Subsynchronous resonance

Article By:

Farmer, Richard G. Department of Electrical Engineering, Arizona State University, Tempe, Arizona.

Agrawal, Bajarang Arizona Public Service Company, Phoenix, Arizona.

Last reviewed:January 2021

DOI:https://doi.org/10.1036/1097-8542.664550

The resonance between a series-capacitor-compensated electric system and the mechanical spring-mass system of a turbine-generator at subsynchronous frequencies, that is, at frequencies that are less than the synchronous frequency. Beginning about 1950, series capacitors were installed in long alternating-current transmission lines [250 km (150 mi) or more] to cancel part of the inherent inductive reactance of the line. Until 1971, up to 70% of the 60-Hz inductive reactance was canceled by series capacitors in some long lines with little concern for side effects. (If 70% of a line's inductive reactance is canceled, the line is said to have 70% series compensation.) In 1970, and again in 1971, a turbine-generator at the Mohave Power Plant in southern Nevada experienced shaft damage that required several months of repairs on each occasion. This followed switching events that placed the turbine-generator so that it was radial on a series-compensated transmission line. The shaft damage was due to torsional oscillations between the two ends of the generator-exciter shaft. Shortly after the second event, it was determined that the torsional oscillations were caused by torsional interaction, which is a type of subsynchronous resonance (SSR). There are two other types of subsynchronous resonance, the induction generator effect and torque amplification. There has been one reported incidence of the induction generator effect type of SSR related to a wind farm in Texas in 2009, but there has been no reported incidence of torque amplification.

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