The ability of AC/DC converters to generate and consume reactive power

ABSTRACT

The conversion of an input AC power at a given frequency and voltage to an output power at different frequency and voltage can be obtained with static circuits called power converters, containing controllable power electronic devices. Various power converters have been developed to fulfill the requirements of the wind power generation. The most common option for the generators used in wind power generation is “asynchronous”, because support for speed variations is provided. This represents a major advantage, considering that the wind speed can exhibit fast variations, mainly during flurries. These result into significant mechanical stresses, smaller when an asynchronous generator is used as compared to a synchronous one, usually operating at a rated speed. The common way to convert the low-speed, high-torque mechanical power to electrical power is using a gearbox and a generator with standard speed. The Grid Connection Code for RPPs in South Africa has stipulated that all RPP shall be equipped with reactive power control functions capable of controlling the reactive power supplied by the RPP at the POC as well as a voltage control function capable of controlling the voltage at the POC via orders using set points and gradients. The reactive power and voltage control functions are mutually exclusive, which means that only one of the three functions mentioned below can be activated at a time. (a) Voltage control (b) Power Factor control (c) Q control. The Q control is a control function controlling the reactive power supply and absorption at the POC independently of the active power and the voltage.

TABLE OF CONTENTS

1.           INTRODUCTION.. 3

2.           Type 4 wind turbines generators...............................................................................4                                                                                          2.1         Operational behaviour of type 4 wind turbines. 4

3.            SYNCHRONOUS GENERATOR. 7

4.           POWER CONVERTER FOR WIND TURBINE GENERATOR SYSTEM 5

4.1.        Three phase AC/DC converter on the generator side. 6

4.1.1      Three phase  DC/AC converter on the grid side. 6

4.1.2       Reasons for not using diode bridge on the generator side. 7

4.1.3       Diode rectifier based converter. 7

5.            REACTIVE POWER AND VOLTAGE CONTROL. 7

5.1.         Reactive power generation and absorption on the grid side. 9

    5.1.1       Reactive power compensation for wind power plants…………………………….9

5.1.2    Mechanically-switched shunt and regulated reactors...............................................9

5.1.3       STATCOM System.. 10

6.            CONCLUSIONS.........................................................................................................15

                    References…………………………………………………………………………………………..……………15

                    Appendix

                      List of Figures

                      Figure 1 Full conversion type 4 wind turbine …………………………………………………………..4

                      Figure 2: Type 4 WTG Short Circuit Current …………………….. ........................................4

                      Figure 3: Synchronous Generator …………………………....................................................5

                      Figure 4: Output voltage of back to back converter ………......................................... 6

                      Figure 5: Input current of back to back converter………………........ …………………………..6

                      Figure 6: Input current of diode rectifier based converter…………………………………………….7

                      Figure 7: Reactive power control functions for Renewable power plants …………………...8

                      Figure 8: Voltage control for the RPP ………………………………………………..........................8

                      Figure 9: Reactive power requirement of Wind energy facilities…………………………..9

                      Figure10: STATCOM description ………………………………..………………………………………...11

                      Figure11: Operating principle and operation area of STATCOM ………………….…...11

                      Figure12: Voltage (pu values) at wind turbine connection point ……………….…,…….12

                      Figure13: STATCOM Reactive power in MV Ar ……………………………………………….…..…12

                      Figure14: Voltage at wind turbine connection point with no reactive power injection. ….13

                      Figure15: Reactive power injected to the grid by STATCOM …………………….....…....13

                      Figure16: : P-V curves. With reactive power injection of STATCOM.….……………………....14