Study On The Control Strategy Of Unified Power Flow Controller

Unified power flow controller (UPFC) is a Flexible AC Transmission System (FACTS) device that is composed of a shunt converter and a series converter. It is the most versatile and attractive member in the FACTS family for the reason that it not only can use shunt converter as a Static Synchronous Compensator (STATCOM) to compensate the power system for reactive power and keep the bus voltage stability, and series converter as a Static Synchronous Series Compensator (SSSC) to compensate transmission line for the reactance independently, but also can be applied to regulating the bus voltage and controlling the active and reactive power flow on the transmission lines through combining the shunt converter with the series converter. Therefore, the researches on UPFC are proved significative.The control system whose control strategy determines the performance of the system is the heart of UPFC. Designing the control method for the UPFC control system is mainly focused to in this thesis. Considering the UPFC system in the rotating coordinate system (dq coordinate system) being a multi-variable, seriously coupling and nonlinear system, UPFC is separated into a shunt converter and a series converter to design the control systems for them independently so as to reduce the difficulty of designing.According to the mathematical model of UPFC in the dq coordinates system, two control strategies are employed to design its controllers in this thesis. The first control strategy is quoted in the paper that it takes the PI control based on state feedback decoupling method to design the shunt converter system and the PI control based on dual-loop decoupling method to devise the series converter system. The design principle of the first control strategy is simple and its theory is mature. However, it involves much difficulty that too many PI regulators of which the proportional and integral parameters are hard to be fixed are used during the whole designing process. So, the other control strategy that Inverse-System Method(ISM) and Variable Structure Control (VSC) theory are adopted to devise the UPFC control system is proposed in this thesis. In this method, first of all, the shunt converter and series converter are decoupled and linearized to be a pseudo-linear system by ISM. Then, the VSC control laws for the pseudo-linear system are laid out with the VSC control theory. Finally the control systems of the shunt converter and series converter are built by this strategy. Not only does the control strategy proposed in this thesis show a clear idea in the whole design process, but also the parameters of VSC controllers have wide stable scopes. However, the used theories are relatively complex and not mature enough.After finished designing the control system of UPFC, the models of UPFC are established to make simulations under the support of the Simpowersystem packages in the MATLAB software. It is indicated by the simulation results that the control strategy presented in this thesis can make UPFC achieve all its functions and its response performances be similar to the control system which is designed by citing the control methods. In this way, a novel idea can be provided in this paper when one designs the control strategies for UPFC control system.