Control Methods Study Of Unified Power Flow Controller

In this dissertation, the development of actuality and trend of the Flexible AC Transmission System (FACTS) and its significant senses to the country are introduced firstly. Then the principle of UPFC controlling the real and reactive power flow in the transmission line is discussed. The control ability and its chat of UPFC are also elicited.In chapter three, the dissertation makes a presentation to the principle and power control ability of UPFC. Moreover it also presents the SVPWM modulation and its control methods of voltage source converter. The main parts of the dissertation are the chapter four and five. In chapter four, three-phase static reference frame is converted to two-phase synchronous rotate reference frame by abc-dq convert matrix. So it makes the UPFC control scheme simple. New fixed UPFC configuration is proposed. New control schemes are also discussed comparing to the conventional fixed UPFC control strategy. The cross-coupling control methods in conventional fixed and new fixed UPFC are established according to the instant power theory. So the real and reactive power flow in transmission line control can be separated. Two control schemes in new fixed UPFC are proposed. The first control scheme makes the real and reactive power flow controlled by the shunt converter; the stability of the DC-link voltage and node voltage of power system are mastered by the series converter. The control scheme is fully different to the control methods in conventional fixed UPFC. In conventional fixed UPFC, the stability of the DC-link voltage and node voltage are controlled by the shunt converter; real and reactive power flows in the transmission line are controlled by the series converter. The second new control scheme makes the reactive power flow in transmission line and the stability of DC-link voltage controlled by the shunt converter; the stability of node voltage and real power flow in transmission line are mastered by the series converter. This control strategy can be applied to conventional fixed UPFC, but it is not common. In this dissertation, operating equation of state is established. The control blocks of these two control schemes are also deduced according these equations. In chapter five, the models of the conventional fixed and the new fixed UPFC in two generators infinite power system are constructed by Matlab and its Simulink power system toolbox. The simulation parameters are designed. The simulation result proved that the first control scheme in new fixed UPFC has greater capability to regulate the power flow and has short adjusting time. But the result of controlling the stability of node voltage is not as well as the control effect in the conventional fixed UPFC. The second control scheme is also validated that the power flow adjusting ability is better than the conventional one. The mutual interface in adjusting the real and reactive power flow in transmission line is little. The performance on stabilizing the DC-link voltage and node voltage is the same as the conventional control effect. Thus it can be seen that the two control scheme in the new fixed UPFC gets better control effects and overcome some disadvantages in conventional control scheme.In chapter six, the control method of fuzzy-PID has been applied to the first control scheme in new fixed UPFC. The fuzzy-PID control synthesizes the virtue of both the fuzzy and the PID which can be switched between the two control mode according to the given error threshold. In this hybrid control method, the fuzzy control is used for quickly adjusting the dynamic process and the PID control method for accurately accommodating the static process. It has been proved that the control strategy can improve the control performance and makes the transient response better.In this dissertation, new fixed UPFC configuration has been proposed and two control strategies have been verified by Matlab and Simulink toolbox. The fuzzy-PID control scheme is also applied to the first control scheme in new fixed UPFC and gets better control performance. It has some application values.