Kungliga Tekniska Högskolan Lillgrund Wind Farm Modelling and Reactive Power Control Isabelle Boulanger Master Thesis Stockholm 2009 Electrical Machines and Power Electronics, Power Systems Royal Institute of Technology SWEDEN Kungliga Tekniska Högskolan Kungliga Tekniska Högskolan Abstract The installation of wind power plant has significantly increased since several years due to the recent necessity of creating renewable and clean energy sources. Before the accomplishment of a wind power project many pre-studies are required in order to verify the possibility of integrating a wind power plant in the electrical network. The creation of models in different software and their simulation can bring the insurance of a secure operation that meets the numerous requirements imposed by the electrical system. Hence, this Master thesis work consists in the creation of a wind turbine model. This model represents the turbines installed at Lillgrund wind farm, the biggest wind power plant in Sweden. The objectives of this project are to first develop an accurate model of the wind turbines installed at Lillgrund wind farm and further to use it in different kinds of simulations. Those simulations test the wind turbine operating according to different control modes. Also, a power quality analysis is carried out studying in particular two power quality phenomena, namely, the response to voltage sags and the harmonic distortion. The model is created in the software PSCAD that enables the dynamic and static simulations of electromagnetic and electromechanical systems. The model of the wind turbine contains the electrical machine, the power electronics (converters), and the controls of the wind turbine. Especially, three different control modes, e.g., voltage control, reactive power control and power factor control, are implemented, tested and compared. The model is tested according to different cases of voltage sag and the study verifies the fault-ride through capability of the turbine. Moreover, a harmonics analysis is done. Eventually the work concludes about two power quality parameters. Index Terms: Wind Power, Power Electronics, Induction Machine, Controls (Voltage Control, Active and Reactive Power Control, Current Control, DC Voltage Control), Voltage Source Converter (VSC), Power Quality, Voltage Sags, Harmonics, and Grid Code. Kungliga Tekniska Högskolan Acknowledgements This Master thesis was done at Vattenfall Research and Development AB and approved by the Division of Power Systems and the Division of Electrical Machines and Power Electronics belonging to the School of Electrical Engineering at KTH. Both divisions are engaged in this project since it treats different aspects within the whole electrical engineering area. The work was funded by Vattenfall Vindkraft. My supervisors at KTH were Dr. Valerijs Knazkins and Professor Hans-Peter Nee and Dr. Fredrik Carlsson at Vattenfall Research and Development AB. My examiner at KTH was Assistant Professor Mehrdad Ghandhari. I would like to thank some persons that played an important role during the 20 weeks of my Master thesis work. I express my gratitude to Dr. Fredrik Carlsson, Dr. Valerijs Knazkins, and Professor Hans-Peter Nee for their help and guidance during the whole project. Thank to Mehrdad Ghandhari for accepting being my examiner for this Master thesis. I am indebted to Evabritt, Urban Axelsson and Daniel Salomonsson for their help and support. I absolutely want to thank Lovisa Stenberg and Laura Bergholz for being helpful, attentive and who always encouraged me. Especially I am very grateful to Lovisa Stenberg with whom I was sharing more than a room during these 20 weeks and who facilitated so much my integration in VRD. I want to thank my parents for teaching me perseverance and rewards of work and also for encouraging me despite the 2000 km distance between us. Finally I am thankful to Benjamin Boullanger who never gave up encouraging me and helping me. For the productive discussions we had and his relevant suggestions and advices. Kungliga Tekniska Högskolan Table of Contents Page 1 Introduction 1 1.1 Background and prior studies 1 1.2 Lillgrund wind farm 2 1.3 Purpose 3 1.4 Report outline 4 2 Control theory 5 2.1 The control system of the wind turbine 5 2.2 Determination of the DC capacitor 5 2.3 Grid side control 7 2.3.1 The system 7 2.3.2 The inner current controller 9 2.3.3 The DC voltage controller 10 2.3.4 Three different control modes on turbine level and park pilot 11 2.3.5 Problems raised by the close bandwidth of the imbricate loops 13 2.4 Generator side control 15 2.4.1 Introduction to vector control 15 2.4.2 The induction generator 16 2.4.3 Current controller 16 2.4.4 Flux estimation for rotor flux orientation 17 2.4.5 Speed controller 19 2.4.6 Optimal speed control system 20 2.5 Siemens control system 21 3 Introduction to power quality analysis 22 3.1 Introduction to power quality – Grid Code 22 3.2 Voltage sags 23 3.2.1 Definition 23 3.2.2 Studied case 24 3.3 Harmonics 25 3.3.1 Measurements of harmonics 25 3.3.2 Induction machine harmonics 26 3.3.3 Power electronics harmonics 26 3.3.4 Transformer harmonics 27 4 Modelling and implementation in PSCAD 28 4.1 Wind turbine 28 4.1.1 Wind source 28 Kungliga Tekniska Högskolan 4.1.2 Wind turbine 28 4.1.3 Governor 29 4.2 Induction generator 29 4.3 Voltage source converter 30 4.3.1 One module 30 4.3.2 The PWM inverter filter 31 4.4 Control system 31 4.4.1 Grid side control 32 4.4.2 Generator side converter 32 4.4.3 Pitch control in PSCAD 34 4.5 DC-link chopper 34 5 Simulation in PSCAD and analysis of results 35 5.1 Simulation on PSCAD – Introduction 35 5.2 Reactive Power Control and Voltage Control Modes 35 5.3 Results for one turbine – Compliance with IEC 61400-21 36 5.3.1 Voltage sags study 36 5.3.2 Harmonics analysis 39 5.4 Results concerning the voltage sag study for one or several turbines connected to the grid 40 5.4.1 Impact on the different voltages of the system 42 5.4.2 Impact on the wind turbine current 46 5.5 Results concerning the harmonics study for one or several turbines connected to the grid 47 5.6 Comparative analysis between two control modes 49 5.7 Comparison with the Siemens’ study 50 5.7.1 Harmonics study 50 5.7.2 Dynamic simulation study 51 5.8 Island Operation 53 6 Conclusions 55 7 Improvements and future works 56 References 58 Appendices 60 Simulations 65 SIMULATION A: Control mode test on the grid-side inverter connected to the grid 66 SIMULATION B: Test of the generator-side system 71 SIMULATION C: Test turbine with connexion IEC 73 Kungliga Tekniska Högskolan List of symbols Symbol Quantity Unit Vtri PWM triangular signal V fs switching/triangular frequency Hz Vcontrol PWM control/modulation signal V f1 modulating frequency Hz ma amplitude modulation ratio - mf frequency modulation ratio - -------------------------------------------------------------------------------------------------------- C DC-link capacitance F UDC DC-link voltage V IDC DC-link current A EDC energy stored in the DC-link capacitor J PDC DC power W -------------------------------------------------------------------------------------------------------- Vabc 0.69/33 kV transformer input voltage V Iabc converter output current A Vabc_conv converter output voltage V PAC AC active power W QAC AC reactive power VAr Vd,q Park coordinates of Vabc V Vd,q_conv Park coordinates of Vabc_conv V Id,q Park coordinates of Iabc A R PWM filter resistance Ω L PWM filter inductance H X reactance corresponding to L (X = ω.L) Ω Ω angular frequency rad/s αβ axes defining the reference frame - dq axes defining the Park reference frame - θ Park transformation angle rad k Park transformation coefficient - α1C bandwidth of a closed loop system rad/s kp,1C proportional gain of the first current controller Ω Ti,1C time constant of the first current controller s tr,1C rise time corresponding to α s -------------------------------------------------------------------------------------------------------- Plosses losses in the converter W Rvirtual virtual resistance in DC voltage control Ω α1DC bandwidth of a closed loop system rad/s kp,1DC proportional gain of the DC voltage controller Ω Ti,1DC time constant of the DC voltage controller s Kungliga Tekniska Högskolan tr,1DC rise time corresponding to α s -------------------------------------------------------------------------------------------------------- Us induction machine (IM) voltage V Es IM internal voltage V Is IM stator current A ωm mechanical angular speed of the IM rad/s ωr electrical angular speed of the IM rad/s ψs rotor flux Wb ψr stator flux Wb Rs stator resistance Ω Rr rotor resistance Ω Lm magnetising inductance H Lr rotor inductance H Lls stator leakage inductance H Llr rotor leakage inductance H -------------------------------------------------------------------------------------------------------- ρ Park transformation angle for flux oriented frame rad Lσ leakage inductance H cT constant factor of the speed controller Nm/A α2C bandwidth of a closed loop system rad/s kp,2C proportional gain of the second current controller Ω Ti,2C time constant of the second current controller s tr,2C rise time corresponding to α s -------------------------------------------------------------------------------------------------------- T torque N.m J inertia of the IM kg.m2 b damping constant of the IM Nm/s α2ω bandwidth of the closed loop system rad/s kp,2ω proportional gain of the speed controller Ω Ti,2ω time constant of the speed controller s tr,2ω rise time corresponding to α s --------------------------------------------------------------------------------------------------------