Subsynchronous Resonance and Torsional Effects on a Steam Turbine Generator in Transmission Systems With Series Capacitor Compensation

Turbine-generators which connect to transmission systems with series capacitor compensated lines can experience problems with the torsional oscillations of the shaft interacting with the electrical oscillations of the series capacitors and transmission system inductance. This resonant interaction is called Subsynchronous Resonance (SSR) since it involves torsional and electrical oscillations which are below the synchronous operating frequency of the power system. The transmission lines near the Newmont TS Power Plant will soon have series capacitors installed. Modeling studies based on calculated torsional frequencies and estimated torsional damping showed that the turbine-generator could have instabilities of the torsional oscillations under some operating conditions. After the modeling studies were completed, actual tests on the steam turbine generator were conducted to measure the frequency and mechanical damping of the torsional oscillations. Field measurements validated modeling studies and showed the SSR risk may be higher than originally estimated. As a result of these investigations, a torsional stress relay is being installed to protect the unit from subsynchronous torsional effects of the series capacitors. The torsional stress relay will trip the unit if damaging torsional oscillations or torsional instabilities occur.