Staged Fault Testing: A Necessary Precaution

After almost 10 years of study, the number of protective relay misoperations associated with high voltage facilities remains unacceptably high. The electric power grid is designed on a single failure basis. Each misoperation results in the isolation of an unfaulted facility at the same time that the faulted facility is isolated.


One misoperation per month will not result in a wide area blackout. However, when several simultaneous misoperations occur, the probability of a wide area blackout that crosses state lines and involves several metropolitan areas increases exponentially.


In 2013, NERC revealed that slightly more than 10% of protective relay operations were misoperations. In 2019, NERC determined that slightly less than 10% of protective relay operations were misoperations. The lack of change in the number of misoperations is troubling. The goal should be to achieve Six Sigma metrics, i.e., one misoperation per 1,000,000 challenges.


To reduce the amount of protective relay misoperations, electric utilities should implement staged fault testing practices after new facilities are first energized, but before they are accepted for use. Read on to learn more about why staged fault testing is a necessary precaution, and how Prescient recommends utilities implement it.


Causes of Protective Relay Misoperations


Many misoperations could be avoided if utilities conducted staged fault testing before a new facility or new protective application was accepted for service. Table 1 shows misoperations categorized by cause code; most, if not all, of these occurrences could have been avoided, especially with preemptive staged fault testing.

The integrity of protective relaying schemes is determined by their response to lightning storms, hurricanes, other severe weather events, which often occur weeks or months after facilities are energized. These events often lead to misoperations that could have been avoided, had preemptive staged fault testing occurred.


Consider the following three examples:


Example 1: Incorrect Transmission Line Impedance


Figure 1 shows a misoperation that occurred when a fault occurred on an adjacent transmission line. During the first month that Substation B was in service, a close in fault occurred on 230 KV transmission line 2, and 230 KV circuit breaker 101 tripped, incorrectly, at Substation A.

This misoperation, classified as an overtrip, was detected immediately. The subsequent root cause investigation determined that incorrect transmission line impedances were used to develop short circuit studies, protective relay settings, and load flow studies.


Example 2: Mismatched Relay Wiring


Figure 2 shows a misoperation that was uncovered several days after faults appeared to be correctly isolated. During an ice storm, 138 KV transmission line 22 tripped nineteen times. When protective relay engineers determined that the line tripped by primary microprocessor protective relays each time and the redundant, backup microprocessor protective relays did not attempt to trip circuit breakers, an apparent cause evaluation was initiated.

This misoperation, classified as a failure to trip, was detected after digital recordings were downloaded and studied. Subsequently, it was determined that both microprocessor protective relays were wired with output 104 as the trip output. The backup microprocessor protective relays were, however, programmed to use output 102 as the trip output.


Example 3: Faulty Relay Logic


Figure 3 shows six misoperations that occurred when a single fault occurred. Electromechanical protective relays for four 500 KV transmission lines were replaced with microprocessor-based protective relays. During acceptance testing, a problem with data displays was uncovered and a firmware patch was applied at all eight line terminals without retesting the microprocessor relays.


The firmware patch inadvertently inserted an instantaneous, 400 amp, non-directional, overcurrent ground trip setting at all eight line terminals. When a single phase to ground fault occurred the day after these microprocessor-based relays were placed in service, all eight circuit breakers tripped.

These misoperations, classified as overtrips, were detected immediately. The root cause investigation revealed that when firmware patches were installed, retesting was not required.


All three of these misoperations would have been detected during staged fault testing, if staged fault testing had been performed after all work was completed and before the facilities were accepted for use.


Next Steps: Perform a Staged Fault Test


Staged fault testing is a necessary precaution that all utilities should implement to the reduce the number of protective relay misoperations. In our next post, we’ll outline procedures for performing a staged fault test for a single phase fault and a three phase fault. We’ll also discuss the equipment needed to implement staged fault testing.


Visit Prescient’s website for more information on Prescient’s wide area blackout risk assessment service, or to learn more from our blackout blog collection. Contact us to continue to discussion on next generation power system concepts.

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