A contribution to modelling and analysis of SSSC-based power flow controls and their impact on SSR

Abstract

This paper presents an initial examination of the impact of SSSC-based closed-loop control of AC power transfer on the damping of the subsynchronous torsional modes of a turbine-generator. The paper begins with a review of the SSR characteristics of the SSSC itself, and how the understanding of these characteristics has grown since the device was first proposed, to demonstrate that the SSSC is not inherently SSR immune, and that its own SSR characteristics, and those of any supplementary controllers employed around it, need careful attention. A detailed mathematical model is then presented of a representative study system, based on the IEEE SSR First Benchmark Model, in which the impact of an SSSC and its supplementary power flow controls on torsional modes can be studied.Detailed frequency-domain analysis, eigenvalue scans and time-domain simulations using this study system model show that the addition of a power flow controller around an SSSC has significant effect on the damping of the generator torsional modes, and that this effect is itself influenced by the speed of response of the power flow controller and by its mode of operation: in particular, in the constant-power mode of operation a fast-responding power flow controller is found to add damping to all the torsional modes whereas in the constant-angle mode of operation a fast-responding controller tends to add damping to some modes and destabilise others. The results suggest that in SSR-susceptible situations, use of power flow controllers in constant-power mode would be preferable.