transformer and a tripping cb...

[zog]

How about that is what we are doing?....we haven't touched the system yet...

From this post I was under the impression someone already cranked up the settings

no settings on this breaker...and yes, turning up the trip settings on the 400, 200 and 125 amp breakers did stop the problem...

now, any helpful ideas on why the breaker is only tripping on a transfer from generator to utility?

No, not until we have some details on the system and test results on the breaker. There have been several good theories already, but until we know more anything is just a guess.

 

[emahler]

sorry zog, i thought you were implying that we adjusted the trip levels...

sometimes relaying a scenerio on here isn't easy or simple...I do appreciate the ideas...

some more background - we were called in by the IT guy at the company (who's somehow responsible for this setup since it controls the UPS systems)...

his first words to me were: "we narrowed it down to a faulty 75kVA transformer. We want you to replace it"...

so, I guess my goal here is to find out what the problem is, since I don't believe it's a faulty transformer...I'm going to the site in a week or so to run tests...if I can pinpoint it to the generator transfer switch - it goes back to the manufacturer...

if it is the transformer - it gets changed...existing transformer is a GE...

if it's a wiring issue - loose connection, bad bonding, etc...we'll figure it out on site...

so the ideas put forth here are great places to verify and test...in the hopes that we can determine the real issue...

on tues of this week, I'm meeting with the EE at the new site, and I'll be sure to pick his brain about the 90A breakers there and the set up at the existing site...

but, this is what I truly love about this industry...i'm in it for 15 yrs, my family for 40+, and heck, we still learn new things every single day...

thanks all

 

[zog]

Megger the transformer including DAR/PI. Winding resistance, and TTR. Will tell you if it is the transformer that is the issue.

 

[emahler]

Megger the transformer including DAR/PI. Winding resistance, and TTR. Will tell you if it is the transformer that is the issue.

that is on the agenda...as well as verifying all connections, grounding, etc...

but, i would make a horrible CSI agent, because I'd bet it is in the transfer...whether it's a simple adjustment necessary or more, not sure...

like I told the customer...I would not want to change a transformer, only to have the same problem....

 

[hillbilly1]

Instantaneous trip levels can be a trick to coordinate for a transformer inrush, an engineer should be consulted with to determine the ITL's of all these breakers...............................................

In one case we had a 750 kva 480 D X 208/120 Y that was tripping a 900 amp on the 480 every time we had a power failure and power was restored, the transformer manufacture sent out an engineer who had us change the breaker ITL setting to .9 from the .4 it was set at and trip time to 1 second from the .6 it was set for, on a GE series 2k MDP but where those numbers came from I have no idea, well I have a dangerous inkling

I agree if the breaker has specific settings such as those, but the breakers the OP has written about have magnetic trip adjustment only, with no absolute setting. This would be very hard for an engineer to recommend a setting. The minimum setting would just be an err on the safe side, while the maximum would be used for high inrush loads such as motors and transformers, just like using NON fuses versues TD fuses, its just you can adjust it.

 

[boboelectric]

In rush current.

 

[emahler]

UPDATE:

finally got into the issue today, here is what we found...

Generator service company was onsite and scoped the system, synchronization is correct.

Megged the transformer - no problems there.

megged the feeds - no problems there

found CB feeding transformer to be 'burnt' on A phase - 100A Siemens NGK

customer had new CB on site, we replaced it after testing...

Additionally:
1) no ground from primary panel to transformer, they used the EMT
2) no bond between XO and ground
3) ground from secondary panel was a #6 connected to a bonding bushing in the secondary panel.
4) building steel was brought to the ground bar in the secondary panel - not connected to transformer.
5) originally, there was a bond in the secondary panel from the ground to the neutral bar, this was removed by a previous electrician
6) loose neutral lug in secondary panel
7) loose lug on phase A in transfer switch on generator feed and on feed to generator panel (primary panel)
8) utility voltage of 499V L-L and 288V L-N
9) generator voltage of 480V L-L and 278V L-N

after verifying the transformer was ok, tightening the loose connections and grounding the transformer, we put it too a test....

load was approx 13A primary and 40-50A secondary (fluctuation I believe was due to 2 small UPS systems fed from the secondary panel)

when utility power was sent to transformer, inrush current was approximately 25A

when switching from utility to generator, under load (we did not shut down building) inrush current varied from 30-45A

when switching from generator back to utility, under load, inrush current varied from 75A to 150A (not sure why the variation)

the transfer occurred without a problem, no more tripping 100A CB...so the IT guy from the company decided to test it again...

again, no problem...

then a 3rd time....and the breaker tripped going from generator to utility...inrush was approx 100A when it tripped.

he tried a 4th time, same problem...

now, the 4 attempts were all within about 15 mins...could the time have an impact on the test? i.e. thermal buildup?

we switched everything back to utility, let the system run for 30 mins...switched to generator - no problem...back to utility - no problem...

so, where it stands is that we need to investigate more...

any thoughts?

 

[SG-1]

I agree with your thermal build up theory.

The utility inrush is higher is because the utility voltage is higher.

 

[jim dungar]

Is this a relatively new transformer conforming to the latest TP-1 energy efficiency requirements? If so it may have a very high inrush current and may need to be fed by a 125A breaker.

Is there a period of power loss when transferring from utility to generator? This would give time for the magnetic field of the transformer to reduce.

Is there an immediate transfer from generator to utility?
Unless there is 'paralleling gear' involved it is unlikely that the generator is absolutely synchronized with the utility. An above normal inrush may occasionally occur if the residual magnetic field is out sync with the utility.

 

[mull982]

My 2 cents:

1) As others have mentioned the inrush magnitude is a function of the voltage magnitude during energization. If the Utility voltage is higher than the generators as you pointed out, then the inrush current will be higher when energizing from the utility. Also as mentioned the system impedance can play a role.

2) It seems like sometimes you are sucessfull energizing with energizing the transformer and other times you are not. (Although the inrush currents you reported seem low ranging from 25A-100A, may not have a fast enough meter) The magnitude of trasformer inrush depends on where on the voltage waveform you energize the transformer with the phase closest to the voltage zero crossing being the worst case. So it is possible that during the sucessfull energizations you are closing each phase away from the zero crossings and closer to the voltage peaks, while for the unsucessfull energizations you are closing on of the phases near the zero crossing.

3) The inrush magnitude is also a function of how much residual flux remains in the core during energization with the amount of residual flux remaining being a function of what point in the cycle the transformer was de-energized. It is possible that if the transformer is de-energized at a point where a good amount of residual flux remains in the core then during the energization of the unit the inrush current will be higher to overcome this residual flux.

4) As others have mentioned depending on the switching type between generator and utility you may have to add a time delay. It sounds like your transition is an open transition transfer in which case you would need to allow time for the magnetic fields of all the motors and other machines on the system to decay to avoid closing in the transformer out of phase. A time delay of 5 sec should be adequate but if you want to be safe then 10s should do the trick.

5) To avoid inrush issues tripping breakers I usually ensure that the priamary breaker is at least 125% of transformer primary full load. In this case a 125% of the 90A full load is 112A. The 100A breaker may be too small and sitting right on the edge. I would make sure I had a minimum size 125A breaker on the primary to avoid these possible nusiance trips.

 

[RoberteFuhr]

have a situation where a 125kw generator was installed in a facility...it feeds a 400A panel with a Siemens Sentron 400A main...480/277V

in the panel is a 200A sentron breaker feeding a 480/277V sub panel
a 125A sentron breaker feeding a UPS system

and a 100A 3P Siemens NGB feeding a 75kVA 480V Delta to 208/120V wye transformer.

The problem apparently occurs when switching from generator power to utility power, not vice versa...

switches to generator without a hitch...when it switches back to utility, it trips the 100A NGB breaker...

any thoughts?

As stated previously, this is very common. You may need to replace the controller in the ATS so that the ATS will transfer back to utility power when both the utility and generator are nearly in synch (same voltage and frequency and phasor position.) This option is available from the ATS manufacturer.