SUBSYNCHRONOUS RESONANCE COUNTERMEASURE USING PHASE IMBALANCE

Subsynchronous Resonance (SSR) in series compensated power systems is a phenomenon implying an undesirable energy exchange between the elect rical and mechanical sides of turbine-generator sets, at a frequency b elow the power frequency (60 Hz in the USA). Since the discovery in 19 70 that SSR was the main cause of the shaft failures at the Mohave pla nt in Southern Nevada, extensive research and development efforts have been devoted to the development of effective SSR mitigation measures. This paper presents a contribution to these efforts by providing a ra ther new concept for simple and reliable SSR mitigation. The basic ide a is to reduce the energy exchange between the two sides of turbine-ge nerator sets at subsynchronous oscillations by weakening the electrome chanical coupling. This is attained by creating phase imbalance at sub synchronous oscillations. The imbalance diminishes the capability of t he three phase currents to develop interacting electromagnetic torques and, therefore, suppresses energy exchange between the electrical and mechanical sides of turbine-generator sets. The basis of this idea ha ve been presented by the author in a previous IEEE paper, where the ph ase imbalance was associated with the series capacitor banks. The obje ctive of this paper is to study the influence of having the phase imba lance associated with the generator to be protected from potential tor sional interaction problems.