Breaker tripping on retransfer

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Ken9876

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I have a bit of a problem with a 100 amp 480 volt Westinghouse breaker tripping when transfering back from generator power. L1 49A, L2 56A, L3 48A, voltage is good. One thing I belive that might be causing this problem is awhile back they added more heat to this circuit. There are three transformers feed off this breaker, serving rec. lighting and some heating loads, no motor loads other than what's in the heaters. I was there last night and had to dump two of the breakers feeding transformers, to get the ATS to transfer back without tripping. The transfer switch is an old Allen Bradley ATS. Took some inrush reading today with a Fluke 434 the inrush goes upto 400 amps, but it does the same thing when transfering to generator and that breaker holds. Any thoughts?
 
Ken9876 said:
I have a bit of a problem with a 100 amp 480 volt Westinghouse breaker tripping when transfering back from generator power. L1 49A, L2 56A, L3 48A, voltage is good. One thing I belive that might be causing this problem is awhile back they added more heat to this circuit. There are three transformers feed off this breaker, serving rec. lighting and some heating loads, no motor loads other than what's in the heaters. I was there last night and had to dump two of the breakers feeding transformers, to get the ATS to transfer back without tripping. The transfer switch is an old Allen Bradley ATS. Took some inrush reading today with a Fluke 434 the inrush goes upto 400 amps, but it does the same thing when transfering to generator and that breaker holds. Any thoughts?
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Settings on the breakers? Or are they fixed? Do a primary injection test on the breakers and compare trip times to TCC.
 
Is the re-transfer immediate or is there a time delay?

Is the transfer swich a break before make switch?

It could be possible that if your transfer is two fast, and your voltage are not synchronized that there is some residual voltage on the system when transfering and the utility voltage is out of phase with this residual voltage thus causing a current surge?
 
Retransfer happens about 20 min after normal power is back

"Is the transfer swich a break before make switch?"
Yes, they are interlocked so both can't be closed at the same time.

"It could be possible that if your transfer is two fast, and your voltage are not synchronized that there is some residual voltage on the system when transfering and the utility voltage is out of phase with this residual voltage thus causing a current surge? "

It could, this is an old setup and doesn't have any controls to synchronize the power. I don't know because it hasn't tripped like this in the past, from what I'm told
 
I once re-closed a 800amp brk into a generator backfeeding a MCC....guess what?

It tripped......:D BAM...Then I realized what I had done, then proceeded to head for the restroom...
 
Someone I worked with that has retired, manualy switched an ATS with the power still on once, he had trouble seeing after for a few minutes.
 
We have had this happen at many sites.Clue:utility fault current is much higher then diesel fault current,that is partially why it happens transferring back to utility only

Exide/Powerware has published a good article(several years ago) describing this in great detail-good luck:smile:
 
Three things

Put a meter across "A" utility to "A" generator and "C" utility and "C" generator do both meters reach peak or zero at the same time, if not you may have rotation right but have phasing messed up.

Use an amp clamp with min max and put one meter across "A" utility to "A" generator does the ATS transfer at the zero cross over (or close to that). If this is an ASCO or similar ATS (fast transfer no center off) you may need an in-phase-monitor. This option may be on this ATS just not activated.

Check if you can get a larger CB (NEC legal) or a CB with adjustable instantaneous
 
Thanks for all the good info.,

Brian this is an old Allen Bradley transfer switch, there are really no controls. The dashpot timer times out after power comes back on and it transfers back to normal power.

"Use an amp clamp with min max and put one meter across "A" utility to "A" generator does the ATS transfer at the zero cross over (or close to that). If this is an ASCO or similar ATS (fast transfer no center off) you may need an in-phase-monitor. This option may be on this ATS just not activated."



Are you saying to put on on A utility and another on A generator or both in the same clamp?
 
brian john said:
Three things

Put a meter across "A" utility to "A" generator and "C" utility and "C" generator do both meters reach peak or zero at the same time, if not you may have rotation right but have phasing messed up.

Use an amp clamp with min max and put one meter across "A" utility to "A" generator does the ATS transfer at the zero cross over (or close to that). If this is an ASCO or similar ATS (fast transfer no center off) you may need an in-phase-monitor. This option may be on this ATS just not activated.

Check if you can get a larger CB (NEC legal) or a CB with adjustable instantaneous
Click to expand...

This method works really well with Fluke 87 meters, watch the analog bars at the bottom of the display, they should scroll in the same direction if you are in phase.
I beleive magnetic trip adjustments are not standard until you reach the 200 amp frame size or larger, It's definetly the transformers causing the problem. Since this apparently just started to do this, has a transformer been added recently? Or has the POCO upgraded their transformer or lines increasing the available fault current? If it is not life safety, you could contactor one or more of the transformers with a time delay, thus reducing the initial inrush.
 
breaking tripping on retransfer

breaking tripping on retransfer

what i would do since all transformer have inrush current and that might be the problem delay each transformer so when at a power loss they do not start at the same time or just shed some load
 
Ken9876 said:
I have a bit of a problem with a 100 amp 480 volt Westinghouse breaker tripping when transfering back from generator power. L1 49A, L2 56A, L3 48A, voltage is good. One thing I belive that might be causing this problem is awhile back they added more heat to this circuit. There are three transformers feed off this breaker, serving rec. lighting and some heating loads, no motor loads other than what's in the heaters. I was there last night and had to dump two of the breakers feeding transformers, to get the ATS to transfer back without tripping. The transfer switch is an old Allen Bradley ATS. Took some inrush reading today with a Fluke 434 the inrush goes upto 400 amps, but it does the same thing when transfering to generator and that breaker holds. Any thoughts?
Click to expand...
Ken -
Inrush, as measured by 434 is the same either direction of transfer.

CB did not used to trip, now it does.

Load has changed by adding electric heating, load is now about 50% of cb rating.

Are you confident your measurement is showing the true peaks, maybe down to the 8ms range?

I'd recommend keeping focused on what has changed or what could have changed.

The only possible timing issue I can see, is the mechanism is worn/out-of-adjustment to where the cbs don't transfer correctly going back to utility. Other than that, I don't see how timing or synchronization coulod matter. Check the adjustments.

Maybe shut off the added electric heat and try the transfer a couple of times. I'll bet on, "still trips" same as with the heat on.

Next look hard at the CB. Customer does not want to pay for testing. Okay - how about swapping the gen and utility cbs? See if the problem follows the CB. If not, then I'd be back looking at my measurement technique to see what I missed.

cf
 
When the utility drops out everything goes dark for many seconds until the generator comes up to speed. This means the transfer from utility to generator is occurring at zero volts and the residual magnetism of the transformers has had a chance to dissipate.

The transfer from the generator to the utility is always from one live voltage source to a different live source. Unless the transfer switch has a dedicated 'off' position there probably will be problems if the two sources are not in sync. The issue is not inrush magnitude only, it is also the inrush phase angle.

Sometimes transformers have more inrush problems when they are lightly loaded and other times when they are heavily loaded, so it is possible that the additional heaters is the cause. Also, is the generator able to maintain the correct speed now that it has additional load?
 
jim dungar said:
. Unless the transfer switch has a dedicated 'off' position there probably will be problems if the two sources are not in sync. The issue is not inrush magnitude only, it is also the inrush phase angle.
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Used to work fine - Doesn't work now.

I have never been a fan of redesign if the system used to work. I have always thought one should fix what is broke first.

cf
 
jim dungar said:
Sometimes transformers have more inrush problems when they are lightly loaded and other times when they are heavily loaded,
Click to expand...
I haven't seen this. I checked my Blackburn - nothing there. Is this anecdotal experience, test data, mfg information, or maybe a paper? I okay with any - just curious as to the source.

cf
 
I used to have the same problem with a Liebert 10kw UPS that had an input transformer to isolate it from the utility. 2 out of 3 transfers from generator to utility would result in a triped input breaker.

It turned out to be that the transfer switch would just blindly transfer from generator to utility as long as voltages where correct, it did not care if they were synced.

Added the inphase transfer option to the transfer switch and the problem was cured, now the transfer switch sits with a display "waiting for source sync" and when the phase angles match it transfers.

The explanation was the transformer field does not colapse during a fast transfer and it was like we were puting 2 misphased lines together.
 
The generator is big enough, it's an 800 amp cat. This is for a draw bridge, the 100 amp breaker is for the house loads, it then feeds out to two 50 amp 2pole breakers which feed, two transformers, the third load is feed thru two 100 fuses which then feed another transformer. A few things have changed which I belive is the problem, one they had a contractor change the two 15 kva transformers, not sure of the new rating, second they added heaters to these transformers where there was no heat before.



"When the utility drops out everything goes dark for many seconds until the generator comes up to speed. This means the transfer from utility to generator is occurring at zero volts and the residual magnetism of the transformers has had a chance to dissipate. "

I'd agree but, when I test the generator it's transfering with the utility power still on.

If I turn off the two breaker feeding the two smaller transformers it will transfer just fine.

Again thank you for all the info.
 
Ken9876 said:
I'd agree but, when I test the generator it's transfering with the utility power still on.

If I turn off the two breaker feeding the two smaller transformers it will transfer just fine.
Click to expand...

Does it trip when you test from utility to 'live' generator with the transformers on-line?
 
Typically the generator cannot supply high in rush currents equivalent to the bus from the utility.

Use an meter with an analog scale (as noted) I carry a Fluke 87 and a Simpson 260, then use an amp clamp for current measurement.
 
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