Breaker sometimes trips on transformer inrush current 7

[Andy32821]

I have a technical question. I'm hoping you can help me out.

At the local convention center they have seventy-five 300 kva GE K Factor transformers for temporary show power.

Each transformer is protected by a GE 500 amp SGLA36AT0600 current limiting circuit breaker.

These transformers have no loads for days at a time and the convention center would like to turn the transformers off during downtime.

(They are unloaded during switching and installed in an air conditioned tunnel).

This could save about $500 a day on the power bill.

I have reports of the breakers sometimes tripping when the transformers are re-energized. (I assume this is inrush current at a bad time in the cycle.)
I can find no problems with the transformers.

My Question:

Will re-energizing the tripped breaker two or three times in a row damage the breaker or damage the transformer?

Is it ok to use this breaker to turn this unloaded transformer on/off a few times a week?

Thanks,

Andy

 

[slavag]

Hi Andy.
For my pinion, no problem (for both Q's).
Problem only, why CB triped?
Regards.
Slava

 

[dpc]

Tripping under high current conditions isn't doing the breakers any good. It would be much better to adjust the instantaneous trips or otherwise fix the root problem which is that the breakers are too small and/or the settings are too low. Repeated tripping under high current will definitely shorten the expected life of the breakers.

 

[Eleceng01]

If my memory is still good, I read recently in the IEEE Blue Book that MCCBs should be replaced after two trips under full tripping current. Granted this may be difficult to determine, but it does support dpc's comment. Thanks again dpc!

 

[cranky108]

With the small size of the transformers, there probally hasen't been much study of inrush completed. But it very well could be inrush. Question: Has there been tripping at other times? Like maybe other inrush events?

You also did not state the type of protection, but I would expect time and inst. over current. It is possible that they could not be set properly for inrush.

Your other question about closing and tripping breakers several times, there is a wear and tear factor anytime you open and close breakers, no matter what type of breakers. So to operate them more will require more maintenance or replacments as time goes.

My thoughts are you should see what is behind the difficulties to close issue, and the $500 a day seems significent, so investigate the cost of maintenance, and make a compairson. In any event, operating the breakers like that will require more maintenance, and eventual replacments, so the maintenance group needs to be up to the task.

 

[slavag]

Thanks Folks.
I learned something about MCCB and IEEE requerements.
Regards.
Slava

 

[dpc]

It's almost certainly transformer inrush. On a 500 A breaker, it should be possible to increase the instantaneous pickup enough to stop this from happening.

 

[itsmoked]

Andy32821; You understand what's happening correct?

The transformers can get disconnected while on, say, the peak negative of the cycle. This leaves the core magnetized in that state. Then later someone closes the breaker while the negative part of the cycle is in effect. Now the flux continues to build and the transformer promptly saturates. Current skyrockets 12X is not uncommon. Breaker trips.

If you can monitor or control when you turn OFF the breakers so the transformer shuts down unmagnetized this wouldn't be an issue.

Anyone heard of a device for this?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[dpc]

Keith,

It's done with large high voltage breakers in some situations, but it requires single-pole switching. I've never seen it done for an application like this.

For this voltage and breaker type, the practical solution is to size the breaker's instantaneous trip to ride through the maximum inrush. These types of problems are errors in design or in establishing proper breaker settings.

 

[itsmoked]

Thanks for the enlightenment dpc.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[davidbeach]

Actually, you can't shut the transformer off at zero voltage, even using single pole tripping because you can only interrupt on a current zero and without out unity power factor you never have voltage zeros and current zeros at the same time. What you can do with single pole switching, generally at 500kV and sometimes at 230kV, is you can close back in where you want on the voltage wave, pole-by-pole, and minimize the inrush current. It is an interesting concept and totally non-applicable for the application of the OP.

The transformer voltages aren't stated, but assuming 480V, I'd have sized the breaker at 800A, with an instantaneous setting of 8000A, that 500A is rather small. I'd expect peak inrush currents between 4300A and 5400A and that 500A breaker won't let 5400A through without tripping.

 

[prc]

Magnetisation inrush current is maximum when the switching is done at zero point of the sinusoidal voltage wave, with residual flux remaining in core from previous switching off .This is because the flux is lagging voltage by 90 degrees.In EHV breakers this inrush can be reduced by switching the breaker exactly when the voltage is at the peak.This is called controlled switching of breakers.This power electronic device is prohibitively costly and limited for use with EHV breakers.Theoretically it can be used with any breaker.
Transformer in rush current always contain 10-60 % second harmonic component depending on the angle of voltage at which switching is done.In differential relays, harmonic restraint feature is provided( ie relay will not respond when there is second harmonics in the over current )to avoid maltripping during energisation.I dont think overcurrent relays can incorporate that feature.For such small breakers the only solution seems to go for time delay with higher instantaneous ratings as David mentioned.

 

[jraef]

That breaker has adjustable (digital) instantaneous over current (IOC) trip settings. The NEC allows you to set them for upwards of 13X the load FLC if you demonstrate that the breaker is nuisance tripping on inrush of inductive loads. I'd say you have demonstrated that. Crank up the IOC setting and quit exercising those breakers; they aren't that fat.

 

[slavag]

Hi PRC.
I heard about this device ( I think made by Sweden).
Could you please send any link.
Regards.
Slava

 

[prc]

ABB is supplying such device.Please go to abb.com and type controlled switching under search -you will get all information.

 

[slavag]

Thanks PRC.
I found.
Regards.
Slava

 

[Andy32821]

Hi Guys,

This breaker is a GE sprctra RMS electronic breaker but does not have the digital readout. Only an inst pot that is at max.

My main concern is damaging the breaker with resets. Since this is an electronic breaker I am hoping that resets are not a problem.

I assume that cycling the breaker 50 time a year to turn the transformer off when not in use will have little impact on the breaker's life. But I get myself in trouble assuming things.

As far as the tripping goes, I can change the 500 amp plug to a 600 amp plug if this will help. The transformers have primary and secondary protection. It's probably not cost effective to add any other hardware.

I will do more studies on whats causing the trips but I need to come up with a procedure on what happens when a breaker does trip. (A rare event).

The whole idea is to save money and at $500 a day a $3000 breaker is not a big deal. (However a $20,000 transformer might get the maintenance manager irked).



I am thinking of this procedure for the switching technician:

If the breaker trips when turning the transformer on then open the secondary breaker and try the primary breaker again.

If the primary breaker trips again wait 5 minutes and try once more.

If the breaker trips again leave it off and call an electrician.

Is this asking too much of the breaker or is this in the normal design duty of this style breaker?

Thanks for any help you can offer.

Andy

 

[waross]

Switching the breaker on should not be a problem. The issue is that the breaker is probably interrupting 5000 amps or more when it trips on instantaneous. That is hard on the breaker.
Try the 600 amp trips on one or two breakers. Knowing that there are inrush problems, I would try two or three resets before calling the electrician, however you are on site and it is ultimately your call as to how many times to retry.
The inrush is a fact of life and cannot be reasonably avoided. The breaker tripping is detrimental to the breaker and should not be happening. Step one is checking the settings. Step two is changing the rating plug. Step three is changeing the breaker. All transformers have inrush current. Transformer breakers should withstand the inrush without tripping.
Switching with no load on the transformer is probably a good idea.
respectfully

 

[dpc]

The problem is the wear and depletion of the contacts - the trip unit type is not an issue and does not make a difference to the longevity of the breaker.

If the 500 A trips are at max instantaneous, you will need to look at larger breakers - since it is a 600 A frame, they can probably be changed out to 600 A, but only if the feeder ampacity is adequate.

Frequent tripping of a 500 A breaker on xfmr inrush would not be an acceptable situation for me.

 

[jraef]

Changing the rating plug will give you an additional 1000A of instantaneous trip range, so that may do it. The problem with changing the rating plug however may be the conductor size, but you appear to be someone who would have thought of that.

I hadn't looked closely at the specs on that breaker before, but now I see that the 500A rating plug only allows adjustment up to 5000A; 10X the rating. A 300kVA transformer is probably around 361A FLC at 480V (you never said) so if the inrush is 13X, then the 5000A setting should have sufficed. But those "K-Factor" transformers may have different magnetic characteristics that end up pushing the worst-case inrush over the edge of that 5000A limit.