A Bus Cleared and A Hidden Breaker Insulation Failure Revealed
On June 27, 2023, lightning struck a transmission line approximately one mile from Substation A. Both ends of the line tripped correctly. While the breaker at Substation A was still open, lightning hit the transmission line again. The impulse from the lightning strike broke down the open breaker’s insulation, allowing current to flow to ground. The current flow through the open breaker caused the high-impedance bus differential to trip the bus.
About half an hour later, while the bus was still locked out, lightning struck a different line coming into Substation A about three miles from the substation. This lighting strike caused another breaker’s insulation to be compromised, allowing current to flow through the open breaker. The high-impedance bus differential relay was blind to this event because its CTs were shorted from the previous event, but the percentage-restrained bus differential relay saw the current flow and attempted to trip the de-energized bus. The attempt of the percentage-restrained bus differential relay to clear the bus when all the breakers were already open obviously did not help clear the fault, but the protection at the remote end of the line cleared this second fault correctly.
The insulation failures of the two breakers at Substation A were not immediately apparent to transmission system operators. Initially, it was thought that lightning hit the bus at Substation A. The bus was restored to service, and both breakers were able to function at load voltage and current levels. After deeper relay event analysis, it was discovered that there was a high probability that the lightning strikes had damaged the breakers. Both breakers were removed from service, replaced, and inspected.
This paper will discuss lightning strikes and the associated transient recovery voltage impacts on system protection. The paper will also deep-dive into the analysis of this event using relay event reports and a lightning strike report. This event required extra in-depth analysis since the fault was in the bus differential relays’ zones of protection but not on the bus itself. The paper discusses why one of the breakers showed visible signs of arcing while the other breaker did not. The paper highlights the different operating principles of high-impedance and percentage-restrained bus differential schemes. It also brings to focus the importance of insulation coordination studies and the use of lightning arrestors.