800A CB tripping when closing a 225A CB transformer feeder. Any helpful suggestions?

J

jbrown

Member
All SqD Equip w/ cu conductors single line setup:
2000A 480 3Ph Main Service --->800A I-Line Sub-Feed CB ---> 800A 480v MGP36800 Sub-Panel Main CB (Trip Unit adjustment set at lowest, #2) ---> 800A lightly loaded I-Line sub-panel ---> 225A I-Line branch Ckt Brkr ---> a (SqD 150T3HF, 180A pri; 416A sec) 150Kva 480x208Y/120v dry type transformer ---> 400A 120/208Y distribution panel Main Breaker.

The transformer was just replaced with a used identical spare from on-site storage with unknown history but told was good. All cabling was checked for opens and shorts as okay. With the transformer's 1st downstream 400A breaker open, energizing the transformer's 225A feed breaker, the 800A sub-panel feed main CB instantly trips. The 225A CB does not trip in the closing process. The 800A main CB's Trip Unit Adjustment hasn't been changed and is set at it's lowest #2 position.

The transformer was changed out due to a failed coil suffered from a nearby lightning strike and had created the same energizing indications we are experiencing now when initially trying to restore power. Does anybody have any helpful experience with this situation they could share? From what I've gathered so far, there might an in-rush current condition with starting up a stale transformer. How is that dealt with if it keeps tripping such a large capacity breaker? Should the trip setting be temporarily adjusted to a longer delay? The instantaneous delay adjustment spans 2-10 times max trip setting. The long time trip is fixed.

Any help or suggestions will be truly appreciated. It's been a lump doing this swap-out so far... A hum is much better than a clunk and silence.
 
Sounds like the instantaneous trip is what is tripping. I would try increasing the IT setting. But there may be a reason it is set there such as keeping the arc flash incident energy down. Might want to look into that before fiddling with it.
 
Good point Personra. I was concerned about the same because the disconnect and transformer are next to each other in a corner. After normal business hours, I can try opening the 800A feeder at the remote bldg service, adjust the IT at the 800A sub-panel and try re-energizing at the service in a safe location... and if it hums, dial back the IT to the original setting. I'll let you know happens. Thanks for the idea!
 
I agree the problem is the instantaneous setting of the 800 A breaker. They come from the factory at minimum, but as Persona said it may be there for a reason.

Transformer inrush is not really load dependent. Load current does not really flow until the inrush settles and the hunk of metal begins to act like a transformer.
I have seen some situations where having the secondary closed reduces the inrush on the primary side. This would be like a normal utility power loss and return. Its worth a try.
 
Another good point, Jim, thank you. I hadn't thought about the utility outage scenario. I'll close the secondary side on my first try. Thanks again, I'm not typically involved at this level of transformers on a daily basis..
 
You didn't say exactly which 225 amp breaker you have, but I looked up a JDA series breaker and it's magnetic trip setting is 2250 and I don't believe it is adjustable.

The 800 amp breaker you mentioned magnetic trip range is 1600-8000, so it's lowest setting is likely lower than that of your 225 amp breaker.
 
Okay friends, here's my update, but 1st, You're right-on kwired. The 225A is a KA36225, now replaced with a JGA36225 and has the same higher trip range than the lower 800A range setting, good find there..
So after business hours, I opened the I-line panel feeder breaker at the bldg service, closed the low side 400A CB, closed the 225A transformer CB and stepped up the 800A IT setting from 2 to 6. Closing the feeder CB at the service still tripped the 800A, as did gradual increases up to the max which exceeding the 225A trip range. There may be problems with the breakers also as the 225 never tripped. Interestingly, the 800A CB at the service is the same CB but in I-line configuration with the IT set at 2. (Ref note; 2x500mcm cu/phase, apx 100' svc to sub-pnl) and never tripped.
I think the replacement transformer is also faulty and I'm ordering backup breakers with a new transformer to be on hand if we need them now, or later on.
Thank you everyone who sent good points and ideas, they are honestly appreciated and add to my knowledge base. Cheers and be safe! Jeff
 
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