Transformer Tripping Breaker Upon Energization

[Pitt123]

Have recenly installed a transformer that is tripping the breaker upon energizing the transformer. Circuit Details are as follows:

Transformer is a 25kVA single phase transformer with a 480 primary to 240/120 secondary. Transformer primary OCPD is a 70A breaker located about 800ft away. Transformer primary feeders are a #2AWG sized to account for voltage drop.

Transformer secondary is feeding load center who's main breaker is open when energizing the transformer, so there is no secondary load. As soon as we energize the transformer the breaker trips instantly. We checked primary cables and transformer windings to ground with megger and they all seem to be clear.

We are thinking that either something is wrong with primary windings, or primary breaker is too small to handle the inrush. We believe primary windings are all correct but are going to re-check.

Do you think the 70A breaker is too small for inrush? The breaker is not-adjustable. When sizing primary breaker I usually take transformer primary current and then size cables for this value. (Except for voltage drop cases) Then for primary breaker I incerase 125% to account for voltage drop. Is this correct? Ignoring voltage drop, do you just use 100% of primary current for sizing feeders.

BTW, the breaker is a 3-pole breaker, on which we are only using to phases.

 

[Fordean]

Inrush

Inrush

Have recenly installed a transformer that is tripping the breaker upon energizing the transformer. Circuit Details are as follows:

Transformer is a 25kVA single phase transformer with a 480 primary to 240/120 secondary. Transformer primary OCPD is a 70A breaker located about 800ft away. Transformer primary feeders are a #2AWG sized to account for voltage drop.

Transformer secondary is feeding load center who's main breaker is open when energizing the transformer, so there is no secondary load. As soon as we energize the transformer the breaker trips instantly. We checked primary cables and transformer windings to ground with megger and they all seem to be clear.

We are thinking that either something is wrong with primary windings, or primary breaker is too small to handle the inrush. We believe primary windings are all correct but are going to re-check.

Do you think the 70A breaker is too small for inrush? The breaker is not-adjustable. When sizing primary breaker I usually take transformer primary current and then size cables for this value. (Except for voltage drop cases) Then for primary breaker I incerase 125% to account for voltage drop. Is this correct? Ignoring voltage drop, do you just use 100% of primary current for sizing feeders.

BTW, the breaker is a 3-pole breaker, on which we are only using to phases.

There isnt no load how can there be Inrush. Btw if you fusing 480 side at 70amp you way over fused anyway. You must be 240 steppin up 480 v Why 240?

 

[Pitt123]

There isnt no load how can there be Inrush. Btw if you fusing 480 side at 70amp you way over fused anyway. You must be 240 steppin up 480 v Why 240?

There will be inrush on the transformer regardless of secondary load in order to establish magnetic field in transformer.

Primary current on 25kVA at 480V is 52A. 125% of this is aprox 65A. Therefore 70A breaker should be adequate.

 

[Volta]

Do you think the 70A breaker is too small for inrush? The breaker is not-adjustable. When sizing primary breaker I usually take transformer primary current and then size cables for this value. (Except for voltage drop cases) Then for primary breaker I incerase 125% to account for voltage drop. Is this correct? Ignoring voltage drop, do you just use 100% of primary current for sizing feeders.

BTW, the breaker is a 3-pole breaker, on which we are only using to phases.

Aside from VD, the conductors will need to have an ampacity greater than 60 amps to serve a 52 amp load protected at 70. That said, you have an ampacity of 95 installed, so you can change the OCP to 100 with that wire, and still protect the transformer per Note 3 of T450.3(B).

But with the likely inrush problem, simply going to 100 amps may not be enough. Is it a simple mid-point tap, suggesting secondary as LV, as you said? Or are there two LV coils, which are able to be paralleled or placed in series, but are actually designed to be the primary of a step-up transformer? If reverse-fed it may explain a higher than expected inrush.

 

[gar]

100304-2013 EST

Pit123:

I calculate your steady state full load current at about 25,000/480 = 52 A RMS.

Peak worst case in rush current might be 30 to 50 times 52 for at most 1/2 cycle. This peak will not always be this large. It is a function of the residual flux density in the transformer core from the last turn off, and the point in the AC cycle when you turn on the transformer.

As an illustration of transformer inrush current see my photo P6 at http://beta-a2.com/EE-photos.html

The trip time curve and rating of your breaker will determine whether it is likely to trip when turning on your transformer.

You may need a different breaker, or series resistance to limit current, and a time delayed contact across the resistor to short the resistor after a fraction of a second.

.

 

[jim dungar]

I have seen many installations where an 'open-secondary' transformer will trip the primary breaker, but by closing onto a load the breaker will hold.

I am sure it has to do with the total impedance of the equivalent circuit, but I have never tried to analyze it, I just know it works sometimes.

 

[Sierrasparky]

Wait a minute folks you need to get the specs from the transformer company.
I had a similar issue with a 75KVA trans. When I turned it on the conductors rattled and everything was ok. I tuned if off and buttoned it up and when I turned it on it popped the breaker. Well I went outside to see what happened and nothing was wrong. I cycled the breaker and the whole plant shut down kicked a 1500 amp main off.
Ok now I was really scared. I knew everything was ok. I borrowed a Megger from the local utility company and everthing checked out ok. Called GE and they were going to send out a new one. I could not by the fact it was bad. I called people all over the place to get help. Finally the engineer from GE called back after his vacation and told me OH you have one of those! well w put a 225 KVA winding inside so we can get the special place a water resitent cover over it. We don't generally let anyone know this because they may order one and load it up to 225kva. He then faxed over a inrush table. It was pulling 1000 plus amps at the right portion of the cycle. What he told me to do is set the trip setting a bit higher on the breakers. It's been working fine for 10 years now.

Woosh It was pretty embarassing.

 

[Pitt123]

But with the likely inrush problem, simply going to 100 amps may not be enough. Is it a simple mid-point tap, suggesting secondary as LV, as you said? Or are there two LV coils, which are able to be paralleled or placed in series, but are actually designed to be the primary of a step-up transformer? If reverse-fed it may explain a higher than expected inrush.

It is a 2 winding transformer on both the primary and secondary. I belive it is connected correctly but will need to verify tomorrow. Why wouldn't you think 100A would help with an inrush issue?

I have seen many installations where an 'open-secondary' transformer will trip the primary breaker, but by closing onto a load the breaker will hold.

I am sure it has to do with the total impedance of the equivalent circuit, but I have never tried to analyze it, I just know it works sometimes.

This is interesting I may try this. Unfortunately there are no secondary loads ready to be energized. We are simply energizing the transformer at this point to test. I'd be interested in the theory with this though.

Is 125% primary breaker typical for the primary of transformer? Should a particular type of breaker be used to deal with inrush issues?

Does the distance of the breaker in regard to the primary of the transformer make a difference?

 

[ptonsparky]

Inrush current. 125% for.primary isn't written in stone.

 

[Volta]

It is a 2 winding transformer on both the primary and secondary. I belive it is connected correctly but will need to verify tomorrow. Why wouldn't you think 100A would help with an inrush issue?

It may help, but maybe not enough because:

Peak worst case in rush current might be 30 to 50 times 52 for at most 1/2 cycle. This peak will not always be this large. It is a function of the residual flux density in the transformer core from the last turn off, and the point in the AC cycle when you turn on the transformer.

Is 125% primary breaker typical for the primary of transformer?

Pretty much. I think your design, as described, should've been, and usually would be, fine.

Should a particular type of breaker be used to deal with inrush issues?

I don't know.

Does the distance of the breaker in regard to the primary of the transformer make a difference?

I don't think so. When you are there again, look at the winding taps, see if you can tell if the high voltage is wound around the low voltage, or vise versa.

 

[templdl]

Is the 25kva an encapsulated or ventilated?
Encapsulated impedances run very low in the 1-1.5 % range which translates to very high inrush where the impedance of ventilated maybe twice that where the inrush is less.
Always remember that the encapsulated transformers are more likely to trip a breaker when energized.
The transformer my cost more but a custom transformer manufacture can increase the impedance of the transformers by increasing the duct space between the coils the essentially moves the HV coils farther away form the core even though the HV coils are wound on the outside (you will note that the taps are on the face of the HV coils) and the LV on the inside.

 

[Cold Fusion]

...Transformer is a 25kVA single phase transformer with a 480 primary to 240/120 secondary. Transformer primary OCPD is a 70A breaker located about 800ft away. Transformer primary feeders are a #2AWG sized to account for voltage drop. ...

... As soon as we energize the transformer the breaker trips instantly. ...

You have lots of good advice, but there are a few things I don't understand. maybe I am just mis-interpreting.

...When sizing primary breaker I usually take transformer primary current and then size cables for this value.

...Then for primary breaker I incerase 125% to account for voltage drop. ...

I don't get these two at all. You size the primary CB so that it will stay closed on the inrush. You size the primary cable such that they are protected by the primary CB

...do you just use 100% of primary current for sizing feeders. ...

No. As mentioned above the primary cables are sized such that they are protected by the primary CB

The guiding NEC principles are T450.3B protects the transformer and 240.4 protects the conductors.

Here is the method I use:
Primary CB per T450.3.B:
Usually a CB 125% of the primary (next size up if between ratings) is enough to handle the inrush. But sometimes not. If not you can go to 250% (NTE) of the primary FLA as long as you have a Secondary main CB set at 125% (next size up if tween).

Primary conductors per 240.4:
The primary conductors must be protected at their ampacity. So they have to be big enough to be protected by the primary CB. Generally, if the CB rating is between cable sizes you can pick the lower cable size. It will still be above the load.

Secondary CB per T450.3.B
If primary CB is more than 125% (excluding next size up if applicable,) then the transformer secondary has to be protected at 125% FLA (next size up applies).

Secondary conductors are selected per 240.4. 240.4.F doesn't help - you don't have a two wire secondary. edit to add: or not you said the secondary was 2 wire and you said 240/120 - so I don't know which.

There are a few other issues in 240.21 that may apply. But they won't make you pick bigger wire.

The issue on VD of the primary you have already figured out.

Now, the 70A primary CB is 135% of the FLA. Assuming it is a standard Thermal-mag, it should hold. As you mentioned, there could be something wrong with the transformer. I'd probably get a DLRO and check both the primary and secondary before I changed the primary CB.

There also could be something wrong with the CB. As others have said, the best way the check the cb is to put in a new one.

But if they checked okay, I'd go right up to the limit - which for the #2 is 125A. That's max for the 250% FLA and max for the #2 ampacity - 115A

If that trips, it's time for some instrumentation - or an ouija board

cf

 

[Volta]

But if they checked okay, I'd go right up to the limit - which for the #2 is 125A. That's max for the 250% FLA and max for the #2 ampacity - 115A
cf

We came up with different max limits. Do you feel that the breakers are undoubtedly rated for 75 deg C terminations?

 

[Cold Fusion]

We came up with different max limits. Do you feel that the breakers are undoubtedly rated for 75 deg C terminations?

First, nothing says I'm not wrong. It's already happened a couple of times today.

But, yes, it would be an odd one that wasn't.

cf

 

[steve66]

If it trips every time, I doubt its just because the breaker is too small to handle the inrush.

I would try Jim's suggestion and put some load on the xformer, even if its only a couple of 500W halogen lights.

If that doesn't help, I think I would start thinking about a bad transformer, or something wrong in the wiring.

Steve

 

[ibew501ed]

If it trips every time, I doubt its just because the breaker is too small to handle the inrush.

I would try Jim's suggestion and put some load on the xformer, even if its only a couple of 500W halogen lights.

If that doesn't help, I think I would start thinking about a bad transformer, or something wrong in the wiring.

Steve

I agree, sometimes a transformer will trip a c/b at startup but if you try to reset it, then it will hold because the partial magnetism reduces the inrush
and its ok till the next time you de-energize the transformer and lose the residual magnetism ( that's my theory)

 

[Cow]

Is there a larger breaker in that panel you can remove the wires from and temporarily connect the transformer to? That'd tell you if it was an inrush problem.

 

[mayanees]

..... I'd go right up to the limit - which for the #2 is 125A. That's max for the 250% FLA and max for the #2 ampacity - 115A


cf

BINGO

That's the answer. NEC compliant, and tolerant of the inrush.

John M

 

[Sierrasparky]

What my post was meaning less?

 

[Cold Fusion]

What my post was meaning less?

Not to me. I thought it was a good anecdote to file away for future reference. I deal with few single phase transformers, so I would have been completely mystified by a winding three times larger than normal, and drawing roughly twice the expected inrush.

Here's why I appreciated your post:
Personally I would call that an "Act of Agression" on GE's part. 75kva transformers are a comodity item. I'd expect all mfgs to be about the same. For one particular model to have twice the inrush as it's competitors and not have that data published is flat mean (on GE's part).

If the inrush does not fit under the curve of a standard i^2t TM CB, the mfg should should say so. For this size stuff the norm is a molded case CB - generally you can't just turn one up, or push out the short time, or go to an i^4t curve to get outside the inrush current curve. So you have to change out the CB and now there is a good chance the primary conductors are too small.

If I discovered about now that the transformer inrush was way over norm and GE knew that, I'd be recommending that GE come get it and put it where it belongs. Think of all the time you have invested in testing to find out, "Yep that's normal", CB is too small, conductors are too small. *O&^*&^#$&^*(&

So, I thought your post was worthwhile.

cf