transformer

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puckman

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ridgewood, n.j.
a customer called about a 75 kva trans stepped up from 208 primary to 480v says the trans is a delta connection.
he has a 3phase 100amp cb feeding the trans. with very little load on it . he uses the higher voltage for testing purposes and at times there is no load at all on trans. the question is would any one have an idea why the main cb would trip when turned on? it does not trip every time. the cb feeds a fused disconnect before trans . you can shut off the disconnect and then turn on the cb, it will hold until you turn on the disconnect then the cb will trip .
going to his site this afternoon. thanks for any input
 
Sounds like they have a standard 480 delta to 120/208 wye and it is backfed. If so, it sounds like the 208 neutral is grounded. If you lift the ground on the neutral, not the egc on the transformer just the neutral to ground connection, your problems will go away.

Note that not everyone agrees that this is ok, and you may also experience nuisance tripping because of higher inrush.

Jim T
 
jtester said:
If you lift the ground on the neutral, not the egc on the transformer just the neutral to ground connection, your problems will go away.
Click to expand...

I would doubt that. But should remove the N-G bond.

The inrush is the problem, normally a 225A breaker would be on the 208V side.
 
I've had the same problem before.

Transformer manufacturer's design a high input impedence on the primary side and a low impedence for the secondary side.
Wire it and use it backwards, you have an input impedence that is so low, the inrush current exceeds the breaker rating.

If I remember correctly there have been other threads on this subject.
 
The problem my post addressed is circulating currents in the 480 side caused by imbalance in the 120/208 side. By removing the ground, he will eliminate the path for the currents to circulate, and the breaker will stop tripping. This is a common mistake when a standard transformer is backfed.

Inrush is a separate condition, and removing the ground will not address it.

Jim T
 
The full load current of this transformer is 208 amps (at 208) inrush can be between 4-6 times (sometimes higher, epically when wired in ?reverse?) the 100 amp circuit breaker most likely has an instantaneous trip of 6 to 10 times the rating (600 to 1000 amps) at 6 times the inrush current would be around 1200 amps. And remember this is a molded case circuit breaker so this 6 ? 10 has a wide margin of error ( margin of error may not be the best terminology but the area in the curve were it may or may not trip).

A manufactures rep. told be that due to the way a ?standard? 480 delta to 208/120 wye transformer is wound that the inrush is higher when wired in ?reverse?. As noted above.
 
Inrush current will for sure start tipping the breaker. Are there adjustments on the breaker? If so the timedelay on the intantaniuos trip should get turned up.
 
On a standard molded case circuit breaker the adjustments is generally Min-Max or 6-10, sometimes 4-10 or in some cases 4-12, and have seen 5-14, this adjustment is for current based upon the rating of the circuit breaker not time delay. There are some circuit breakers commonly referred to as static trip or electronic that has additional adjustments.

Utilize a high speed recorder capture the inrush current on several operations of the CB.

Properly size the circuit breaker and conductors (if necessary) for this transformer

Why is the customer operating the CB on a regular basis?
 
I Just Lost Everything That Took About 25 Minutes To Type Up. Where It Went I Don't Know. So I Will Get To The Point.

After Opening Up The Load Center To Check The 100 Amp Cb Feeding The Trans I Found That Some Bright Person Figured Out 2 Breakers Are Better Then 1. Yes, He Spliced What Looks Like A 2/0 To A #3 And Feed 2 100amp Cb's And Feed The Trans With The 2/0 .
All I Can Say It Check Check Check.
 
Parallel overcurrent protection, a no no by the NEC, though parallel fuses above 600 amps were once used at 208 VAC (never saw them on a 480 VAC system)
 
puckman said:
I Just Lost Everything That Took About 25 Minutes To Type Up. Where It Went I Don't Know. So I Will Get To The Point.
Click to expand...

Boy do I know that frustration!!!

After a few times of loosing a post I learned to copy it (or start it) into Word or Works, that way I can finish it there and paste it back into a post.

Roger
 
roger said:
Boy do I know that frustration!!!

After a few times of loosing a post I learned to copy it (or start it) into Word or Works, that way I can finish it there and paste it back into a post.

Roger
Click to expand...

I've had to do that as well....very frustrating to lose it all.
 
A manufactures rep. told be that due to the way a ?standard? 480 delta to 208/120 wye transformer is wound that the inrush is higher when wired in ?reverse?. As noted above.
Click to expand...

As would be expected, and similar to a "standard" 208V to 480V transformer.
Wouldn't inrush be expected to be higher with the 208V coils?

Feeding the 208V side of a 75kvA transformer (reverse-fed) with a 100A cb would be like feeding the 480V (normal-fed) with a 60A cb. Wouldn't you expect trouble?
 
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