Effects of unified power flow controllers on transient stability

The paper investigates the mechanism of the three control methods of unified power flow controllers, namely in-phase voltage control, quadrature voltage control and shunt compensation, in improving transient stability of power systems and examines the utilisation of the voltampere ratings by the three control methods. The study is based on a single machine infinite bus system with a unified power flow controller connected in series with the transmission lines. The potential of the three control methods in transient stability enhancement is identified. Analytical results indicate that significant reduction in the transient swing can be obtained with any of the above methods by using a simple proportional feedback of rotor angle deviation. The analysis of the transient energy function shows that the transient stability margin can also be substantially improved with in-phase voltage control and quadrature voltage control, however, to a smaller extent with shunt compensation. The analysis is verified by transient stability simulation. The simulation results show that the quadrature voltage control, apart from being more effective in reducing the transient swing than the in-phase voltage control, incurs a substantially smaller real power flow in the excitation converter. This makes a larger voltampere capability of the excitation converter available for shunt compensation