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Staged Fault Testing: A Proactive Approach to Grid Reliability

Updated: May 23

Staged fault testing is key to preventing protection system misoperations. This is not only important to prevent nuisance misoperations; it is also key to preventing cascading faults from leading to a wide area blackout.

In this series on staged fault testing, we’ll explore the equipment necessary to perform a staged fault test, as well as associated costs. We’ll also share how to analyze staged fault test results.

Before we get into these details of staged fault testing, let’s explore current protection system misoperation data and gain a better understanding of what leads to misoperations. We’ll also share a few misoperations that could have been identified via staged fault testing. First, let’s review the basics of a naturally occurring fault versus a staged fault.

What is a Staged Fault?

Lightning strikes, tree falls, and other naturally occurring short circuits are referred to as faults. Unlike naturally occurring faults, which are random in nature, staged faults are intentional short circuits of electrical facilities used for testing purposes.

All faults stress electrical systems and validate the adequacy of design, operation, and maintenance practices. Staged faults are proactive events. In contrast, naturally occurring faults are reactive events; electric utilities can only react to any issues that arise due to naturally occurring faults.

Electric utilities have been reluctant to perform staged fault testing. The prevailing sentiment is that all systems have been exhaustively tested before facilities are energized, and that no good thing can happen when an intentional short circuit is placed on an electric system.

However, staged fault testing is important because it can uncover missteps before a wide area blackout occurs.

NERC Misoperation Data

The North American Electric Reliability Corporation (NERC) has developed reporting instructions and misoperation information data analysis systems that allow electric utilities to report misoperations and cause codes reactively, after an event occurs. This is an important step in understanding the number of misoperations in any given year.

During the last decade, NERC has been developing graphs and charts with electric utilities’ reported protection system misoperations. The chart from NERC, below, titled “M-9 Protection System Misoperations” shows little change in the misoperation rate across NERC regional entities from 2017 to 2021.

Graph from the North American Electric Reliability Corporation (NERC) shows little change in the misoperation rate across NERC regional entities from 2017 to 2021.

The lack of change in the number of misoperations is disheartening, especially when such misoperations could be minimized or even eliminated with staged fault testing.

Too Much Dependability?

Although protective relaying schemes are designed to be both dependable and secure, the scale has been tipped a little too much towards dependability. The predominant reasoning is that a single protective relaying scheme will detect and correctly isolate a short circuit 99% of the time; redundant protective relaying schemes will detect and correctly isolate a short circuit 99.99% of the time. This certainly meets the dependability challenge.

The majority of misoperations, however, are sympathetic trips in which protective relay schemes trip circuit breakers that do not need to open to isolate a short circuit. In other words, when a short circuit occurs, protective relays unnecessarily open circuit breakers connected to other nearby facilities. This is due to electric utilities’ overemphasis on dependability.

The prevailing sentiment is that most of the time, opening circuit breakers connected to nearby facilities is a nuisance event with no significant consequences. However, this is not the case during peak load periods when the simultaneous loss of two energy transfer paths can create wide area low voltage conditions, which can preface a wide area blackout.

Experience Shows: Staged Fault Testing Can Identify Errors

As an electrical engineer with decades of experience, I know many old sayings within the industry. One saying is, “God acts quickly when protective relay engineers forget the decimal point.” This colloquialism is due to the fact that protective relay applications are tested through natural causes, such as in the following examples that I experienced:

In one instance, shortly after a new 230 KV transmission line was energized, lightning struck an adjacent transmission line and circuit breakers on one end of the new transmission line opened. The subsequent root cause investigation (RCI) revealed that the actual impedance of the new transmission line was only 80% of the impedance used to develop protective relay setpoints. The preparer, reviewer, and approver all used the same incorrect data when determining setpoints... oops.

In another instance, during the replacement of electromechanical relays with microprocessor-based relays on three 500 KV lines, a firmware issue was resolved by inserting a patch that was developed by the relay manufacturer. After this patch was installed, a 230 KV insulator flashed over at a nearby substation, and protective relays tripped all terminals of all three 500 KV lines.

The subsequent RCI revealed that, after the patch was inserted, the relays were not retested and unintended changes in functionality went undetected. If this flashover occurred in July, rather than in April, three 500 KV lines, two 1200 MW nuclear power plants and two 800 MW coal fired power plants would have tripped at the same time, resulting in a wide area blackout.

These are just two examples of missteps that would have been uncovered and resolved by staged fault testing.

Cross Industry Comparison: Automobile Crash Testing

The National Highway Traffic Safety Administration performs crash tests on vehicles as part of the agency’s 5-Star Safety Ratings system under its new car assessment program. This proactive approach to car safety allows consumers to see how well their next potential vehicle will withstand a car accident.

If NHTSA followed the logic of most electric utilities, there would be no need for crash tests because reliable, redundant braking systems are installed in automobiles.

However, unlikely events happen – cars get in accidents, lightning strikes electrical lines, insulators flashover. It is prudent for electric utilities to follow the standards of the automobile industry by testing their protective relay settings and responses to a fault before a naturally occurring fault happens.

A Successful Staged Fault Test

Sometimes, so many nuisance trips occur that staged fault tests are authorized. In the late 1970s, staged fault testing was performed near Hazleton, PA to understand the response of the Bell Telephone communications network when faults occur. This test revealed that leased telephone lines in Pennsylvania could not be used as a communication channel for protective relaying schemes. Bell’s Special Services Maintenance Bureau participated and advised that Bell discontinue offering this service.

Today’s Technology Supports Staged Fault Testing

Today, we have access to a plethora of technology that was not available in the 1970s: SF6 insulated circuit breakers, microprocessor-based protective relays, laptop computers and tablets, fiber optic networks, and more. Additionally, with the widespread use of air conditioners, rooftop solar panels, and wind turbine energy farms, the risk of misoperations leading to a wide area blackout is much greater than at any point in electric utilities’ history.

It’s time to recognize the need to refine electric utility testing and maintenance activities. Staged fault testing must be included in facility acceptance testing.

Want to learn more about staged fault testing? Follow along with our blog as we explore the ins and outs of staged fault testing over the next few weeks. Or check out our previous posts:

And contact us with any questions!

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