A Collection of Bedtime Stories For Protection Engineers - Volume 1

This technical paper is a collection of recent events on Con Edison’s power system. Each event will have its own section for background, root cause analysis, fault calculation study and corrective action plan. These events demonstrate the uniqueness of the Con Edison power system, challenges, and lesson learns. Highlights of each are below:

Event #1 occurred during an external phase to ground fault. A line current differential scheme mis-operated due to false differential current being introduced at one terminal’s CTs on account of inter-core coupling on a GIS installation. Temporary and permanent corrective action plans will be discussed.

Event #2 also occurred during an external phase to ground fault. Multiple transmission lines in the vicinity over-tripped due to mis-operation of legacy pilot wire relaying. The focus will be on event analysis of all these over-trips and the corrective action plan.

Event #3 occurred during the commissioning process of a group of new power system elements. An undetected single-phase condition occurred when one pole of a circuit breaker remained stuck closed after the breaker had been switched open. This began a chain of events due to the unusual power system condition. Several faults developed, and protection systems operated to clear them.

Event #4 occurred during a mechanical failure of power distribution transformer’s load tap changer (LTC). It led one particular type of directional phase time overcurrent relay element to mis-operate on circulating current. The discussion will focus on calculation and comparison of different operating principles of different directional elements, revealing a limitation of quadrature polarizing. The corrective action plan will also be described.

Event #5 occurred during an external phase to phase fault. One terminal that was 3 buses away from the fault over-tripped on a line current differential scheme that had been in service for more than 5 years. One of the circuit breakers on that line, however, had recently been replaced and a CT polarity error was introduced in the wiring during that breaker replacement. The discussion will cover how this error was uncovered during the root cause analysis.