On the sign of the feedback applied by power system damping controllers

This paper studies the sign of the feedback applied by Power System Damping Controllers (PSDCs). The sign of the feedback to be applied by power system stabilizers (PSSs) is well established from the understanding of the physical mechanisms of their damping action. However, the complexity of the damping action provided by FACTS devices does not allow to determine easily the sign of the feedback to be applied by their damping controllers. This paper discusses the sign of the feedback applied by PSDCs in the context of their design using eigenvalue sensitivities. As the eigenvalue sensitivity approach to design PSDCs focuses only on the poorly damped electromechanical modes, they can be properly damped whereas one or several unstable modes may arise if the sign of the feedback is not proper. This paper suggests a criterion to determine the sign applied by PSDCs. It is based on the analysis of the frequency response of the open loop transfer function combined with conclusions drawn from the numerical pole zero analysis of several open loop transfer functions of a small power system. The application of the criterion is illustrated in a small power system widely used for fundamental studies of power system electromechanical oscillations.