MV circuit breaker trips

[AusLee]

Hello,

I have an installation running for more than 5 years. No new loads have been added, still the same from day 1 in service.

For some reason, the local civil engineer claims that the transformer is heating. A site investigation reveals air no hotter than leaving a normal house hold winter time radiator. The 1,250 kVA dry type transformer is "Class F", it is allowed to overheat 100 degrees above ambient temperature.

The transformer is equipped with a thermal protection relay, and last year the MV breaker was changed (just because the civil engineer deemed it had operated enough).

Now, for some reason, the new MV circuit breaker is tripping occasionally, once every 2-3 days. The load on the transformer is about 700A, less than 40% (it is a well oversized transformer). The civil engineer wants to change the transformer as well.

I have checked the MV voltage on site, it is near exact match with the tap setting, no over or undervoltage. Since the load is still the same, i assume either the new MV breaker is faluty, or is set to trip at way below the rated current, or the thermal relay is faulting and causing the breaker to trip.

Can you please advise on what may be the reason of these nuisance trippings, and what tests can be made on site to point the real reason for the tripping?

 

[jghrist]

First you have to determine what relay is tripping the breaker. I assume that there is an overcurrent relay in addition to the transformer thermal relay. What relay targets are there after the trip?

Did the civil engineer set the relays? If so, blame him. If not, blame him anyway for not setting them. ;-)

Many relays will record the current at the time of the trip. See if this is the case.

 

[itsmoked]

Seems you should check the ACTUAL temperature not a <100 degree estimate. 100 degree rise is still fits into some absolute maximum. You will look pretty foolish if you go chasing off down the "defective breaker" trail and you have gathered a circulating current that's heating your xformer up.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jpankask]

Find out which relay is tripping the breaker, breaker type, voltage, etc., then you will get a more focused response.

Not that he can't learn, but what is a Civil Engineer doing telling you what to do with your electrical system? You would have to operate that breaker a LOT to reach the limit of operations on todays circuit breakers. I agree with jghrist, it's the civil engineers fault one way or another but it sounds like your trip settings on the relays may need to be calibrated, not a changeout of the transformer. See if you can get more details.

 

[ScottyUK]

Obviously it is the building which is at fault. Knock it down and design the replacement yourself. Point out to the CE that you're as qualified to tell him to demolish the building as he is to demand replacement of the transformer.

Does the relay which initiates the trip not have either a mechanical flag or an LED indicator to identify that it has tripped? Consider bringing in a relay testing company to check-cal the relay, once you are happy with the settings. Where are the CTs for the relay - can you check they are the correct ratio?


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[Zogzog]

You are taking advice from a CE on this? Where are you located? Maybe I or someone else can recommend a testing firm to come out and resolve this for you. Could be something very simple.

P.S. How many times was "too many" times for the MV breaker to have operated? What type of breaker and what type of operations?

 

[oldfieldguy]

I'm trying to get a picture of "it had operated enough". I know of applications where 15 kV breakers operated twice a day, every day, for years.

Once a quarter we went in and checked contact wear in accordance with the manufacturer's procedures, and after eight years of this, wear started to APPROACH the limits, so we swapped that breaker from its service as a motor controller (8000 horsepower across the line) to semi-retirement as a transformer feeder breaker, where operation was more likely once every three years.

The "it had operated enough" was determined by specific measurement, not opinion.

I know of a multitude of breakers in transformer and generator service that have been in continuous service for fifty years.

Medium voltage circuit breakers are typically tripped by protective relays. You need to determine which protective devices you have, and which device is tripping your breaker. A breaker tripping frequently like yours points to a problem, and it's unlikely to be the breaker.

A good testing agency can test the breaker to manufacturer and industry standards, checking it for proper mechanical and electrical operation. Past that, they can test protective devices for proper settings and help you determine the cause of the tripping.

old field guy