EFFECTS OF UNIFIED POWER-FLOW CONTROLLERS ON TRANSIENT STABILITY

The paper investigates the mechanism of the three control methods of u nified power flow controllers, namely in-phase voltage control, quadra ture voltage control and shunt compensation, in improving transient st ability of power systems and examines the utilisation of the voltamper e ratings by the three control methods. The study is based on a single machine infinite bus system with a unified power now controller conne cted in series with the transmission lines. The potential of the three control methods in transient stability enhancement is identified. Ana lytical results indicate that significant reduction in the transient s wing can be obtained with any of the above methods by using a simple p roportional feedback of rotor angle deviation, The analysis of the tra nsient energy function shows that the transient stability margin can a lso be substantially improved with in-phase voltage control and quadra ture voltage control, however, to a smaller extent with shunt compensa tion. The analysis is verified by transient stability simulation. The simulation results show that the quadrature voltage control, apart fro m being more effective in reducing the transient swing than the in-pha se voltage control, incurs a substantially smaller real power flow exc itation converter. This makes a larger voltampere capability of the ex citation converter available for shunt compensation.