POWER QUALITY ASPECTS IN A WIND POWER PLANT IMAMHUSEN M PATIL. MASTER OF TECHNOLOGY IN ELECTRICAL ENGINEERING (Computer Controlled Industrial Power)

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1 POWER QUALITY ASPECTS IN A WIND POWER PLANT A THESIS Submitted by IMAMHUSEN M PATIL In partial fulfillment for the award of the Degree of MASTER OF TECHNOLOGY IN ELECTRICAL ENGINEERING (Computer Controlled Industrial Power) Under the guidance of Dr. ASHOK. S DEPARTMENT OF ELECTRICAL ENGINEERING NATIONAL INSTITUTE OF TECHNOLOGY NITC CAMPUS P.O, CALICUT KERALA, INDIA MAY 2008

2 ACKNOWLEDGEMENTS I express my profound sense of gratitude to my guide, Dr. Ashok. S, Assistant Professor, Department of Electrical Engineering, NIT Calicut, for his systematic guidance and valuable advices. His encouragement and suggestions were of immense help to me throughout my project work. I would like to express my sincere gratitude to Dr. Paul. K. Joseph, Professor and Head of the Department, Department of Electrical Engineering, N.I.T. Calicut, for providing me with all the necessary facilities for the work. I would also like to thank all the faculty and staff members of EED, especially the staff of EED computer center (P.G.) and library, who extended full cooperation for completion of this work. I take the opportunity to thank all my friends who helped me through their patient discussions and suggestions and for their timely help at various stages in completion of this work. Imamhusen M Patil

3 DECLARATION I hereby declare that this submission is my own work and that, to the best of my knowledge and belief, it contains no material previously published or written by another person nor material which has been accepted for the award of any other degree or diploma of the university or other institute of higher learning, except where due acknowledgment has been made in the text. Place: NIT Calicut Date: Name: Imamhusen M Patil Roll. No: M060308EE

4 C E R T I F I C A T E This is to certify that the thesis entitled: POWER QUALITY ASPECTS IN A WIND POWER PLANT submitted by Mr. IMAMHUSEN PATIL to the National Institute of Technology Calicut towards partial fulfillment of the requirements for the award of the Degree of Master of Technology in Electrical Engineering (Computer Controlled Industrial Power) is a bona fide record of the work carried out by him under my supervision and guidance. Dr. Ashok.S (Project Guide) Assistant Professor Department of Electrical Engineering National Institute of Technology, Calicut Dr. Paul. K. Joseph Professor and Head Department of Electrical Engineering National Institute of Technology, Calicut Place: NIT, Calicut Date:

5 CONTENTS Abstract i List of symbols iii List of figures v List of Tables vi List of Abbreviations vii CHAPTER 1:INTRODUCTION 1.1 Overview Objectives Outline 3 CHAPTER 2: POWER QUALITY AND GRID INTEGRATION OF WIND FARMS 2.1 Literature survey Introduction Voltage quality Classification of disturbances in the grid Wind integration grid code requirements Slow Voltage Variations Flicker Voltage Dips Harmonic Voltage Fault Ride-Through and Frequency Range Frequency Control Reactive Range and Voltage Control Capability 14 CHAPTER 3:WIND ENERGY CONVERSION SYSTEM 3.1 Wind power technology Main generating systems Squirrel Cage Induction Generator Doubly Fed Induction Generator Direct drive Synchronous Generator 19

6 3.3 Power System Interaction Reactive Power Generation and Voltage Control Behavior incase of disturbances on the Grid Frequency Control 21 CHAPTER 4: POWER QUALITY AND NETWORK INTEGRATION ISSUES IN INDIAN WIND FARMS 4.1 Power Quality Grid Impact Scenario of Indian wind farms Factors affecting capacitor compensation Cases with switching over voltages Conclusions of study Recommendations for grid integration Recommendations for wind turbines 34 CHAPTER 5: MODEL OF GRID CONNECTED WIND FARMS 5.1 Single line diagram of test system Voltage variations Impact on power quality Dynamic fault mitigation Voltage collapse Modeling of wind farms Sudden islanding 39 CHAPTER 6: SIMULATIONS AND RESULTS 6.1 Simulation models Model implementations and simulation results Wind turbine characteristics Voltage variations Effect of compensation Effect of reactive compensation to other buses Voltage fluctuations due to reactive compensation Voltage fluctuations due different X/R ratio 48

7 6.2.7 STATCOM characteristics Modeling of wind plants Induction Generator in Islanding condition Harmonic analysis Current distortions 61 CHAPTER 7:CONCLUSIONS 7.1 Conclusions Work for the future scope 63 REFERENCES 64 APPENDICES 66

8 ABSTRACT Wind power is gaining rapid momentum in the world energy balance. The integration of large scale (higher penetration level) wind power into power system raises issues that must be clarified and that have to be addressed. Power quality has to ensure the stability and reliability of power system it is connected to and to satisfy the customer connected to the grid is desirable. Grid codes should be fulfilled by wind power plant to minimize their impact on the grid. Up to date experience and knowledge coupled with accurate load flow and dynamic simulation models (encompassing all significant air dynamical, mechanical, and electrical factors) are necessary to evaluate the impact of wind farms on power systems. Power system operators and planner need to understand how wind power generation interacts with power system and what analytical tools are available for system studies to asses wind turbine performance and ensure grid compatibility and compliance with relevant codes and regulations. At the same time, the lack of rules, standards, and regulations during early wind development has proven to be an increasing threat to the stability and power quality of the grid connected to a wind power plant. Fortunately, many new wind power plants are equipped with state of the art technology, which enables them to provide good service while producing clean power for the grid. The advances in power electronics have allowed many power system applications to become more flexible and to accomplish smoother regulation. Applications such as reactive power compensation, static transfer switches, energy storage, and variable-speed generations are commonly found in modern wind power plants. Although many operational aspects affect wind power plant operation, which is connected to the grid, it is very important to understand the sources of disturbances that affect the power quality.the voltage and current distortions are created by harmonics and self-excitation is due to loss of line are to be considered. In general, the voltage and frequency must be kept as stable as possible. Presently, the wind energy penetration in the power systems is often highest in rural areas with weak grids. In India, the wind energy is concentrated in rural areas with a very high penetration. In these cases, the wind power has an increasing influence on the power quality on the grids. Another aspect is the influence of weak grids on the operation i

9 of wind turbines. The grid abnormalities reduces the financial attractiveness of wind farm investment. In this project, the power quality issues have been studied with the power plants consists of Squirrel Cage Induction Generators (SCIG) integrated to power system network as a case, the various parameters which affect the voltage fluctuations have been investigated and preliminary guidelines for grid integration of wind turbines in weak grids have been formulated. The voltage and current distortions are created by harmonics and self-excitation is due to loss of line (islanding mode) are investigated with different loading conditions. Causes for the harmonics has been investigated and model for the harmonic analysis of grid connected induction generator is verified. ii

10 LIST OF SYMBOLS P in V r T el Symbol Description Electrical power input generator Reference power wind speed Electrical counter torque s V wind R X V f cos(φ) P Q V A C p P lt P st β n R S SN SSC Z(C,h) Y(C,h) Z line Z xfmr Z gen Slip Wind velocity Resistance of transmission line Reactance of transmission line Voltage of transmission line Frequency Power factor Real power Reactive power Change in voltage Cross section of the air mass flow Aerodynamic efficiency Long term flicker level Short term flicker level Blade pitch angle Number of identical wind mills Stator phase winding resistance Rated power of wind power plant Short circuit Power of the grid Impedance function of capacitance and harmonics Admittance function of capacitance and harmonics Impedance of line Impedance of transformer Impedance of generator iii

11 K f (Ψ k) E pl Pw ρ η t η g C p Flicker step factor Flicker emission Electrical power generated Factor to account for air density(1.225 kg/m 2 at sea level Turbine efficiency Generator efficiency Power coefficient (0.35 for a good design) A Wind turbine rotor wept area in m 2 K Grid stiffness factor S ZW Short-circuit power at the point of load connection (SSC). E M Voltage across magnetising branch Z c Impedance of the capacitor circuit iv

12 LIST OF ABBREVIATIONS WECS Wind Energy Conversion System IG Induction Generator DFIG Doubly Fed Induction Generator PMSG Permanent magnet synchronous generator RPM Rotations Per Minute WT Wind Turbine WG Wind generator VAR Volt ampere reactive SVC Static VAR Compensator STATCOM Static synchronous Compensator SCIG Squirrel Cage Induction Generator ASVC Advanced Static VAR Compensator VS-WTG Variable Speed Wind Turbine Generator PCC Point Common Coupling STR Single Turbine Representations MTR Multiple Turbine Representations TSO Transmission System Operators VSC Voltage Source Converter IEC International Electro-technical Commission STR Single Turbine Representation MTR Multi Turbine Representation STATCOM Static Synchronous Compensator TSO Transmission System Operators IEC International Electrotehnical Commission EN Power Quality Standard HVDC High Voltage Direct Current viii

13 CHAPTER 1 INTRODUCTION 1.1 OVERVIEW Wind power is gaining rapid momentum in the world energy balance. The integration of large scale (higher penetration level) wind power into power system raises issues that must be clarified and that have to be addressed. Power quality has to ensure the stability and reliability of power system it is connected to and to satisfy the customer connected to the grid is desirable [1]. In India, the wind energy is concentrated in rural areas with a very high penetration. In these cases, the wind power has an increasing influence on the power quality on the grids. Another aspect is the influence of weak grids on the operation of wind turbines. In India it is found that grid abnormalities reduces the financial attractiveness of wind farm investments. Wind power is one of the natural resources used for generating electricity. In conventional generation, synchronous generator has been used for generation for more than one hundred years. There is a major different between conventional power plant and wind power plant. In a conventional generation, a prime mover is connected to a synchronous generator while in a wind power generation, the wind turbine is the prime mover and it is connected to an induction generator. There are two major types of induction generator used one is the so-called fixed-speed (squirrel cage induction generator) and the other one is the wound rotor induction generator. Although there are other types of generators used (permanent magnet, synchronous generator etc.), but only Squirrel Cage Induction Generator will be discussed. There are many types of wind turbine generator available on the market. In this project we limit our scope to a typical wind turbine presently used in many wind farms in the country like India. We chose a Fixed-Speed Squirrel Cage Wind Turbine Generator (FS-WTG) at 1.5 MW with pitch control. The Fixed-Speed Squirrel Cage Wind Turbine Generator (FS-WTG) is commonly used in wind turbine generator.the subject of voltage and frequency stability 1

14 is very critical to the customers at the receiving end of the electrical grid. Customers want to have good power quality electricity from the grid namely constant voltage and constant frequency all the time. In practice, these two attributes cannot be maintained constant all the time. Variation in loads, generations, switching of auxiliary equipments (transformer taps, capacitors), circuit changes (planned and unplanned) happen all the time. Thus, there is always imbalance in the net real power and net reactive power. Small imbalances (wind fluctuations, small load changes etc.) create a degraded power quality of the available electrical energy, but larger imbalance (fault, loss of line, loss of generation etc.) threatens stability of the grid. If the net real power is zero there will be a stable and steady frequency, and similarly if the net reactive power is zero, there will be a stable and steady voltage on the grid. On the contrary, if the wind turbine generates more power additional frequency will be seen on the generator bus and vice versa.the same phenomenon is applicable for reactive power. If the generator generates more reactive power, a voltage rise will be seen on the generator terminals. Technical constraints in relation to wind power in weak grids may be associated with limited thermal capacity in parts of the grid and the effect that wind power has on voltage quality and stability. Indeed, if the weak grid is a small island grid, frequency control could also be a constraint. In this case however, the weak grid is assumed to be part of a large interconnected power system where wind power will not have a significant effect on the frequency control. Hence, the focus of this project is on issues related to voltage quality, power fluctuations, harmonics and distortions [2]. The wind regimes are located far away from load centers and connected through the weak grids. Large wind farms consumes reactive power through weak grids affects the voltage quality. In this project, the power quality issues have been studied with the power plants consists of Squirrel Cage Induction Generators (SCIG) integrated to power system network as a case, the various parameters which affect the voltage fluctuations have been investigated and preliminary guidelines for grid integration of wind turbines in weak grids have been formulated. Effect of reactive power compensation on voltage quality is investigated. The voltage and current distortions are created by harmonics and self-excitation is due to loss of line (islanding mode) are investigated with different loading conditions. Causes for the harmonics has been investigated and model for the 2

15 harmonic analysis of grid connected induction generator is verified. Results from the simulations will be presented. Normal and abnormal condition will be investigated and the worst possible conditions will be used in the simulation. The grid simulated will be a weak grid, thus the voltage variation can be obviously seen on the traces of the voltage. Therefore, voltage and current distortions created by harmonics, Self-excitation, which may occur in a wind power plant due to loss of line, will also be considered [3], [4]. 3