3PH transformer blows breaker on power up

[winnie]

Does the operating state if the 75kVA unit affect the startup tripping of the 51kVA unit?

Jon

 

[FOG1]

Does the operating state if the 75kVA unit affect the startup tripping of the 51kVA unit? Jon

 

[FOG1]

No it’s been off when this has been occurring.

 

[winnie]

As I requested above, I think that it makes sense for you to post a summary of the complete situation if you need further answers. However my take:

You have a 240V 200A service. That is 83 kVA.

You are successfully starting a 75 kVA step up transformer but not a 51 kVA transformer with similar configuration.

You have tried 3 different 51 kVA transformers. One of those 3 was identified as defective. One had high no-load current. One sometimes starts by frequently (usually??) trips the 200A service breaker on inrush.

The proper installation has been confirmed.

You don't have scope traces of the inrush waveform, nor of the no-load current waveform.

Given the existing data, IMHO there is no strong reason to think that anything is 'broken' with the latest transformer nor with its installation.

Code would permit the 51 kVA transformer to be installed with a 300A primary breaker, specifically because of inrush current issues. The fact that the 75 kVA transformer does start is weak evidence of a problem, because many factors influence inrush.

At this point I believe you have several options:
'Live with it'. Turn the transformer on (possibly at different times of day) and once you get it on, leave it on.
'Continue investigating'. For this I believe you need to get scope traces.
'get a larger service'. 51 kVA + 75 kVA would seem to be a problem on an 83 kVA service, but I have no idea what your actual loads are.
'inrush current limiting'. Get some hardware upstream of the transformer specifically to limit the inrush current.

-Jon

 

[FOG1]

240 volt 400amp service split at the meterback into two 200 amp breaker disconnect feeds, one 200amp feeds the 75kva and one other machine, the other feeds the 51kva and the rest of the shop.

75 kva turns on and starts daily never an issue it has delay fused disconnect

51 kva trips 100 amp breaker when turning on with unfused disconnect.

51kva leave disconnect on and reset breaker, breaker holds no tripping

51kva bypass 100 amp breaker, turn on by the unfused disconnect and it will trip the 200 amp main outside at meter.

One of 3 was assumed defective by DMG Mori the machine builder, never confirmed back to me.

FED Pacific sent another 51 KVA in installed turned off and on 10-12 times no trip, came in Monday morning and it trips every time when turning on by disconnect.

DMG MORI found in one of their showrooms a 2005 build trans 51kva, same Fed /Pacific brand sent it to us. We installed same thing turned it off and on no less than a dozen times on Friday, came in Saturday and it now trips.

Called Electrician that DMG Mori hired to inspect the install and try to troubleshoot, told him what had transpired. He asked me to check no load amp draw it was higher, 1st 51 kva was as high as 9amps, 2nd 51kva was lower at 3-4 amps, 3rd 51 kva (old build) was as high as 18 amps on the highest leg, this is all without and secondary load.

Removed #3 51kva and reinstalled #2 51kva, still trips while turning on with unfused disconnect switch.

I have a small handheld cheap scope, not well versed enough to use it like you would need, sorry.

I am re-attaching pictures of the transformer labels as ask.

 

[Barbqranch]

Are you sure the breaker is operating as rated? Can you switch the 200 amp breakers between the two transformers, one breaker could be weak and open on low current, or the other one might not open at the same point as the first.

 

[FOG1]

Are you sure the breaker is operating as rated? Can you switch the 200 amp breakers between the two transformers, one breaker could be weak and open on low current, or the other one might not open at the same point as the first.

the 200 amp never blows when switching the transformer on with the breaker in three phase panel or with unfused disconnect, it would only blow when bypassing the breaker in the 3ph panel. Turning trans on with unfused disconnect.

 

[ptonsparky]

Are the currents taken at the same time or clamp one, move it and clamp two, then three?
or
Three different meters?
Did the electrician have a three phase PQ Meter?

 

[winnie]

I think that Barbqranch is asking is if the 200A breaker currently feeding the 51 kVA unit has become 'weak' because of frequent tripping on the (presumed defective) earlier transformer. This is possible, but requires the electrician to test.

Thanks for sending the summary. I'd not realized that there are essentially 2 separate 200A systems.

-Jon

 

[FOG1]

Are the currents taken at the same time or clamp one, move it and clamp two, then three?
or
Three different meters?
Did the electrician have a three phase PQ Meter?

One clamp on meter moving between phases.

He did not but DMG MORI brought theirs in.

 

[FOG1]

I think that Barbqranch is asking is if the 200A breaker currently feeding the 51 kVA unit has become 'weak' because of frequent tripping on the (presumed defective) earlier transformer. This is possible, but requires the electrician to test.

Thanks for sending the summary. I'd not realized that there are essentially 2 separate 200A systems.

-Jon

The 200 amp breaker was only tripped 2 times when we did breaker panel bypass test.

Also I have reached out to DMG to try and get the Power Quality Meter Files from December 9th that they took.

 

[gadfly56]

The 200 amp breaker was only tripped 2 times when we did breaker panel bypass test.

Also I have reached out to DMG to try and get the Power Quality Meter Files from December 9th that they took.

All the engineers here really, really what to find the root cause of the problem. That's the way we are hard-wired. As a practical matter, is there a huge problem just leaving the transformer energized all the time?

 

[synchro]

Have you looked closely at that disconnect for proper operation?

the 200 amp never blows when switching the transformer on with the breaker in three phase panel or with unfused disconnect, it would only blow when bypassing the breaker in the 3ph panel. Turning trans on with unfused disconnect.

Just a guess, but maybe there is a slight delay before one of the poles in the unfused disconnect makes contact relative to the other two. And that might induce an initial asymmetry in how the core of the transformer gets magnetized, leading to a higher level of core saturation and a larger inrush current.
Like I said this is speculation, but I'm just trying to connect all of your observations with a possible root cause.
And so replacing that unfused disconnect would be something to try, but there's no guarantee that it will help.

 

[FOG1]

All the engineers here really, really what to find the root cause of the problem. That's the way we are hard-wired. As a practical matter, is there a huge problem just leaving the transformer energized all the time?

I got the files from DMG of course you will need the Fluke Power Log 430-II software to view it, see link for zip file. https://www.dropbox.com/s/d4j8cqebcmp80jc/Power Readings.zip?dl=0

 

[winnie]

I don't see any smoking guns in the power logs, but I am not experienced with that power monitoring hardware, so if there is something subtle I'd miss it.

Depending upon the test I see upwards of 500A inrush current, which is reasonable/low for a transformer with a full load current of 125A. The PQ software is averaging over 250 mS, but reports maximum, so I _think_ the numbers represent the peak current during a 250 mS period.

-Jon

 

[gar]

220328-2007 EDT

FOG1:

I can not access your link. But based on what winnie posted I suspect that you do not have good information on peak current in the first half cycle.

You can get a good reading on peak current for this application with a suitable current probe, and a Fluke 87 in peak mode. The 87 in peak mode is faster than 1 millisec, a 60 Hz half cycle is 8,3 milliseconds.

When using a shunt for fast measurements it is important to minimize a 1 turn coil in the connection of wires to the scope from the shunt. One time I made a special shunt to minimize this problem. This was related to measuring thousands of amperes to a 15 A circuit breaker.

In this motor experiment we need to see the first half cycle current, and you will not necessarily get it with a single random measurement. The largest peak value will occur for an unloaded transformer when the applied sine wave starts on a voltage zero crossing, with the correct slope for the residual flux state, and the transformer left in its highest residual flux level from its last turn off.

You and most electricians on this forum lack the capability to make the measurement. Further most electricians here have no more knowledge of this problem, or how to measure it than you do.

.

 

[winnie]

No magnets, nothing in my shop is DC, put the new 2021 build transformer back in that 2005 drawed stupid amps with no secondary load.

Have you successfully run the supplied machine with the system?

Does the machine include a variable speed drive?

Does your clamp on current meter have DC measurement capability? (Only some current clamps will measure DC current.)

My current 'out on a limb' guess is that there is a bad VFD rectifier causing DC current flow in one of the transformer limbs. This causes a much larger than normal residual flux which triggers greater inrush.

Jon

 

[ptonsparky]

Have you successfully run the supplied machine with the system?

Does the machine include a variable speed drive?

Does your clamp on current meter have DC measurement capability? (Only some current clamps will measure DC current.)

My current 'out on a limb' guess is that there is a bad VFD rectifier causing DC current flow in one of the transformer limbs. This causes a much larger than normal residual flux which triggers greater inrush.

Jon

How could a bad rectifier be influencing a transformer with no connected load at startup? Did I misunderstand that there is NO load on the transformer secondary or just that no machine start button has been pushed?

 

[winnie]

Transformer inrush current is affected by the timing of applying power and by residual flux in the core. The residual flux is set by the previous operation of the transformer.

My out on a limb guess is that the previous operation of the machine is somehow setting up the transformer for a bad startup the next time around.

The only way I could see this happening is if the machine were somehow causing a DC imbalance, and even this is a pretty weak theory.

But you do have a history of installing multiple transformers, having them appear to work, and then having them unable to start because of excessive inrush.

Jon

 

[ptonsparky]

Transformer inrush current is affected by the timing of applying power and by residual flux in the core. The residual flux is set by the previous operation of the transformer.

My out on a limb guess is that the previous operation of the machine is somehow setting up the transformer for a bad startup the next time around.

The only way I could see this happening is if the machine were somehow causing a DC imbalance, and even this is a pretty weak theory.

But you do have a history of installing multiple transformers, having them appear to work, and then having them unable to start because of excessive inrush.

Jon

How long does it take for the residual flux to disappear?