SUBSYNCHRONOUS RESONANCE MITIGATION USING TCSC

Abstract

Sustained oscillation below the fundamental system frequency can be caused by series capacitive compensation. This phenomenon is known as sub synchronous resonance (SSR). The turbine generator mechanical system comprises large masses connected by steel shafts. This rotating system has torsional resonance frequencies at which adjacent masses tend to twist back and forth for any shock. A shock to a turbine generator unit on its own and connected radially to a power system via a series compensated line can cause the turbine generator unit to oscillate at one or more of its modes of torsional oscillations. These oscillations can produce peak torques in the shaft system several times the normal torque corresponding to the rated power. Under some circumstances shaft twisting may become excessive at one of the resonance frequencies and potentially damage the shaft even without any series capacitor compensation. Traditional techniques to mitigate SSR include the use of Power System Stabilizers (PSS), which have been reported to be moderately effective for this purpose. In the 1980’s, the EPRI initiated the FACTS technology where various power electronics based FACTS devices were used to improve power transmission capability over existing transmission corridors. Subsequently, these devices were increasingly used for improving transient stability and for SSR mitigation. In particular, series FACTS Controllers can actively mitigate sub synchronous resonance either by their non-capacitive characteristics in the sub synchronous frequency range of interest or by active damping action. This consists of thyristor modulated series compensator like TCSC where control of sub synchronous oscillations is achieved by modulating the thyristor switch firing angles using various signals such as generator shaft speed, line current, etc. In this project, the phenomenon of SSR is first demonstrated in the IEEE first benchmark model and is subsequently mitigated using PSS and thyristor controlled series capacitor (TCSC).