ANALYZE RELAY FAULT DATA
TO IMPROVE SERVICE RELIABILITY
Schweitzer Engineering Laboratories, Inc.
Pullman, WA USA
Protective relays are at the core of maintaining electric service to as many customers as possible
during a system disturbance. For this reason, an improvement in protection quality is directly
reflected as an improvement in customer service. To improve protection quality, it is useful to
identify and record performance measures that can be evaluated for system impact.
Using 18 months of data (January 1996–August 1997), detailing every relay operation on an
anonymous utility system (1400 operations), this paper analyzes the faults and protective system
operation to determine relay operation and performance of electromechanical, solid-state, and
digital relays. Specific system quality measurements include relay misoperations; relay failures to
operate; relay delayed operations; and accessory component failure, such as fault recorder, trip
circuit, communications system, or targeting system. Results are compared to the values given by
Working Group I17, Transmission Relay System Performance Comparison for 2000 and 2001
For each protection quality measurement, examples of negative responses and their power system
impact are presented, as well as statistical values detailing the probability of a negative response.
In conclusion, a discussion of reasonable mitigation techniques and their cost and benefits, based
on the data, is presented.
Looking at actual system events provides fascinating details in the life of a power system. A
massive explosion at an industrial facility breaks windows a half mile away and causes
electromechanical contacts to bounce closed. Pelicans fly into a subtransmission line and relays
clear the fault after a time delay. Individual events are significant in that they form part of a
pattern; the analysis of which can be used to improve relay operation.
The power system data used in this paper include electromecha