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Dissertation thesis

TRANSILVANIA UNIVERSITY OF BRAŞOV FACULTY OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCE Study Program: Advanced Electrical Systems. Dissertation thesis Control of a small wind turbine for integration in an autonomous microgrid .  Student: Ing . MĂLĂCEA Alexandru – Costinel

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Dissertation thesis

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  1. TRANSILVANIA UNIVERSITY OF BRAŞOV FACULTY OF ELECTRICAL ENGINEERINGAND COMPUTER SCIENCEStudy Program: Advanced Electrical Systems Dissertation thesis Control of a small wind turbine for integration in an autonomous microgrid.  Student:Ing.MĂLĂCEA Alexandru – Costinel   Project advisor:Șef. Lucr. Dr. Ing. ȘERBAN Ioan Brașov 2012 1/24

  2. TABLE OF CONTENTS 1. Motivation 2. Introduction in Wind Energy Conversion Systems (WECS) 3. Project description 4. System Description and Analysis 5. System Modeling 6. Results & Discussion 7. Conclusions 2/24

  3. 1. Motivation • The objective of this project is to analyze the behavior and control of a gearless small power wind turbine of 5 kW with a variable speed permanent magnet synchronous generator (PMSG), diode bridge rectifier, boost converter, single – phase inverter, low pass filter, integrated in a single – phase autonomous microgrid (MG) with ac loads. • The main focus in this report is to develop a proper control system for the wind turbine, including the maximum power point tracker (MPPT) and the power flow control to the microgrid. For this end, the system is modeled and simulated in the Matlab/Simulink environment, including the SimPowerSystems and Simulink as most often used libraries. 3/24

  4. 2. Introduction in Wind Energy Conversion Systems (WECS) • Wind energy is one of the most spread renewable energy source (RES), being practically inexhaustible. • Wind turbine technology is one of the most emerging renewable technologies. • The working principle of a wind turbine includes two conversion processes, which are carried out by its main components: • the rotor, which extracts kinetic energy from the wind and converts it into a mechanical torque; • the generating system, which converts the wind power into electricity. General working principle of wind power generation 4/24

  5. 3. Project description The boost converter (step-up) must always provide on the inverter DC-link side a voltage between 350-400 V in order the inverter to operate properly. The purpose is to supply microgrid loads through a single phase 230 V, 50 Hz inverter. The studied 5 kW wind turbine transformerless configuration connected to the MG 5/24

  6. In order to better analyze and simulate the behavior of hardware components, the system is divided in two parts, the primary side (wind turbine, generator, rectifier and boost converter) and the grid interface side (inverter and filter). They are modeled and simulated both separately and combined, resulting in a comprehensive analysis of the studied system. 1 Primary side of the studied system 2 Secondary side of the studied system 3 6/24 The overall wind turbine system interconnected to a single-phase AC autonomous MG

  7. 4. System Description and Analysis The two inputs of the wind turbine are the generator speed in p.u. and the wind speed in m/s. The output represents the mechanical torque produced by the wind rotor and applied to the generator shaft (Tm). Simulink model of the wind turbine The mechanical power Pm as a function of generator speed, for different wind speeds and for blade pitch angle β = 0 degree (fixed-pitch rotor), is shown in the left figure. 7/24 Wind turbine power characteristics

  8. The step-up (boost) converter control The boost converter must provide and maintain a voltage between 350-400 V on the DC link. An adequate controlling method based on MPPT must be used, in order to maximize the electric output power. The boost converter model The boost converter operates with an MPPT algorithm based on a static characteristic and MPPT derating. The DC-DC boost converter control model with MPPT based on a static characteristic and MPPT derating 8/24

  9. The single – phase inverter control The inverter control is developed considering the following set of measurements: the DC voltage (Vdc) from the DC – link side, the current (io) and voltage (vo) from the AC side, at the output of the inverter. The adopted method for controlling the inverter (current control mode) and implemented in Matlab/Simulink model is called the PI – dq single – phase control method. The single-phase inverter model 9/24 PI – dq single – phase inverter control block diagram

  10. 5. System Modeling – primary side Three simulation cases are hereinafter presented. 10/24 the primary side of the simulated system

  11. 5. System Modeling – inverter side 11/24 The secondary side of the simulated system

  12. 5. System Modeling – overall 12/24 The overall wind turbine system interconnected to a single-phase AC autonomous MG

  13. 6. Results & Discussion Regarding the first simulation model, some measurements were performed in Simulink for different wind speeds. The range of the wind speed is set to be from 4 m/s as the cut-in speed up to 12 m/s, from 0.5 to 0.5. The results prove that the primary side system operates as expected and can be integrated in the overall model. In order to draw a MPPT efficiency characteristic, another set of measurements for different wind speeds starting from 4 m/s up to 10.5 m/s with a step of 0.5 are required. The results show an acceptable accuracy of the adopted static MPPT. MPPT static characteristic diagram for I_ref function of V_in MPPT static characteristic diagram for P_in function of V_in MPPT static characteristic diagram for P_in function of wind speed 13/24 Characteristic diagram for Pm function of wind speed Characteristic diagram for MPPT_eff function of wind speed

  14. Regarding the second simulation model, some measurements were performed for different values of the reference active power Pref. The values of Pref are given in a range of 500 W up to 4500 W, from 500 to 500. The following voltage harmonics of 3rd (5%), 5th (4%), 7th (3%), 9th (2%) and 11th (1%)orders were included in the Simulinkmicrogrid block, in order to consider an adverse operating case. There were considered two cases, without and with voltage harmonics. chart characteristic for THDI function of P_ref when no voltage harmonics are introduced chart characteristic for THDI function of P_ref when voltage harmonics are introduced (THDV=7.4%) 14/24

  15. Figure below shows the active and reactive power waveforms when a step of 4500 W is applied on the active power reference at t=0.3 s. It can be noticed that the measured active and reactive powers stabilize after approximately 0.1 s. Measured active and reactive power waveforms for P_ref set at 4500 W 15/24

  16. Regarding the third simulation model, when the two subsystems are linked together,the measurements are given for different values of the wind speeds that are introduced in a constant on the turbine block model in Simulink. The range of the wind speeds is from 5.5 m/s up to 12 m/s, from 0.5 to 0.5. Characteristic diagram for P_meas function of wind speed 16/24

  17. In order to simulate the variability of wind speed with time, in this third simulation a ramp function was considered for the wind speed to replicate a wind gust. The introduced ramp-up function Active and reactive power when a ramp-up function is used 17/24

  18. The left figure illustrates the rotor mechanical power in comparison with the ideal conditions, while the right figure (top) shows the generator rotor speed in comparison with the ideal conditions. The MPPT efficiency is shown in the right figure (bottom). In steady state the MPPT efficiency is approximately 95 %, whereas in dynamic conditions it decreases down to 70 %, mainly because of the rotor inertia, which limits the slope of the rotor speed. The rotor mechanical power in comparison with the ideal conditions The generator rotor speed in comparison with the ideal conditions and MPPT efficiency 18/24

  19. In order to improve the quality of the last simulation model related to the overall wind turbine system interconnected to the autonomous single-phase MG and to have a more realistic perspective of the entire control of frequency in terms of maintaining stability of the MG, a fourth simulation model was developed which includes the MPPT derating feature and a new MG model of 20 kW. 19/24 The wind turbine system interconnected to a single-phase AC autonomous 20 kW dynamic MG

  20. In case of system imbalance of active power, MPPT control method can be modified to reduce the amount of power that can be extracted from the wind turbine. The frequency of the dynamic MG it’s passed through the MPPT derating algorithm to be adjusted in order to provide the reference current for the boost converter. kmd The kmdfunction of frequency characteristic The MPPT control subsystem with the MPPT derating feature The DC-DC boost converter control model with MPPT based on a static characteristic and MPPT derating 20/24

  21. A ramp function was considered in this fourth simulation model for the wind speed to replicate a wind gust. In order to highlight the proposed power limitation method, the MG is subjected to an adverse operating scenario, where the generation level exceeds the consumption and the frequency substantially increases. The considered ramp function 21/24

  22. The analysed features of this simulation regard the MG frequency, the inverter active and reactive powers and also the MPPT derating coefficient (kmd). These features will be given for two cases but on the same diagram for a better comparison. In one case the simulation will be performed by enabling the MPPT derating block, and in another case by disabling it. MPPT derating coefficient for the analyzed two cases MG frequency for the analyzed two cases 22/24 Active & reactive powers for the analyzed two cases

  23. 7. Conclusions • The project presents a study regarding the integration of a small wind turbine of kW-range within a MG. • A maximum power point (MPPT) algorithm that aims to extract as much wind energy as possible but also taking into account the limited power absorbing capabilities of a MG, controls the boost converter. • The H-bridge inverter is current-controlled by means of a control scheme developed in the synchronous reference frame (dq) with PI controllers. • The system was modeled and simulated in Matlab/Simulink/SimPowerSystem. Three models were developed: for the primary side (wind turbine, generator, rectifier and boost converter); the inverter (hardware and control); the entire system (primary side and inverter). • The simulations were mainly focused on the wind turbine performances under variable wind speeds and on the inverter output power quality (with active power step and voltage harmonics). Adequate results were obtained, which proved the effectiveness of the system. • Further developments were performed in a fourth simulation of the overall WT system, which includes an original MPPT derating when the MG frequency increases above a certain limit (51 Hz). 23/24

  24. THANK YOU! 24/24

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