and torque under limits). • Minimize mechanical loads in the drive train. Design of this strategy is a very complicated task strongly related with the aerodynamic and mechanical design of the turbine and indeed only known by the manufacturers. In this section, only the aspects related to energy extraction and speed–power control will be discussed [81]. Figure 2.14 shows a general control scheme for the VSWT, where the two degrees of freedom are the generator torque and the pitch angle. This control is independent of the generator technology and can be simulated without modeling the electrical machine, power converters, and their associated controls by only including the torque dynamics as a first-order system. Moreover,

β

Tblade

β* Pitch Regulator

dβ/dt

Vω Wind Turbine control strategy

Tm

Tem*

Generator torque

Tem

Electric variables

Regulator

Aerodynamic, pitch and Mechanical system

Fig. 2.14 Pitch-regulated variable speed wind turbine control scheme

26

2

Wind Energy Conversion System

for DFIG-based wind turbines, this limitation also serves to limit the slip of the electrical machine, and therefore, the voltage must provide the rotor converter. The following subsections describe the wind turbine control strategy and the control objectives.

2.7.5

Turbine Speed Control Regions

Two distinct operating regions of a variable-speed, variable-pitch DFIG-based wind turbine system can be defined based on incoming wind speed and the generator power output required [82]. The output power from wind turbine Pe can be defined as shown below. Assuming Pe varies when wind speed Vω varies between cut-in Vc and rated Vr wind speeds, the closed form expression for energy production is: Pe = 0, Pe = a þ

bV kω ,

Pe = Pr ,

for ðV ω ≤ V c Þ

ð2:7Þ

for ðV c ≤ V ω ≤ V r Þ

ð2:8Þ

for ðV ω ≥ V r Þ

ð2:9Þ

where Pr is the rated electrical power, k is Weibull shape parameter = 2 for the p vk studied system, and the coefficients a and b are given as a = vk -r cvk and b = vk p-r vk . c r r c As shown in Fig. 2.15, the variable speed WECS can be operated in the maximum power point tracking (MPPT) mode and blade pitch control mode (constant power mode) depending upon the velocity of wind to extract maximum power from the wind as well as to regulate the power output from the wind turbine.

Fig. 2.15 Power curve of a variable speed wind turbine [83]

2.7

Main Components of a Wind Turbine

2.7.6

27

Operating Modes of Wind Turbine

Variable-speed, variable-pitch wind turbine can be operated in three distinct operating modes depending upon the wind speed available and the amount of power output needed from the wind turbine system. Mode I: Maximum Power Point Tracking From the plot shown in Fig. 2.16, it can be stated that for β = 0°, λopt = 8.1 and Cp_max = 0.48. From Eqs. (2.1), (2.2) and (2.3), the mechanical torque can be expressed by: Tm =

2 1 2 RC p ðλ, β ÞρAt V ω

λ

ð2:10Þ

Since Cp(λ, β) = Cp-max(λ, β) constant, in this mode, from Eqs. (2.1) and (2.2): Pm = kV 3ω

ð2:11Þ

1 k = ρAC p-max ðλ, βÞ = constant 2

ð2:12Þ

where

From Eq. (2.10), if the wind generator is run at a particular speed that corresponds to wind speed Vω in such a way that wind turbine operates at maximum power point, as shown in Fig. 2.16, then we can extract maximum available power from the available wind speed via wind turbine.

Fig. 2.16 Wind turbine characteristic curve [84]

28

2

Wind Energy Conversion System

1 Cp-max

Output Mechanical Power (pu)

Vω = 5m/s Vω = 7m/s Vω = 9m/s Vω = 11m/s Vω = 12m/s

0.8

0.6

0.4

0.2

0 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Generator Speed (pu)

Fig. 2.17 Wind Turbine mechanical power output vs rotor speed [84]

The objective of the MPPT operation mode is to maximize power extraction at low-to-medium wind speeds by following the maximum value of the wind power coefficient (Cp_max) as shown in Fig. 2.17. Mode II: Pitch Control (Rated Power Operation) When the wind speed increases beyond the rated value, the electromagnetic torque is not sufficient to control rotor speed because this leads to an overload on the generator and the converter. To prevent the rotor speed from becoming too high, the extracted power from incoming wind must be limited. This can be done by reducing the coefficient of performance of the turbine (Cp value). The Cp value can be manipulated by changing the pitch angle (see Fig. 2.16). Altering the pitch angle β means slightly rotating the turbine blades along the axis. The wind speed is varied, turbine speed is maintained at rated speed (ωt = ωt rated), and corresponding λ is calculated using (2.13). The power output is maintained at rated power (P = Prated). The Cp corresponding to rated power is calculated using (2.14): λ=

ωt - rated R Vω

ð2:13Þ

ωPrated 0:5 AρV ω

ð2:14Þ

Cp =

2.7

Main Components of a Wind Turbine

29

Mode III: Power Regulation With the increased penetration level of wind power in a power system, it is not always possible to operate wind turbines in the MPPT mode and constant power mode only. For maintaining regulated frequency and voltage in the power system, the generated power should be equal to the demanded power. Hence in that condition, the variable-speed, variable-pitch wind turbine should be operated in the power regulation mode. When the load decreases, the power output from the turbine should be reduced to match the load. When the wind speed is less than the rated speed, the pitch angle is always kept at zero (β = 0), and the λ is varied and the corresponding Cp is calculated to obtain the demanded power output from the wind turbine. Then, the wind speed is calculated based upon the demanded power output P given by (2.16): Vω = ωr =

P 0:5 Aρ Cp

1 2

λ Vω  Gear ratio R

ð2:15Þ ð2:16Þ

Figure 2.18 shows the rotor speed with wind speed operating points for various demanded power outputs P. As shown in Fig. 2.18, DFIG wind turbine system can be operated at more than one speed to generate particular power output P from the wind turbine. In this case, the wind turbine should be operated in the closest possible speed from the previous speed to minimize the transient operation period. In this way, disturbance to the pitch controller is also reduced significantly.

Fig. 2.18 Power regulation operation with wind speed less than the rated wind speed [84]

30

2

Wind Energy Conversion System

Fig. 2.19 Power regulation operation with wind speed more than the rated wind speed [84]

If the wind speed is more than the rated speed, then the turbine speed is maintained at rated turbine speed (ωt = ωt-rated), and wind speed is varied. The corresponding λ is calculated and pitch angle is obtained for various values of demanded power P, which gives corresponding Cp using (2.18): λ=

ωt - rated R Vω

ð2:17Þ

P 0:5 AρV ω

ð2:18Þ

Cp =

Figure 2.19 shows the pitch angle with wind speed operating points for various demanded power outputs.

2.7.7

Doubly Fed Induction Generator

The DFIG is currently the system of choice for multi-MW wind turbines. The aerodynamic system must be capable of operating over a wide wind speed range in order to achieve optimum aerodynamic efficiency by tracking the optimum tip speed ratio. Therefore, the generator’s rotor must be able to operate at a variable rotational speed. The DFIG system therefore operates in both sub- and supersynchronous modes with a rotor speed ranging around the synchronous speed. The DFIG consists of stator winding and rotor winding equipped with slip rings. The stator is provided with three-phase insulated windings making up a desired pole design and is connected to the grid through a three-phase transformer. Similar to the

2.7

Main Components of a Wind Turbine

31

stator, the rotor is also constructed of three-phase insulated windings. The rotor windings are connected to an external stationary circuit via a set of slip rings and brushes [85]. By means of these components, the controlled rotor current can be either injected into or absorbed from the rotor windings. The stator and rotor windings are usually coated with insulation and are mechanically assembled to form a closed structure to protect the machine from dust, dampness, and other unwanted pollutants ensuring proper magnetic coupling between rotor and stator windings. The dynamics of the DFIG is represented by a fourth-order state space model using the synchronously rotating reference frame (q-d frame) as given in (2.19), (2.20), (2.21), and (2.22) [85]. Figure 2.19 shows the equivalent circuit of the DFIG in the d-q synchronous reference frame where all the rotor quantities are referred to the stator side. d ɸ dt qs d V ds = r s I ds þ ωe ɸqs þ ɸds dt d V qr = r s I qr þ ðωe - ωr Þ ɸdr þ ɸqr dt d V dr = r s I dr þ ðωe - ωr Þ ɸqr þ ɸdr dt V qs = r s I qs þ ωe ɸds þ

ð2:19Þ ð2:20Þ ð2:21Þ ð2:22Þ

where Vqs, Vds, Vqr, and Vdr are the q- and d-axis stator and rotor voltages, respectively; Iqs,Ids,Iqr, and Iqr are the q- and d-axis stator and rotor currents, respectively; ɸqs, ɸds, ɸqr, and ɸdr are the q- and d-axis stator and rotor fluxes, respectively; ωe is the angular velocity of the synchronously rotating reference frame; ωr is the rotor angular velocity; and rs and rr are the stator and rotor resistances, respectively. The flux linkage equations are given as: ɸqs = Ls I qs þ Lm I qr

ð2:23Þ

ɸds = Ls I ds þ Lm I dr

ð2:24Þ

ɸqr = Lm I qs þ Lr I qr

ð2:25Þ

ɸdr = Lm I qs þ Lr I dr

ð2:26Þ

where Ls, Lr, and Lm are the stator, rotor, and mutual inductances, respectively, with Ls = Lls + Lm and Lr = Llr + Lm as the self inductance of stator and the self inductance of rotor, respectively. Solving (2.23), (2.24), (2.25), and (2.26) in terms of current equations: I qs =

1 L ɸ - m ɸ σLs qs σLs Lr qr

ð2:27Þ

32

2

I ds =

1 L ɸ - m ɸ σLs ds σLs Lr dr

ð2:28Þ

I qr = -

Lm 1 ɸ þ ɸ σLs Lr qs σLr qr

ð2:29Þ

I dr = -

Lm 1 ɸ þ ɸ σLs Lr ds σLr dr

ð2:30Þ

where leakage coefficient σ =

2.7.7.1

Wind Energy Conversion System

Ls Lr - L2m Ls Lr .

Dynamic Modeling of DFIG in State Space Equations

The dynamic modeling in state space form is necessary to carry out simulation using different simulation tools such as MATLAB. The basic state space form helps to analyze the system in the transient condition. According to the basic definition, the space whose coordinate axes are the “n” state variables with time as the implicit variable is called the state space. The variables of the state space (state variables) are involved in determining the state of the dynamic system. Basically, these are the energy-storing elements contained in the system like inductor and capacitor [86]. The fundamental equations of the state space are as follows: X_ ðt Þ = AX ðt Þ þ BU ðt Þ

ð2:31Þ

Y ðt Þ = CX ðt Þ þ DU ðt Þ

ð2:32Þ

Equations (2.31) and (2.32) are for linear time invariant system, where A, B, C, and D are state, input, output, and feed forward matrices, respectively, X is the state vector, and Y is the output vector. Equations (2.33) and (2.34) are for linear time variant system, where A, B, C, and D are time-dependent matrices: X_ ðt Þ = Aðt ÞX ðt Þ þ Bðt ÞU ðt Þ

ð2:33Þ

Y ðt Þ = C ðt ÞX ðt Þ þ Dðt ÞU ðt Þ

ð2:34Þ

In the DFIG system, the state variables are normally currents or fluxes. In the following section, the state space equations for the DFIG in the synchronously rotating frame have been derived with flux linkages as the state variables [86]. Substituting (2.27), (2.28), (2.29), and (2.30) into (2.19), (2.20), (2.21), and (2.22) gives the DFIG dynamics in the state space form as: d r rL ɸ = - s ɸqs - ωe ɸds þ s m ɸqr þ V qs σLs σLs Lr dt qs

ð2:35Þ

2.7

Main Components of a Wind Turbine

33

r rL d ɸ = ωe ɸqs - s ɸds þ s m ɸdr þ V ds σLs σLs Lr dt ds

ð2:36Þ

rL r d ɸ = r m ɸ - ðωe - ωr Þɸdr - r ɸqr þ V qr σLr dt qr σLs Lr qs

ð2:37Þ

d rL r ɸ = r m ɸ - r ɸ þ ðωe - ωr Þɸqr þ V dr dt dr σLs Lr ds σLr dr

ð2:38Þ

Equations (2.35), (2.36), (2.37), and (2.38) are written in state space matrix form as:

ɸ_ qs ɸ_ ds ɸ_ qr

- ωe

ωe

-

= r r Lm σLs Lr

ɸ_ dr

r s Lm σLs Lr

rs σLs

0

0

rr σL r

r r Lm σLs Lr

0

þ

2.7.7.2

rs σLs

-

0 r s Lm σLs Lr ðωe - ωr Þ

ðωe - ωr Þ

1

0

0 0

V qs

0

1

0 0

V ds

0

0

1

0

V qr

0

0

0

1

V dr

-

rr σ Lr

ɸqs ɸds ɸqr ɸdr

ð2:39Þ

Active Power, Reactive Power, and Torque Calculation

All the equations above are induction motor equations. When the induction motor operates as a generator, the current direction will be opposite. Assuming negligible power losses in stator and rotor resistances [87], the active and reactive power outputs from stator and rotor side are given as: 3 2 3 Qs = 2 3 Pr = 2 3 Qr = 2 Ps = -

V qs I qs þ V ds I ds

ð2:40Þ

V qs I ds - V ds I qs

ð2:41Þ

V qr I qr þ V dr I dr

ð2:42Þ

V qr I qr - V dr I dr

ð2:43Þ

The total active and reactive power generated by DFIG is:

34

2

Wind Energy Conversion System

Ptotal = Ps þ Pr

ð2:44Þ

Qtotal = Qs þ Qr

ð2:45Þ

If Ptotal and/or Qtotal is positive, DFIG supplies power to the power grid, else it draws power from the grid. In the induction machine, the electromagnetic torque is developed by the interaction of air-gap flux and the rotor magnetic-motive force (mmf). At synchronous speed, the rotor cannot see the moving magnetic field; as a result, there is no question of induced electromotive force (emf) and the rotor current, so the torque becomes zero. But at any speed other than the synchronous speed, the machine experiences torque. This is true in case of motor, whereas in the case of WTGs, electromechanical torque is provided by means of prime mover, which is wind turbine in DFIGbased WECS [87]. The rotor speed dynamics of the DFIG is given as: p d ð T - T e - C f ωr Þ ω = dt r 2J m

ð2:46Þ

where p is the number of poles of the DFIG, Cf is the friction coefficient, J is the inertia of the rotor, Tm is the mechanical torque generated by wind turbine, and Te is the electromagnetic torque generated by the DFIG, which can be written in terms of flux linkages and currents as follows: Te =

3 λ I - λds I qs 2 qs ds

ð2:47Þ

where positive (negative) values of Te means DFIG works as a generator (motor).

2.7.7.3

Power Flow of DFIG

The DFIG stator is connected to the grid with fixed grid frequency fs at fixed grid voltage Vs to generate constant frequency AC power during all operating conditions, and the rotor is connected to the frequency converter voltage source converter (VSC) having a variable (slip/rotor) frequency ( fr = Sfs). At constant frequencyfs, the magnetic field produced in the stator rotates at constant angular speed (ωs= 2πfs), which is the synchronous speed of the machine. The stator rotating magnetic field will induce a voltage between the terminals of the rotor. This induced rotor voltage produces a rotor current Ir, which in turn produces a rotor magnetic field that rotates at variable angular speed (ωr = 2πfr). Usually, the stator and rotor have the same number of poles p, and the convention is that the stator magnetic field rotates clockwise. Therefore, the stator magnetic field rotates clockwise at a fixed constant speed of ωs (rpm) = 120fs/p. Since the rotor is connected to the variable frequency VSC, the rotor magnetic field also rotates at a speed of ωr (rpm) = 120fr/p [88].

2.7

Main Components of a Wind Turbine

2.7.7.4

35

Operating Modes of DFIG

• Sub-Synchronous Speed Mode Figure 2.20 illustrates the case where the rotor magnetic field rotates at a slower speed than the stator magnetic field. The machine is operated in the sub-synchronous mode, i.e., ωm < ωs. If and only if its speed is exactly equal to ωm = ωs - ωr > 0 and both the phase sequences of the rotor and stator mmf’s are the same and in the positive direction it is referred to as positive phase sequence (ωr ≥ 0).This condition takes place during slow wind speeds. In order to extract maximum power from the wind turbine, the following conditions should be satisfied: The rotor side converter (RSC) shall provide low frequency AC current (negative Vr will apply) for the rotor winding. The rotor power shall be supplied by the DC bus capacitor via the RSC, which tends to decrease the DC bus voltage. The grid-side converter (GSC) increases/ controls this DC voltage and tends to keep it constant. Power is absorbed from the grid via the GSC and delivered to the rotor via the RSC. During this operating mode, the GSC operates as a rectifier and RSC operates as an inverter, hence power is delivered to the grid by the stator [88]. The rotor power is capacitive, and the torque speed characteristic of doubly fed induction generator is shown in Fig. 2.22. • Super-Synchronous Speed Mode The super-synchronous speed mode is achieved by having the rotor magnetic field rotate counterclockwise as shown in Fig. 2.21. However, in order to represent the counterclockwise rotation of the rotor, which is analytically equivalent to inverting the direction of the rotor magnetic field, a negative sign can be introduced

Vs , f s

Constant Grid

ωs Stator Field

Stator Power Ps

ωr Stator

Rotor Field

Rotation Mechanical Power Pm

ωm

ω m = ω s- ω r

Vr , f r ω r> 0

Rotor Power Pr

Fig. 2.20 Sub-synchronous operating mode of DFIG

Variable

36

2

Wind Energy Conversion System

Vs , f s

Constant Grid

ωs Stator Field

Stator

Stator Power Ps

ωr Rotor Field

Rotation Mechanical Power Pm

ωm

ω m= ω S+ ω r

Vr , f r ω r
ωr, if and only if its speed is exactly ωm = ωs – (-ωr) = ωs+ ωr > 0, and the phase sequence in the rotor rotates in opposite direction to that of the stator, i.e., negative phase sequence (ωr < 0). This condition takes place during the condition of high wind speeds. The following conditions need to be satisfied in order to extract maximum power from the wind turbine and to reduce mechanical stress: The rotor winding delivers AC power to the power grid through the VSCs. The rotor power is transmitted to DC bus capacitor, which tends to raise the DC voltage. The GSC reduces/controls this DC-link voltage and tends to keep it constant. Power is extracted from the RSC and delivered to the grid. During this operating mode, the RSC operates as a rectifier and the GSC operates as an inverter. Hence, power is delivered to the grid directly by the stator and via the VSCs by the rotor [88]. The rotor power is inductive, and the torque speed characteristic of doubly fed induction generator is shown in Fig. 2.22. • Synchronous Speed Mode The machine is operated in the synchronous speed mode, i.e., ωm = ωs, if and only if its speed is exactly ωm = ωs – 0 = ωs > 0, and the phase sequence in the rotor is the same as that of the stator, but no rotor mmf is produced (ωr = 0). The following

2.7

Main Components of a Wind Turbine

37

Fig. 2.22 Torque–speed characteristics of induction machine

conditions are necessary in order to extract maximum power from the wind turbine under this condition: The rotor side converter shall provide DC excitation for the rotor so that the generator operates as a synchronous machine. The RSC will not provide any kind of AC current/power for the rotor winding. Hence, the rotor power is zero (Pr = 0). A substantial amount of reactive power can still be provided to the grid by the stator. As per the operating modes described above, at any wind speeds, a wide range of variable speed operation can be performed to achieve maximum wind power extraction [88]. Example 2.2 A 1.0 MW, 575 V, 60 Hz, 2160 rpm DFIG is used in a variable-speed wind energy conversion system. The parameters of the generator are: • • • •

Number of pole pairs = 2 Rated mechanical torque, Tm= -4421 N.m Stator winding resistance, Rs= 3.654 mΩ Stator leakage inductance, Ls = 0.1304 mH

The generator operates with an MPPT scheme, and its stator power factor is unity. Assuming that the DFIG operates at a super-synchronous speed of 2160 rpm, determine the following: (a) (b) (c) (d)

The generator mechanical torque and power The rms stator current The rms magnetizing voltage and current The rms rotor current and voltage

38

2

Wind Energy Conversion System

(e) The equivalent resistance and reactance for the rotor side converter (f) The maximum torque and the corresponding slip Solution (a) The generator mechanical torque and power The rotor mechanical speed 2π = 226:19 rad=s 60

ωm = 2160 × The rotor electrical speed

ωr = ωm × p = 226:19 × 2 = 452:39 rad=s The rated rotor mechanical speed ωm,r = 2160 ×

2π = 226:19 rad=s 60

The stator electrical speed ωs = ωm = 2π × 60 = 376:99 rad=s The pu rotor speed ωm,pu =

ωm ωm,r

=

226:19 226:19

= 1:0 pu

The generator mechanical torque at 1.0 pu rotor speed T m = T m,r × ωm,pu

2

= - 4421 × ð1:0Þ2 = - 4421 N:m

The rated mechanical power Pm,r = ωm,r × T m,r = 226:19 × ð- 4421Þ - 1000 × 103 w The generator mechanical power at 1.0 pu rotor speed P = Pm,r × ðωm,r Þ3 - 1000 × ð0:1Þ3 = - 1000 × l03 w (b) The stator current

Is =

Vs ±

V 2s 2Rs

4Rs T m ωs 3P

= - 829:18 ðrmsÞ

2.7

Main Components of a Wind Turbine

39

(c) The magnetizing branch voltage 575 V m = V s - I s ðRs þ jωs Ls Þ = p ∠0∘ - 829:18∠180∘ × 3:654 × 10 - 3 þ j120π × 0:130 × 10 -3 3 = 337:48∠6:94∘ VðrmsÞ

The magnetizing current can be calculated by Im =

Vm = 217:28∠ - 83:1∘ AðrmsÞ jωs Ls

(d) The rotor current I r = I s - I m = 829:18∠180∘ - 217:28Z - ∠83:1∘ = 882:19∠165:85 ° A ðrmsÞ The rotor voltage V r = sV m - I r ðRr þ jsωs Lr Þ = 67:97∠165:83 ° VðrmsÞ where s = (ωs - ωr)/ωs = (376.99 - 452.39)/376.99 = -0.2 (e) The equivalent impedance for the rotor side converter is given by Z eq =

Vr = 0:06782 þ j0:03656 Ω Ir

from which Req = 0.06782 Ω and Xeq = 0.03656 Ω (f) The slip at which the maximum torque occurs can be obtained from the following equation

STmax = ±

Rr þ Req

2

þ X 2eq

R2s þ ðxs þ xr Þ2

= - 0:8497

s = +0.8497 is omitted because of the super-synchronous mode of operation. The maximum torque T max =

1 × 2ωs ðX S þ X r ÞX eq P Rs þ Rr þ Req

= - 16214 N - m ð3:6675 puÞ

3V 2s ðX s þ X r Þ2 þ R2s × 1 þ

X 2eq Rr þ Req

2

40

2

Wind Energy Conversion System

Example 2.3 A variable-speed WECS employs a 6.0 MW, 4000 V, 50 Hz, 1170 rpm DFIG. The parameters of the generator are: • • • •

Number of pole pairs = 3 Rated mechanical torque Tm= -48,971 N.m Stator winding resistance, Rs = 26.86 mΩ Stator leakage inductance, Ls = 0.23142 mH

The generator operates with an MPPT scheme, and its stator power factor is unity. Assuming that the DFIG operates at a super-synchronous speed of 1170 rpm, find the following: (a) (b) (c) (d) (e)

The rms and q-d axis stator current The rms magnetizing voltage and current The rms and q-d axis rotor currents The rms and q-d axis rotor voltages The equivalent resistance and reactance for the rotor side converter

Solution (a) The rms and q-d axis stator current The rotor mechanical speed ωm = 1170 ×

2π = 122:52 rad=s 60

The rotor electrical speed ωr = ωm × p = 122:52 × 3 = 367:56 rad=s The rated rotor mechanical speed ωm,r = 1170 ×

2π = 122:52 rad=s 60

The stator frequency ωs = 2π × 50 = 314:16 rad=s The pu rotor speed ωm,pu =

ωm ωm,r

=

122:52 122:52

= 1:0 pu

The generator mechanical torque at 1.0 pu rotor speed T m = T m,r × ωm,pu

2

= - 48971 × ð1:0Þ2 = - 48971 N:m

2.7

Main Components of a Wind Turbine

41

The stator current

Is =

Vs ±

V 2s -

4Rs T m ωs 3P

2Rs

= - 733:93 A ðrmsÞ

The stator current phasor is I s = I s ∠ - φs = 733:93∠ - 180∘ A ðrmsÞ The dq-axis stator currents can be given by I ds = I s cos∠I s = 733:93 × cosð180∘ Þ = - 733:93 A ðrmsÞ I qs = I s sin∠I s = 733:93 × sinð180∘ Þ = 0 A ðrmsÞ (b) The rms magnetizing voltage and current The magnetizing branch voltage 4000 V m = V s - I s ðRs þ jωs Ls Þ = p ∠0∘ - 733:93∠180∘ 3 -3 × 26:86 × 10 þ j100π × 0:23142 × 10 - 3 = 2329:73∠1:3∘ VðrmsÞ The magnetizing current can be calculated by Im =

Vm = 286:23∠ - 88:69∘ AðrmsÞ jωs Ls

(c) The rms and dq-axis rotor currents I r = I s - I m = 733:93∠180∘ - 286:23∠ - 88:69∘ = 793:86∠158:87 ° A ðrmsÞ The dq-axis rotor currents can be given by I dr = I r cos∠I r = 733:93 × cosð158:87∘ Þ = - 740:49 A ðrmsÞ I qr = I r sin∠I r = 733:93 × sin ð158:87∘ Þ = 286:16 A ðrmsÞ (d) The rms and dq-axis rotor voltage Vr = sVm - Ir ðRr þ jsωs Lr Þ = 381:05∠ - 176:23 ° VðrmsÞ where s = (ωs - ωr)/ωs = - 0.17

42

2

Wind Energy Conversion System

The dq-axis rotor voltages can be given by V dr = V r cos∠V r = 381:05 × cosð- 176:23 ° Þ = - 380:23 VðrmsÞ V qr = V r sin∠V r = 381:05 × sinð- 176:23 ° Þ = - 25:1V ðrmsÞ (e) The equivalent impedance for the rotor side converter is given by Z eq =

Vr = 0:43538 þ j0:20211 Ω Ir

from which Req = 0.43538 Ω and Xeq = 0.20211 Ω

2.7.8

Power Converters of DFIG

The power converter of DFIG is made up of a back-to-back converter connecting the rotor circuit and the grid as shown in Fig. 2.23. The converters are typically made up of voltage source inverters equipped with insulated gate bipolar transistor (IGBTs) provided with freewheeling diodes, which enable a bidirectional power flow. A resistance-inductance RL-filter is provided on the GSC output to minimize switching harmonics supplied to the grid [84].

2.7.8.1

Rotor Side Converter

The power rating of the RSC is determined by two factors, namely maximum slip power and reactive power control capability. The rotor-side converter can be seen as Rotor Side Converter

DC link Capacitor

Grid Side Converter

Filter

Fig. 2.23 The ac/dc/ac bidirectional power converter in DFIG

2.7

Main Components of a Wind Turbine

43

a current-controlled voltage source converter. The control objective of RSC is to regulate the stator side active power (or rotor speed) and the stator side reactive power independently [84].

2.7.8.2

Grid-Side Converter

The power rating of the GSC is mainly determined by maximum slip power since it usually operates at a unity power factor to minimize the losses in the converter [89]. The GSC is normally dedicated to controlling the DC-link voltage only. The amount of energy stored in the dc-link capacitor can be written as: Ec =

Pdt =

1 CV 2DC 2

ð2:48Þ

where P is the net power flow into the capacitor, C is the DC-link capacitor value, and VDC is the capacitor voltage. P is equal to Pr - Pg, where Pr is the rotor power inflow and Pg is the grid power outflow. After studying this chapter, we can answer the following questions: 1. 2. 3. 4. 5. 6.

What are the types of wind farms? What are the classification of wind farms? What are the main components of a wind turbine? How to control variable speed wind turbines? What are the turbine speed control regions? What are the operating modes of DFIG?

Chapter 3

Power-Quality and Grid Code Issues of Wind Energy Conversion System

3.1

Introduction

A power-quality problem is any change displayed in voltage, current, power, or frequency that causes a failure in customer devices, which is caused due to non-sinusoidal wave forms [90]. The power quality in relation to a wind turbine describes the electrical performance of wind energy generating system. It reflects the generation of grid interference and the influence of a wind turbine on power and voltage quality of grid. There has been an extensive growth and quick development in the use of wind energy in recent years [90]. It has been suggested that today’s industrial development is related to the generalized use of computers, adjustable speed drives, and other microelectronic loads. It also becomes an increasing concern with power quality to the end customer. The presence of harmonics and reactive power in the grid is harmful because it will cause additional power losses and malfunction of grid components. The massive penetration of electronically controlled devices and equipment in the low-voltage distribution network is responsible for further worsening of power quality problems [91, 92]. The wind power in the electric grid system affects the voltage quality. To assess this effect, the knowledge of the electrical characteristic of wind turbine is needed. This means that by having the actual parameter values for a specific wind turbine, the expected impact of the wind turbine on voltage quality is found. Today, the measurement and assessment of the power-quality characteristics of the grid-connected wind turbines are defined by IEC Standard 61400-21. This chapter displays power-quality problems of grid-connected wind energy system; special emphasis has been given to harmonics as they are the most important issue of power quality.

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 W. A. Hafez, A. A. Elbaset, Power Quality Enhancement of Wind Energy Systems, https://doi.org/10.1007/978-3-031-43243-9_3

45

46

3.2

3

Power-Quality and Grid Code Issues of Wind Energy Conversion System

Need of Power-Quality Studies

With the advancement in fast switching power devices, there is a trend for power supply size reduction. The current harmonics due to switching converters make supply current distorted. The increase in electronic controllers in drives, furnaces, household equipment, and switch mode power supplies (SMPS) increases harmonics and reactive power in the electric supply. The distribution transformers apart from reactive loads draw reactive current from the supply to meet the magnetizing current. The ever-increasing demand for power is not fulfilled by an increase in generation and particularly in distribution for various reasons such as environmental issues, increasing cost of natural fuel, and opposition to nuclear power plants. This puts excessive burden on the electric supply, resulting in poor power quality [90]. The term “power quality” here refers to the variation in supply voltage, current, and frequency. The excessive load demand retards the turbines at the generation plant. This results in reduction in voltage and a severe reduction in the supply frequency. The increasing problems and advances in power electronic technology have forced to change the traditional power system concepts. The use of fast reactive power compensators can improve the power system stability, and hence, maximum power transfers through the electric system.

3.3

Voltage Sag and Swell

A voltage sag happens when the root mean square (RMS) voltage decreases between 10 and 90 percent of nominal voltage for onehalf cycle to one minute. Voltage swell is an increase in the RMS (effective) voltage to between 1.1 and 1.8 pu of nominal from half-cycle to 1 min depending on the European Standard EN 50160 – IEEE 1159 [93]. These problems are considered in the power quality and wind turbine generating system (WTGS) operation and computed according to the rule given in the IEC 61400-3-7 standard, “Assessment of emission limit for fluctuating load.” The startup of wind turbine causes a sudden change in voltage. The relative % voltage change due to switching operation of wind turbine is calculated as: d = 100 K u ðΨk Þ

Sn Sk

ð3:1Þ

where d is the relative voltage change, Ku(Ψk) is the voltage change factor, Sn is the rated apparent power of wind turbine, and Sk is the short-circuit apparent power of grid. The voltage dips of 3% in most of the cases are acceptable. When evaluating flicker and power variation, around 95% of maximum variation band corresponding to a standard deviation is evaluated.

3.3

Voltage Sag and Swell

47

2 1.5 Amplitude

1 0.5 0 –0.5 –1 –1.5 –2 0

0.05

0.1

0.15

0.2

0.25 Time (s)

0.3

0.35

0.4

0.45

0.5

Fig. 3.1 Voltage sag waveform

Example 3.1 Generate a voltage sag sine wave signal with the following data using m-file: (a) (b) (c) (d) (e) (f)

Voltage = 160 V Voltage sag = 70% t = 0–0.5 s f = 50 Hz t1 = 0.2 s t2 = 0.3 s

Solution To generate a voltage sag sine wave signal, the following equation is used (Fig. 3.1): V ðt Þ = 1 - aðuðt - t 1 Þ - uðt - t 2 ÞÞ sinðωt Þ The MATLAB code is as follows: t=0:0.0001:0.5; %period time f=50; %frequency V=160; %voltage magnitude W=2*pi*f; x=V*sin(W.*t); u=inline('t>=0'); %(unit step function) a=0.7; %(depth of voltage sag) t1=0.2; % (moment of the sag starting and ending) t2=0.3; % (moment of the sag starting and ending) x1= (1-a*(u(t- t1)-u(t- t2))).*x; plot(t,x1) title('Voltage sag') xlabel('time(s)') ylabel('Amplitude')

48

3

Power-Quality and Grid Code Issues of Wind Energy Conversion System

2 1.5

Amplitude

1 0.5 0

–0.5 –1

–1.5 –2

0

0.05

0.1

0.15

0.2

0.25 Time (s)

0.3

0.35

0.4

0.45

0.5

Fig. 3.2 Voltage swell waveform

Example 3.2 Generate a voltage swell sine wave signal of the following data using m-file. (a) (b) (c) (d) (e) (f)

Voltage = 160 V Voltage swell = 60% t = 0–0.5 s f = 50 Hz t1 = 0.2 s t2 = 0.3 s

Solution To generate a voltage swell sine wave signal, the following equation is used (Fig. 3.2): V ðt Þ = 1 þ aðuðt - t 1 Þ - uðt - t 2 ÞÞ sinðωt Þ The MATLAB code is as follows: clear all clc t=0:0.0001:0.5; %period time f=50; %frequency V=160; %voltage magnitude W=2*pi*f; x=V*sin(W.*t); u=inline('t>=0'); %(unit step function) a=0.6; %(depth of voltage swell) t1=0.2; % (moment of the swell starting and ending) t2=0.3; % (moment of the swell starting and ending) x1= (1+a*(u(t- t1)-u(t- t2))).*x; plot(t,x1) title('Voltage swell') xlabel('time(s)') ylabel('Amplitude')

3.6

3.4

Harmonics

49

Voltage Variation

If a large proportion of the grid load is supplied by wind turbines, the output variations due to wind speed changes can cause voltage variation and flicker effects in normal operation. The voltage variation can occur in specific situations as a result of load changes and power produced from the turbine. These are expected in particular in the case of generators connected to the grid at fixed speed. The large turbine can achieve significantly better output smoothing using variable-speed operation, particularly in a short time range. The speed regulation range is also a contributory factor to the degree of smoothing with the large speed variation capable of suppressing output variations [94].

3.5

Switching Operation of Wind Turbine on the Grid

Switching operations of wind turbine generating system can cause voltage fluctuations and, thus voltage sag, voltage swell that may cause significant voltage variation. The acceptances of switching operation depend not only on grid voltage but also on how often this may occur. The maximum number of the above-specified switching operation within 10-min period and 2-h period is defined in the IEC 61400-3-7 standard [90]. Voltage sag is a phenomenon in which the grid voltage amplitude goes below and then returns to the normal level after a very short time period. Generally, the characteristic quantity of voltage sag is described by the amplitude and the duration of the sags. The IEEE power-quality standards define the voltage sag when the amplitude of voltage is 0.1–0.9 pu, and its duration is between 10 ms and 1 min. A voltage sag is normally caused by short-circuit faults in the power network or by the starting up of induction generators/motors. During bad weather conditions, such as thunderstorm, single-phase earthed faults are the causes of voltage sags. In addition, large electric loads such as large electrical motors or arc furnaces can also cause voltage sags during the startup phase with serious current distortion. The adverse consequences are the reduction in the energy transfer of electric motors. This results in the disconnection of sensitive equipment, and thereby leading to the termination of the industrial process.

3.6

Harmonics

Harmonics are the most important issue of power quality; the definition of harmonics is given in IEEE Std.519-1992 as “A sinusoidal component of a periodic wave or quantity having a frequency that is an integral multiple of the fundamental frequency” [95].

50

3

Power-Quality and Grid Code Issues of Wind Energy Conversion System

For example, if the waveform has a fundamental frequency F1, the third harmonic will have a frequency of 3*F1 and the fifth harmonic will have a frequency of 5*F1, i.e., for the fundamental frequency of 50 Hz, the third, fifth, and seventh harmonics would have frequencies 150, 250, and 350 Hz, respectively. The harmonic distortion caused by nonlinear load such as electric arc furnaces, variable-speed drives, large concentrations of arc discharge lamps, saturation of magnetization of transformer, and a distorted line current. The current generated by such load interaction with power system impedance gives rise to harmonics. The effect of harmonics in the power system can lead to degradation of power quality at the consumer’s terminal, increase of power losses, and malfunction in communication system. The degree of variation is assessed at the point of common connection, where consumer and supplier area of responsibility meet. The harmonic voltage and current should be limited to acceptable level at the point of wind turbine connection in the system [96]. This fact has led to more stringent requirements regarding power quality, such as Standard IEC 61000-3-2 or IEEE-519. Conventionally, passive inductor-capacitor (LC) resonant filters have been used to solve power-quality problems. However, these filters have the demerits of fixed compensation, large size, and the resonance itself. To overcome these drawbacks, active filters appear as the dynamic solution. The IEC 61000-3-6 standard provides guidelines for assessing and setting harmonic current limits. According to standard IEC 61400-21 guidelines, harmonic measurements are not required for fixed-speed wind turbines where the induction generator is directly connected to the grid. Harmonic measurements are required only for variable-speed turbines equipped with electronic power converters. In general, the power converters of wind turbines are pulse-width modulated inverters, which have carrier frequencies in the range of 2–3 kHz and produce mainly interharmonic currents. The harmonic measurement at the wind turbine is a problem due to the influence of the already existing harmonic voltage in the grid. The wave shape of the grid voltage is not sinusoidal. There are always harmonic voltages in the grid such as integer harmonic of the fifth and seventh orders that affect the measurements. Today’s variable-speed turbines are equipped with self-commutated pulse width modulation (PWM) inverter system. This type of inverter system has the advantage of controlling both active and reactive powers, and it also produces a harmonic current. Example 3.3 Draw the voltage waveform that consists of a fundamental frequency, third harmonic, fifth harmonic, and seventh harmonic with the following data using m-file: (a) Voltage = 4 V (b) f = 50 Hz (c) t = 0–0.08 s Solution If the waveform has a fundamental frequency f, the third harmonic will have a frequency of 3*f, the fifth harmonic will have a frequency of 5*f,and the seventh harmonic will have a frequency of 5*f. For a fundamental frequency of 50 Hz, the third, fifth, seventh harmonics would have frequencies 150, 250, and 350 Hz, respectively.

3.6

Harmonics

51

To draw a voltage waveform, the following equation is used: V ðtÞ = Vmððsin ωt Þ=π ÞÞ þ Vmððsin 3ωt Þ=3π ÞÞ þ Vmððsin 5ωt Þ=5π ÞÞ þ Vmððsin 7ωt Þ=7π ÞÞ

Where: ω = 2πf The MATLAB code is as follows: clear all clc fs=10000 t=0:1/fs:0.08; f=50; v1=4*sin(2*pi*f*t)/pi; v3=4*sin(2*pi*3*f*t)/(3*pi); v5=4*sin(2*pi*5*f*t)/(5*pi); v7=4*sin(2*pi*7*f*t)/(7*pi); v=v1+v3+v5+v7; plot(t,v,'b');grid on title('Voltage') xlabel('time(s)') ylabel('Amplitude')

Figure 3.3 shows a square wave whose combination series of sine-wave harmonics from fundamental to seventh harmonics are added together. Example 3.4 Draw a fundamental frequency, third, fifth, and seventh harmonics of voltage waveform with the following data using m-file:

Voltage

1.5

Amplitude

1 0.5 0

–0.5 –1 0.01

0.02

0.03

0.04 Time (s)

Fig. 3.3 Summation of fundamental and harmonic content

0.05

0.06

0.07

0.08

52

3

Power-Quality and Grid Code Issues of Wind Energy Conversion System

(a) Voltage = 4 V (b) f = 50 Hz (c) t = 0–0.08 s Solution To draw a fundamental frequency V(1), third V(3), fifth V(5), and seventh harmonic V(7) voltage waveform in the following equations are used: V(1) = Vm (sin ωt)/π V(3) = Vm (sin 3ωt)/3π V(5) = Vm (sin 3ωt)/3π V(7) = Vm (sin 3ωt)/3π The MATLAB code is as follows: clear all clc fs=10000 t=0:1/fs:0.08; f=50; v1=4*sin(2*pi*f*t)/pi; v3=4*sin(2*pi*3*f*t)/(3*pi); v5=4*sin(2*pi*5*f*t)/(5*pi); v7=4*sin(2*pi*7*f*t)/(7*pi); plot(t,v1,'b',t,v3,'g',t,v5,'r',t,v7,'k');grid on title('Voltage') xlabel('time(s)') ylabel('Amplitude')

Figure 3.4 shows the third, fifth, and seventh fundamental and various harmonic contents of the square wave

Fig. 3.4 Harmonic waveform from the fundamental to the seventh orders

3.6

Harmonics

3.6.1

53

Effect of Harmonics on Wind Farms

Due to common harmonic presence in wind farms, each element of the wind farms must be examined for its sensitivity to harmonics. It gives allowable harmonic level examination for every component. The main effects of voltage and current harmonics within wind farms are as follows: • Reduction in the efficiency of the generation and transmission of electric energy within a wind farm • Possibility of amplification of harmonic levels resulting from series and parallel resonances • Thermal stress of electrical components with consequent shortening of their useful life or damage • Malfunctioning of wind farm components and protection systems Among the possible external effects of harmonics are an excessive audible noise, degradation in communication systems performance, and harmonic-induced voltages and currents [97].

3.6.2

Types of Harmonics

• Even Harmonics Even harmonics refer to voltage or current waves with frequencies that are even multiples of the fundamental frequency F1, such as the following relationship. F = 2nF 1 ,

ðn = 1, 2, 3 . . .Þ

ð3:2Þ

Even harmonics are less likely to occur at levels detrimental to electrical systems. This is because nonlinear loads normally generate odd harmonics rather than even harmonics. Furthermore, when both the positive and negative half-cycles of a waveform are similar in shape, the Fourier series contains only odd harmonics. • Odd Harmonics Odd harmonics refer to voltage or current waves with frequencies that are odd multiples of the fundamental frequency F1, such as the following relationship. F = ð2n þ 1ÞF 1 ,

ðn = 1, 2, 3 . . .Þ

ð3:3Þ

Odd harmonics are more common in power systems connected to variable-speed wind energy systems and lead to severe consequences if not controlled. Each odd harmonic is associated with one of the sequence components (positive, negative, or zero).

54

3

Power-Quality and Grid Code Issues of Wind Energy Conversion System

• Inter-Harmonics Voltage or current waves having frequency components that are not an integral multiple of the fundamental power frequency are called enter harmonics. Thus, the frequency will be in the following range: ðn - 1ÞF 1 < F < nF 1 ,

ðn = 1, 2, 3 . . .Þ

ð3:4Þ

A special category of inter-harmonics that has frequency values less than the fundamental frequency is called sub-harmonics.

3.6.3

Harmonic Phase Sequence

In a balanced linear load, the phase currents are perfect sine waves consisting only the fundamental frequency, and the neutral current or neutral wire is the sum of the three phases [98]. For the fundamental frequency current component in a three-phase power system: I a1 = I a1 sinðωt Þ

ð3:5Þ

I b1 = I b1 sinðωt - 120 ° Þ

ð3:6Þ

I c1 = I c1 sinðωt - 240 ° Þ = I c1 sinðωt þ 120 ° Þ

ð3:7Þ

These fundamental currents are called positive-sequence currents based on the way their phases are sequenced, which is counter clockwise. When we apply this to the third harmonic: I a3 = I a3 sin 3ðωt Þ

ð3:8Þ

I b3 = I b3 sin 3ðωt - 120 ° Þ = I b3 sinð3ωt - 360 ° Þ = I b3 sin 3ωt

ð3:9Þ

I c3 = I c3 sin 3ðωt - 240 ° Þ = I c3 sinð3ωt - 720 ° Þ = I c3 sin 3ωt

ð3:10Þ

The expression for the third harmonic shows that it is in phase and has a zero displacement angle between the third harmonic currents, and therefore, the sequence is zero. The expression for the fifth harmonic current is: I a5 = I a5 sin 5ðωt Þ

ð3:11Þ

I b5 = I b5 sin 5ðωt - 120 ° Þ = I b5 sinð5ωt - 600 ° Þ = I b5 sinð5ωt þ 120Þ

ð3:12Þ

I c5 = I c5 sin 5ðωt - 240 ° Þ = I c5 sinð5ωt - 1200 ° Þ = I b5 sinð5ωt - 120Þ

ð3:13Þ

3.6

Harmonics

55

a

b Ic2

Ic1 Positive phase sequence

Negative phase sequence

Ia1

Ia2

Ib2

Ib1

c Ia3

Zero phase sequence

Ib3 Ic3

Fig. 3.5 Phase sequence of (a) fundamental, (b) second, and (c) third harmonics

The expression indicates that, for the fifth harmonic, the phase sequence is clockwise and opposite to that of the fundamental. Hence, the fifth harmonic is negative sequence. A similar expression could be derived for the seventh harmonic, and it would reveal that the seventh harmonic has the same phase sequence as the fundamental. Harmonic sequences for different harmonic orders are illustrated in Fig. 3.5, and phase sequence rotations of harmonics are summarized in Table 3.1.

3.6.4

Mathematical Definitions for the System with Harmonics

• Voltage p V ðtÞ = 2V 1 sinðωt þ θv1 Þ þ

1

p

2 V h sinðhωt þ θV h Þ

ð3:14Þ

h=2

• Current p I ðtÞ = 2I 1 sinðωt þ θI 1 Þ þ

1 h=2

where

p

2 I h sinðhωt þ θI h Þ

ð3:15Þ

Harmonic Sequence

Fund Positive

Second Negative

Third Zero

Fourth Positive

Fifth Negative

Sixth Zero

Seventh Positive

Eighth Negative

Etc.

3

Table 3.1 Harmonics and their corresponding sequence components

56 Power-Quality and Grid Code Issues of Wind Energy Conversion System

3.6

Harmonics

57

h is the order of harmonics, V1 and I1 are voltage and current RMS values of the fundamental component; Vh and Ih are voltage and current RMS values of the hthorder harmonics; θV1 and θI1 are voltage and current phase angles of the fundamental component; and θVh and θIh are voltage and current phase angles of the hth-order harmonics. • Power The equation for inter-power relations of a linear circuit with sinusoidal current and voltage is: S2 = P2 þ Q2

ð3:16Þ

The equation for inter-power relations of a system with harmonic current and voltage is: S2 = P2 þ Q2 þ D2

ð3:17Þ

where D implies the distortion power that can be calculated by the equation: D2 = S2 - P2 - Q2

ð3:18Þ

where S is the apparent power, P is the active power, Q is the reactive power, and D is the distortion power.

3.6.5

Total Harmonic Distortion

Total harmonic distortion (THD) is used to quantify the level of harmonics in voltage or current waveforms. It equals the RMS value of all harmonics divided by the RMS value of the fundamental and expressed in percentage [99]: THDV =

THDI =

1 V1 1 I1

1

V 2h × 100

ð3:19Þ

I 2h × 100

ð3:20Þ

h=2 1 h=2

where THDV is the total harmonic distortion of voltage and THDI is the total harmonic distortion of current. These THD values for the current and voltage are given in percentage. For pure sinusoidal waveforms, the total voltage and current distortion are zero.

58

3.6.6

3

Power-Quality and Grid Code Issues of Wind Energy Conversion System

Harmonic Distortion

Harmonic distortion (HD) calculates the ratio of a given harmonic component to the fundamental component; this value is used to determine the magnitude of each individual harmonic [100]. HDV =

Vh V1

ð3:21Þ

HDI =

Ih I1

ð3:22Þ

where HDV is the individual harmonic distortion of voltage and HDI is the individual harmonic distortion of current.

3.6.7

Total Demand Distortion

This term is similar to THD except that the distortion is expressed as a percentage of some rated or maximum value, rather than as a percentage of the fundamental frequency [101]. TDD =

1

1 I max

h=2

I2h

ð3:23Þ

where TDD is the total demand distortion and Imax is the maximum load current.

3.6.8

Crest Factor

The crest factor (CF) is another characteristic of the waveform and the most simple harmonic distortion estimation technique. It is given by [102]: CFV =

V Peak V RMS

ð3:24Þ

CFI =

I Peak I RMS

ð3:25Þ

where CFV is the voltage crest factor and CFI is the current crest factor.

3.6

Harmonics

59

For pure sinusoidal waveform, CF is equal to 1.41, but for a distorted one, it will be different. It measures the effect on the waveform but is too limited to indicate general distortion.

3.6.9

Mathematical Analysis of Harmonics

• Fourier Analysis Current and voltage waveforms in alternative current are supposed to be sinusoidal. Non-sinusoidal waveforms are caused by the presence of nonlinear loads. In order to define and analyze these non-sinusoidal waves, the Fourier analysis method can be used. The harmonic component is an element of a Fourier series, which can be used to define any distortion waveform. As defined by J. Fourier, non-sinusoidal periodic waves are the sum of many sinusoidal waves that are different in frequencies and amplitudes, and additionally, this kind of waveform can be separated into sinusoidal waves whose frequency and amplitude are multiples of the fundamental frequency. Consequently, the series derived by this way is called the “Fourier series.” A distorted waveform can be analyzed using Fourier series representation given as the following equation [103]: f ðt Þ = F 0 þ

1

1 f h ðt Þ = a0 þ 2 h=1

1

½ah cosðhωt Þ þ bh sinðhωt Þ

ð3:26Þ

h=1

where f(t) is the instantaneous value at any time t and F 0 = 12 a0 is the average value of the function f(t). a0 =

1 2π

2π

f ðt Þdðωt Þ

0

ω = 2πf, where f is the frequency and 1/f defines time over which the complex wave repeats itself. ah and bh are series coefficients that can be determined as follows: ah = bh =

1 2π 1 2π

2π

f ðt Þ cosðhωt Þdðωt Þ;

h = 1, 2, 3, . . .

ð3:27Þ

h = 1, 2, 3, . . .

ð3:28Þ

0 2π 0

f ðt Þ sinðhωt Þdðωt Þ;

60

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Power-Quality and Grid Code Issues of Wind Energy Conversion System

Therefore, the Fourier series in Eq. (3.26) can be expressed as: f ðt Þ = F 0 þ f 1 sinðωt þ φ1 Þ þ f 2 sinð2ωt þ φ2 Þ þ f 3 sinð3ωt þ φ3 Þ . . . þ f h sinðhωt þ φh Þ

ð3:29Þ where F0 is the direct component, f1, f2, f3, and fh are the maximum values of the fundamental, second, third, and hth harmonic components, respectively, and φ1,φ2, φ3, and φh are the phase shifts of the fundamental, second, third, and hth harmonic components, respectively. • Harmonic Analysis Simplifications There are special simplifications in the analysis of a complex waveform that can be applied for any given wave: • If the areas of the positive and negative half-cycles are equal, a0 is zero, i.e., there is no direct current (DC) component in that wave. • If f(x + π) = - f(x), then there are no even harmonics. • If f(-x) = - f(x), then bn = 0, i.e., there are no cosine terms. • If f(x) = - f(x), then bn = 0, i.e., there are no sine terms.

3.6.10

Harmonic Standard Limits

After studying harmonics, bus harmonic voltages, line harmonic currents, and the harmonic distortion of voltage and current are measured and compared to limits set by standards. If the limit exceeds, various filter alternatives are simulated and examined to resolve the harmonic problems. • Voltage Harmonic Distortion Limits Limits of allowable voltage distortion set by IEEE 519 are provided in Table 3.2 [104]. 1

THDV n =

h=2

vn

V 2h × 100%

ð3:30Þ

Table 3.2 IEEE 519-1992 voltage harmonic limits Bus voltage at point of common coupling (PCC) (Vn) Vn ≤ 69 kV 69 KV < Vn ≤ 161 kV Vn > 161 kV

Individual harmonic voltage distortion (%) 3.0 1.5 1.0

Total voltage harmonic distortion (THD) (%) 5.0 2.5 1.5

3.7 Flickers

61

Table 3.3 IEEE 519-1992 current harmonic limits Isc/IL h < 11 Vn ≤ 69 kV 1000 15.0 69 kV < Vn ≤ 161 kV 1000 7.5 Vn > 161 kV