Comparison of dynamic phasor, discrete-time and frequency scanning based SSR models of a TCSC

Abstract

Sub-synchronous Control/Torsional Interactions occur in a power system due to adverse interactions between the electrical network, turbine-generators and power-electronic systems. The extraction of time-invariant models of the system for analysis and design is often an onerous task due to the complex dynamics associated with power-electronic systems. The modelling techniques proposed in the literature include the use of discrete-time mapping, dynamic phasors and frequency scanning. Frequency scanning is a black-box approach, wherein a time-invariant model is extracted from the simulated response of the system to a probing signal. The first two techniques derive the model directly from the analytical equations (with several simplifying assumptions). This paper compares the accuracy of these models using the example of a SSR-vulnerable system which includes a TCSC. The presence of several poorly damped torsional modes and the use of a TCSC to damp them make this an excellent test case for modelling accuracy. It is seen that the frequency scanning based state-space model is quite accurate and its extraction using simulation and the vector fitting technique presents no significant difficulties, irrespective of the extent of modelling detail. The results demonstrate that this approach can be both convenient and accurate for sub-synchronous interaction studies.