Determination of the Peak Transient Recovery Voltage

Summary

The inherent try of a system at a given location may be predominantly oscillatory or exponential. Interaction effects of the circuit breaker, other than switching resistors, and high frequency transients due to short line faults or local system elements are of little significance in the peak regime. Exponential try’s are frequently disguised by a major superposed oscillation caused by parts of the faulted sub-system which lie outside the primary fault circuit.

The system try should be assessed beyond the highest peak. With an oscillatory try several 100 ps will suffice but about 2 ms is required for exponential try’s. This suggests a system model extending to a radius of 300 km! Fortunately this is not necessary in complex interconnected systems. The model has to cover the whole of the system directly connected to the fault point at the one voltage, however the models of lines going out from remote busbars can usually be markedly simplified. Care must be taken in modelling local transformers although in general an ‘equivalent line’ model suffices for the system behind a transformer. Local loads can have a very significant effect after a few 100 μs despite several stages of transformation to low voltage.

Some simple rules for the calculation of inherent try using a single phase travelling wave computer program are elucidated.

Presented at the symposium by J. L. Diesendorf