“Interesting” doesn’t do justice to the generator circuit-breaker failure events described by two utility users at the Combined Cycle Users Group’s 2021 virtual conference, although that’s the word they used. Threatening is more accurate. While the events themselves tend to be unique and site-specific, many readers can relate to the pain, and the “lessons from the field” they presented. Above all, it never hurts to be reminded that anything can take you out in a powerplant and everything needs to be checked, rechecked, etc.
Examples included new breakers and old breakers in variety of different plants but the most impactful event was an overheated breaker which took out a 7EA gas turbine/generator on the hottest day in August last year with no replacement power available for purchase anywhere. Root cause: A runback for unit output set higher than the 7EA could produce. Takeaways here are to check runback settings, make sure operators act on alarms, maintain fans, correct failing switches, and make sure spare breakers are available.
In another example, the presenters stressed that contractual issues can be as important as mechanical issues when it comes to maintenance. With non-utility owner/operators, FERC rules, markets, and state/regional ISOs, it’s often not clear who pays for plant substation equipment. The “point of ownership” may not be the same as the “point of interconnection.” Contractual language and accounting rules governing critical plant property can be confusing, but should be understood, not ignored.
What’s more, some breaker suppliers “played fast and loose” in the past with breaker ratings at ambient temperature, and it may be prudent to validate the rating. Some units are forced-cooled “to get more rating out of it.” In this way, a 3000-amp breaker can be “uprated” to 5000 amps.
Age of a breaker has much to say about its failure likelihood. Annual failure probability at six years is 0.1%, but at 24 years, it rises to 2%. And if you want to address 50% of generic underlying performance failure causes (lubrication), make sure you are using the right lubricant, that it is applied correctly, and that it has not degraded (Fig 1).
The slides, available on the Power Users website [link] to registered owner/operators, detail some basic maintenance standards—such as conducting visual inspections every 12 months, minor maintenance every 24 months for 13.8-kV units, and and 36 months for 480- and 4160-V breakers, and major maintenance (Fig 2) every 72 months, regardless of voltage. These are based on ANSI/NETA MTS guidelines. If you’re unfamiliar with these acronyms, NETA is for the InterNational Electrical Testing Assn and MTS for Maintenance Testing Specifications (for Electrical Power Equipment and Systems). MTS is an approved American National Standard.
But realize that established maintenance just tells you whether the breaker will work, not what might be wrong with it. Modern relaying can “speed up troubleshooting time after an electrical trip.”
Finally, the presenters give critical suggestions for conducting opening time tests and measuring and trending contact gaps on the vacuum bottle for an indication of remaining life.
This is a presentation that you’ll want to listen to, not just review the slides; the commentary adds valuable color and texture to the illustrations.