Transient recovery voltage risks and mitigation due to switching for oil sands fault reduction

Current-limiting reactors are often used in applications where high fault currents encroach or exceed rated fault duties of downstream switchgear. Transient Recovery Voltages (TRV) that develop across the vacuum or SF6 interrupter contacts of the circuit breaker directly in series with the reactor during the interruption of reactor terminal faults can exceed TRV limits published for specific circuit breakers. This situation occurred in an Oil Sands application, where distribution system enhancements and expanded generation to meet process power and steam requirements resulted in overdutied medium voltage switchgear and required installation of a current limiting reactor. Using this case study from the Oil Sand application, this paper will examine transient program modeling techniques associated with reactor-limited fault current interruption and the subsequent TRV response that develops using electromagnetic transients program (EMTP) simulations. These TRV responses, subsequently, are compared to TRV envelopes established by harmonized IEC and IEEE Standards and, where available, manufacturer's breaker lab testing results. In limited cases where the standards or published limits are exceeded, as did occur in this Oil Sands case study, TRV-reduction techniques and the associated practical solutions will be examined. Such practical solutions discussed in the paper include the application of surge capacitors, RC snubber circuits or uprating the breaker to a higher voltage rating.