Subsynchronous resonance

(electricity) An electrical resonant frequency on an alternating-current transmission line that is less than the line frequency, and results from the insertion of series capacitors to cancel out part of the line and system reactance.

The resonance between a series-capacitor-compensated electric system and the mechanical spring-mass system of a turbine-generator at subsynchronous frequencies. 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, following switching events that placed the turbine-generator 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. There have been no reported occurrences of two other types of subsynchronous resonance, the induction generator effect and torque amplification. See also Alternating-current generator; Transmission lines; Turbine.

It has been clearly established that subsynchronous resonance can be controlled with the use of countermeasures, thus making it possible to benefit from the distinct advantages of series capacitors. About a dozen countermeasures have been successfully applied such that there has been no reported subsynchronous resonance event since 1971. Subsynchronous resonance countermeasures protect turbine-generator shafts from harmful torsional oscillations by one of two methods. First, the turbine-generator can be tripped when a subsynchronous resonance condition is detected. This limits the number of torsional oscillations experienced by the turbine-generator shafts. This type of countermeasure is relatively inexpensive but is not acceptable if the anticipated subsynchronous resonance conditions are expected to occur frequently. Generally, such a countermeasure will not be applied as the sole subsynchronous resonance protection if it is expected to cause a turbine-generator to be tripped more than once every 10 years. Three types of tripping countermeasures are applied: torsional motion relay, armature current relay, and the generator tripping logic scheme. The second method of protection does not involve turbine-generator tripping, but eliminates or limits harmful torsional oscillations. See also Electric power systems.