Sliding mode active and reactive power decoupled control for Distributed Power Flow Controllers

Abstract

Unified Power Flow Controllers (UPFC) are one of the most useful Flexible AC Transmission Systems (FACTS). They can be used for power flow control in AC transmission grids, allowing simultaneous control of the bus voltage and line active and reactive power. However, due to high costs and reliability concerns, UPFCs have experimented limited use in such applications. Recently, the concepts of Distributed FACTS (DFACTS) and Distributed Power Flow Controller (DPFC) have been introduced as a low cost, high reliability alternative for power flow control. However, DPFCs present cross-coupled (interdependent) and limited regulation of active and reactive power. Therefore, this paper contributions include: (1) a third-harmonic output voltage controller for a full-bridge converter, able to extract active power from third-harmonic currents, to maintain the converter DC voltage constant; (2) DPFC sliding-mode controllers to simultaneously inject active and reactive power at the fundamental frequency, to achieve cross-decoupled (independent) control of active and reactive power flow; (3) applying the sliding mode controlled DPFC to a part of the Portuguese distributed generation and transmission network under study. To provide the required active power to each DPFC device, a PI controlled full-bridge converter acting as a virtual resistance is proposed to extract active power from zero-sequence harmonic frequency currents injected into the line. DPFC models including semiconductor switching, together with line transmission models, were simulated in Matlab/Simulink environment and in PSCAD for comparison purposes. Simulations results show the effectiveness of the full-bridge converter sliding mode controllers in decoupling P and Q control while simultaneously extracting active power from the injected zero-sequence injected currents.