Damping torque estimation and oscillatory stability margin prediction

The damping torque of linearized models of power systems is studied here as a possible on-line security index, based on system identification techniques applied to realistic measurements. First, the theoretical values of damping and synchronizing coefficients of the electromagnetic torque are discussed in detail. These values are then used to investigate the accuracy of damping coefficient identified from on-line measurements using the ordinary least square (OLS) method. It is demonstrated that OLS may not be able to correctly estimate the coefficients due to the nonlinear nature of power system oscillations. Hence, generalized least square (GLS) and robust fitting with bisquare weights (RFBW) are applied to this system identification problem, showing to be better alternatives. Based on these results, the damping coefficient is proposed and studied as an index to calculate the distance to the closest oscillatory instability point. The results obtained from 3 test cases show that the index is an effective tool, and can be of significant help to operators for on-line security monitoring of power systems