Double Closed Loop Control and Analysis for Shunt Inverter of UPFC

The flexible AC transmission system (FACTS) provides a new advanced technology solution to improve the flexibility, controllability and stability of a power system. The unified power flow controller (UPFC), as an outstanding part one for regulating power flow among the FACTS, can control respectively transmission line real power, reactive power and node voltage. In this paper operation principle and model of UPFC are introduced, and control strategy of current feed-forward plus double PI loop for adjusting shunt inverter real power is proposed. According to math model of shunt inverter, a simple model in the synchronous rotating dq coordinates is given and a dq decoupling double close-loop controller is established and designed. The simulation results for a case study indicate that DC bus voltage and node voltage can be controlled efficiently, attest that control scheme and controller design are viable and effective. This paper has a certain instructive significance for UPFC application. It settles foundation in both practice and theory for further manufacturing UPFC laboratory-scale equipment