Transformer question.

11733 Views 63 Replies 8 Participants Last post by brian john, Mar 27, 2007

rod213

Discussion Starter · Mar 20, 2007

Got a transformer fed with 480V three phase. Output is two legs of 115V, 630A with two neutrals and a ground.

Now here's the thing, I've always been taught to bond the cabinet with the ground. So, that's what I did on this one. Well our 200A three phase breaker kept tripping when we were engaging the fused disconnect at the transformer. So we reset it a couple times then we decided to take the equipment ground loose. BINGO! No kicking.

Now this has me wondering what in the world is going on here. I ohmed from the lug we put on the cabinet to the bar in the top of the xfmr for the neutral and it 'beeped', so they're connected somewhere. Same with the ground lug on the transformer itself to the bar on the top of the xfmr.

So is the neutral it's making bonding the cabinet also? The cabinet wasn't bonded in the beginning we added it. Also, the transformer wasn't under load.

Thanks ya'll,

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MDShunk

· #2 · Mar 20, 2007

It's hard to feed a large transformer with a breaker sometimes. Just the current that it takes to saturate the transformer can sometimes trip the breaker. I prefer fused disconnects feeding large transformers.

That said, I'm not sure I have a good handle on your problem. Is this a regular transformer, or an autotransformer? The two neutrals thing is screwing with my head.

rod213

Discussion Starter · #3 · Mar 20, 2007

Well the way I look at it, it looks like two small regular transformers side by side and the cabinets are 'mounted' together. Feed is comming if from left, A & B hooks up in the right one, goes through a short conduit to the other transformer where C lands. I'll take pictures of it tomorrow and post so you all get a better idea of what I'm talkin' 'bout.

Long story short there are two different hots and two neutrals comming from the load side of the transformer. I can ohm from the ground to the xfmr made neutral and it will 'beep'. When the cabinet was bonded it would trip my breaker, when we took the cabinet bond loose it quit tripping.

Question is, why does the equipment ground make this thing trip?

For reference we're comming from a 1200A I-line 200A breaker, to the control panel's 200A shunt trip, out of the shunt trip to a 200A fused disconnect to the xfmr then to each side of the furnace.

Sorry if I'm not able to make it more clear at this time.

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Joe Momma

· #4 · Mar 20, 2007

I'd definately like to see a picture.

Why is the transformer being fed with a shunt trip?
Why are you feeding 3 phase to a single phase secondary xfmr?
And why aren't the '2' neutrals tied together to make one neutral?

brian john

· #5 · Mar 21, 2007

1. Did you megger the XMFR?
2. All of Joe's questions.
3. Size of XMFR
4. K rated?
5. CB Type thermal magntic or static trip, adjustable instantaneous?
6. The load should have no impact on turning on the primary CB.
7. The GEC and EGC should have no impact on the XMFR primary CB tripping.
8. It is VERY possible that the CB is tripping on instaneous (MOST LIKELY) if you are at the limit of the Instantaneous trip it may trip sometimes and not others.
9. Did you utilize a True RMS min/max amp clamp to capture the in rush current?

that should get you started....Oh tough to do, but did you test the CB?

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faber307

· #6 · Mar 21, 2007

MISWIRE
I am not familiar with this set-up, but this is my gut feeling

rod213

Discussion Starter · #7 · Mar 21, 2007

Okay lots of info for me to gather tommorrow it seems!

Here's the breakdown on what I know.

The shunt trip will kick out if there is a temperature overload on the furnace, that way it will shut off everything leaving the control cabinet.
You can see the shunt trip 200A on the right bottom of this picture.

I'm feeding three phase for a secondary power because that's what was originally hooked to it at it's old location and not a cop-out, but that's what I was told to do. Perhaps to save power in my opinion due to the 630A it gets stepped up to comming off the load side of the xfmr.

why aren't the '2' neutrals tied together to make one neutral?

Joe I'd say this is the case, though I'm not 100% sure, I was just paying attention to the load side of the xfmr which has two 'hots' and two neutral lugs. One of each feeding each side of the furnace's elements.

In this picture you can see the transformer in the lower left corner with two 2 1/2" conduits leaving. One conduit feeds left side and the other feeds right side of furnace. Each "hot run" if ran through a 630A fuse.

This picture shows where the hot and neutrals come into the furnace, keep in mind each side got 3 3/0's parrallelled for the 'hots' and 3 3/0's parrellelled for the neutral.

Answer to most of brian johns questions are no or I don't know at the moment. Some electrician I am, huh. Lol.

I'll get better pictures of the whole deal tommorrow guys. Thanks for the answers.

Also, while I'm thinking of it. There was a brand new automated (is that the right term, the one for the multivoltage imputs.) transformer installed in the other side of this shop about 2 weeks ago and they were having a similar problem. They took the ground loose from the cabinet and all the weird voltage problems they were having cleared up. That was a brand new transformer. Not sure of the specs on that one either.

This is the delta or 'corner grounded' 480V power system also, if that matters.

Thanks again

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3

John

· #8 · Mar 21, 2007

Tab Faber said:
MISWIRE
I am not familiar with this set-up, but this is my gut feeling

My 6th sence kicked in on the miswire theroy too.

Rodney can you post a closeup picture of the transformer and side conection box on the transformer? this might be where the problem is.

brian john

· #9 · Mar 21, 2007

A multi voltage transformer is commonly referred to as a multi tap transformer.

The corner grounded is the service feeding the transformer?

If so that may be the source of the problem, IF this is an AUTOTRANSFORMER not automated. Auto transformers have a primary winding common with the secondary, grounding the secondary would be the could be the same as a BOLTED FAULT.

Man one word can change toe world, if that is the case!

rod213

Discussion Starter · #10 · Mar 21, 2007

Lotsa pictures!

Yes this is fed by a corner grounded delta 3 phase system. 480 on A, 0V on B and 480V on C.

I took a picture of the coverplate for the transformer, it's hard to read though in the pic. So I'll rewrite the values for the fields below the pictures.

KVA = 70
Primar V = 415
Sekundar V = 110
Primar A = 174
Sekundar A = 637

Typ(in upper left corner) = PTU 63
Fabr-Nr 880526
Frequenz = 60 HZ
Betrieb = LB
Schaltgr = I10
Isol.Kl. = T40E
Kuhlart = AN
Schutzart = IP 23

Here's an overall side shot of the transformer. It is fed from the left, the load comes out of the right side.

Below is a close up of the left side of the transformer.

Below is a close up of the right side of the transformer.

More images in next post.

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5

rod213

Discussion Starter · #11 · Mar 21, 2007

This shot is a bit closer up on the entire side.

This is an upclose shot of the load side (110V w/neutrals). The fuses have been removed so we could check the secondary voltage without it going to the furnace.

This is the same as above, just a bit further back.

And there you have it!

The ground comes in the left, lands in a terminal on the leftside, had two comming out of that one to a lug to the cabinet and one continuing to the ground lug on the right side of the transformer. Had two comming out of the right side ground lug to another cabinet lug we put on for the right side and to the ground bar on the load side.

After we took the ground off of the cabinet lug the breaker set. It would 'ohm beep' from that bar you see on the tops of the transformers to the cabinet. It would also 'ohm beep' from ground to neutral on the load side.

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3

faber307

· #12 · Mar 21, 2007

1_800 (help) tech support.
I have never come across anything like it
Grounded B phase on primary
Grounded neutral on secondary. somthing is not happy :001_huh:
I can't even guess that moving your input phases around would make a difference.

Can't wait for the smart one to step up and explain this one!
Good post Rod.

rod213

Discussion Starter · #13 · Mar 22, 2007

Tab Faber said:
1_800 (help) tech support.
I have never come across anything like it
Grounded B phase on primary
Grounded neutral on secondary. somthing is not happy :001_huh:
I can't even guess that moving your input phases around would make a difference.

Can't wait for the smart one to step up and explain this one!
Good post Rod.

That's what everyone at the jobsite is saying too.

I'd say it has to do with the corner grounded (delta) system. The cabinet is 'grounded' by the made neutral in the transformer. Still unclear on the whole reasoning behind the earth ground shorting out the transformer neutral.

Joe Momma

· #14 · Mar 22, 2007

Now I see, without reading it all.

You put these in on an ungrounded system, no wonder the grounds are giving you problems. Your XO has the same potential as one of your primary phases.

I thought we told you to address that corner tapped delta system.
And please please please, for heavens sake don't just leave the ground open becasue that fixed the problem.
I'm not sure but you will probably overheat/burn-up the transformer once it's under load.

Go to MikeHolt.com and find the forum and ask about ungrounded systems, and transformer with no GEC/EGC

Joe Momma

· #15 · Mar 22, 2007

That's some nice looking work on probably the strangest transformer I've seen.

But you have to understand, when you bonded this thing to 'ground' or anything else to 'ground' there is no ground in this building (ground meaning a zero potential referance).

The B phase of this system is what your considering 'ground' which is actually charged with 480 volts(as compared to A or C phase).

It's all a matter of referance when establishing a ground and in this case you got a problem.

rod213

Discussion Starter · #16 · Mar 22, 2007

Joe Momma said:
Now I see, without reading it all.

You put these in on an ungrounded system, no wonder the grounds are giving you problems. Your XO has the same potential as one of your primary phases.

I thought we told you to address that corner tapped delta system.
And please please please, for heavens sake don't just leave the ground open becasue that fixed the problem.
I'm not sure but you will probably overheat/burn-up the transformer once it's under load.

Go to MikeHolt.com and find the forum and ask about ungrounded systems, and transformer with no GEC/EGC

Well, that corner tapped delta system is what the customer wants. The ultimate decision on this is up to my boss. I've come here just trying to get an explanation of the situation.

Why would the made neutral that is evidently bonded to the cabinet cause this to trip my main. I can't wrap my head around it yet. Keep in mind there are no neutrals comming in with the main service from the pole.

I'll check out Mike holt's site too.

rod213

Discussion Starter · #17 · Mar 22, 2007

Joe Momma said:
That's some nice looking work on probably the strangest transformer I've seen.

But you have to understand, when you bonded this thing to 'ground' or anything else to 'ground' there is no ground in this building (ground meaning a zero potential referance).

The B phase of this system is what your considering 'ground' which is actually charged with 480 volts(as compared to A or C phase).

It's all a matter of referance when establishing a ground and in this case you got a problem.

Thanks, we try to make it at least look good!

Now the equipment ground I used to bond the cabinet is actually going to 4 ground rods we drove right beside the 1200A service. It's an entirely seperate wire. I fed that transformer with 3 3/0's and 1 #3 for my equipment/earth driven ground. Not sure if that's what you were looking for on the above.

Joe Momma

· #18 · Mar 22, 2007

I've never read the code sections about ungrounded systems simply because I've never had to, but I feel your answer may be in there.

You should do a bit of schooling, at least to understand how a Delta works and how a Wye works. Here's a hint; Delta's don't have a neutral by nature, only if they're center-tapped(like what's fed to your house) to save on wire.

frenchelectrican

· #19 · Mar 22, 2007 (Edited)

KVA = 70
Primar V = 415
Sekundar V = 110
Primar A = 174
Sekundar A = 637

Typ(in upper left corner) = PTU 63
Fabr-Nr 880526
Frequenz = 60 HZ
Betrieb = LB
Schaltgr = I10
Isol.Kl. = T40E
Kuhlart = AN
Schutzart = IP 23

For this listing it kinda spook me a bit

and i dont know if you mention the spec are set for single phase ?? i was going thru the figures twice and are you running the transformer set up in open delta or close delta ?? it do make the diffrence there

I hope you relized that you are actally overdriven the transformer ??

i did read the primay voltage from name plate it say 415 v ,, ok that common in European area system .

secondary side it say 115 volt ok if that feed for furance heating elements

but this transformer is calberated at 60 HZ as i did read it now

and look at one of the photo at the section #11 i did see two fuse holder there and one look bent or overheated

i don't know if you did read the primary amparage with ampmeter ? it should stay near the max primary amparage

but one thing when you increase the voltage on the transformer the amparage will drop but a catch if you have electric heating element the current will go up with voltage as well

Merci , Marc

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brian john

· #20 · Mar 22, 2007 (Edited)

Joe a few comments on your statements:

You put these in on an ungrounded system, no wonder the grounds are giving you problems. Your XO has the same potential as one of your primary phases.

I thought we told you to address that corner tapped delta system.
And please please please, for heavens sake doesn’t just leave the ground open because that fixed the problem.

The system he installed this on is UNGROUNDED; I did not see that stated anywhere. This is installed on a corner grounded system.

Check the NEC

The NEC is not a design manual

But you have to understand, when you bonded this thing to 'ground' or anything else to 'ground' there is no ground in this building (ground meaning a zero potential reference).

There is no ground in the building WHY? This makes no sense

The B phase of this system is what your considering 'ground' which is actually charged with 480 volts (as compared to A or C phase).

And The Earth/Ground/Building Structure cares about this WHY?

ROD:

Are there any drawings for this transformer?
1st based upon the voltage ratio 3.6 to 1 this is not an autotransformer as autotransformers generally are not utilize to make voltage changes of this ratio.

This is a 70 KVA 415VAC to 115VAC, I do not see where you can apply 480 to achieve 120 if this is actually 415 VAC to 115 VAC or 3.6 to 1 Transformer at 480 VAC it would deliver 133 VAC.

Additionally there are 4-coils typically a 3-phase transformer would have 3-coils. Are you sure this is not a 480 VAC single phase transformer with parallel coils to achieve the 70 KVA at 115?

What I would do

Disconnect the primary and secondary leads.
Separate the coil leads.
Use a Transformer Turn ratio meter (TTR) and determine the ratio of the transformer, in lieu of not having a TTR use 120 VAC on the primary and measure the secondary. You could do this at 480 but 120 is safer. At 120 VAC the secondary reading should be 33.33 or if it is actual a 480 VAC primary at 120 VAC it should be 28.7 VAC (4.1 to 1).
Assuming all the coils have the same polarity I would try (at the lower voltage connecting the transformer as a parallel operation.
Check voltage at all coils.

And then back to my original question ANY DRAWINGS of this transformer!

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