Voltage Collapse in Power System

The power system is a highly nonlinear system that operates in a constantly changing environment; loads, generator outputs, voltage controllers, transmission lines, topology, and key operating parameters change continually. The behavior and output of the generators can be affected in a transient conditions. This transient condition can change the excitation and dynamics of the rotor which in turn produces undesirable oscillations of the system frequency and voltage magnitude. When subjected to a transient disturbance, the stability of the system depends on the nature of the disturbance as well as the initial operating condition. The disturbance may be small or large. Small disturbances in the form of load changes occur continually, and the system adjusts to the changing conditions. When power system becomes more complex with heavier loads connected to the system, the control of the voltage becomes an important issue. The complexity of the network and the addition of heavier loads can result to a stressful operating condition which can cause system instability resulting to voltage collapse which leads to power blackout.

 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