Pilot Protection System Failure - an Investigation

Author: Collin Martin, Oncor Electric Delivery

Introduction

Protection system failures are sometimes hard to detect, especially in the case of electromechanical relays with no inherent self-testing or alarming. It often takes a misoperation of the protection system and the following investigation to discover the problem. At this point it is too late and the damage is already done. This paper examines an event on the Oncor Electric Delivery system in which a critical customer, normally served by four 138kV transmission lines, was left hanging on a single feed due to a fault and two relaying system failures. Subsequent investigation revealed that both relaying system failures had been present for extended periods of time. Recommendations for future changes are presented.

Primary relaying system failures are associated with long fault durations because backup relaying must be relied upon to clear the fault. A slow trip and the accompanying long-duration voltage dips can be very costly to sensitive customers, such as semiconductor manufacturers, due to product loss.
An even worse scenario would be to unnecessarily trip one of these sensitive customers and completely put them in the dark.

The area of the system in which this event occurred is especially important for several of Oncor Electric Delivery's critical customers. The Dallas Switching Station is served by four 138kV lines which are protected by permissive overreaching transfer trip (POTT) schemes using various electromechanical and microprocessor relays. POTT schemes over tone are used to provide high speed tripping and increased security against undesirable trips for external faults.
Relay coordination is challenging because of the number of short lines in the area as well as the number of Y-Δ-Y transformers and their FID ratings. Within just the four lines serving Dallas there are twenty- one Y-Δ-Y distribution transformers which are zero sequence current sources.

FID ratings can cause coordination problems because they are blocked from tripping for faults over 16,000 amperes. Relays on both ends of the line must trip for the 16,000 ampere faults because of the POTT pilot logic used. This requires unusually large distance relay reach settings because fault duties in the corridor range from 30,000 - 60,000 amperes.

Oncor Electric Delivery maintains an extensive digital fault recorder (DFR) network. Approximately 230 DFRs are scattered around the system and capture more than 15,000 records a year. After the records are collected, they are automatically converted into a standard report and sorted into low, medium, and high priority groups. The low priority group accounts for 95% of the records and is generally made up of records triggered by remote faults that contain limited useful information. The medium priority group consists of correct fault operations and accounts for about 4% of the records. The high priority group is made up of the remaining 1% of the records that indicate possible slow breakers, slow relays, breaker failures, or carrier problems.

DFRs are powerful tools to use when analyzing system operations, but they are only as good as the information being fed to them.  Sometimes they contain very valuable "hidden" information that can easily be missed at first glance. The rest of this article describes the process through which Oncor Electric Delivery went to get to the bottom of a relaying system failure, and find the true root cause.

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