Main Breaker Tripping? or Destroyed?

[nightfox1925]

I am having a strange case here. Ona 480VAC system, the arrangement goes like this. There is an MCC with the following feeders: 2 motor loads rated for 3kW, and the rest are transformer feeders rated not greater than 45kVA. The total connected load is 433A and this MCC is connected to an ATS connected to normal and emergency supplies with main breakers rated 400A each. Assuming the ATS is at emergency supply, when the normal comes back, the ATS re-transfers in an open transition scheme. At this point, the 400A main breaker according to the report fails (not trip). In their report, the breaker is damaged and claimed that the 400A breaker is having a rating of 3200A and the in-rush is 11000A (quite strange). I told them to deactivate the instantaneous setting of the main breaker and check settings to allow for in-rush to dissipate. However, they claim that the breaker fails to trip and get damaged. For all I know, there is no such thing as a 3.2kAIC breakerat 480V at 400A. What do you think is the problem? I feel estranged by the way they made the report.

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[waross]

Transformer loads on a fast open transition. You may expect extremely high transient currents.
If the transformers are driven into saturation the current may easily damage the breaker.
Depending on the impedance the asymmetric fault current may be 10,000% or more of rated full load current. If the transformers are being saturated the inrush current may be higher.
I would seriously consider installing closed transition or a synchronized very fast transition.
respectfully

 

[jraef]

I agree, sounds like transformer inrush is the culprit. You could also stagger the restart of the transformers, but that involves adding contactors.

But wait, you have a 433A connected load on 400A breakers? Start with that problem my friend. You should have 600A breakers, or at the very least, 500A 100% rated breakers. Better yet, get some good electronic trip circuit breakers with adjustable instantaneous trip settings that will allow for the transformer inrush.

 

[nightfox1925]

Wouldn't be possible to adjust to disable the instantaneous setting and set a time delay on the main 400A main breaker for let say 0.2 or 0.25 seconds to allow for the total transformer in-rush to dissipate? The individual downstream circuit breakers have instantaneous settings and hence an instantaneous setting on this main circuit breaker is not required, and the long time or short time settings will be set to coordinate with downstream devices. Is this a good option?

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[davidbeach]

Didn't you see jraef's response? 433A on a 400A breaker is an inherent problem and must be corrected before worrying about anything else.

 

[nightfox1925]

The 433A was the total connected load. Applying demand factor and diversity would yield below 400A. These guys provided the connected load instead of the demand load. sorry for the misleading information. is the time setting adjustment approach the best solution?

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[jraef]

Still, a 400A breaker can really only be loaded to 320A (again assuming it is not 100% rated), are you sure you have that much of a demand factor?

Not all breakers have adjustable instantaneous trips, in fact most of the standard low cost versions you find in panelboards do not. You may need to change it out. That's why I said to use a good electronic trip breaker, it will accommodate that.

 

[nightfox1925]

Thanks, I will ensure to note that up. Another one, I normally assume the transformer in-rush to be 12x rated primary, is there a possibility that it may be 12x higher? What is the maximum in-rush possibly a 45kVA transformer may yield, any idea?

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[waross]

What is the percent impedance voltage of the transformer?
respectfully

 

[jraef]

Until the transformer establishes a field and adds inductance, the only thing stopping the line connection from being a dead short is the winding resistance. The 10 - 12X inrush for .10 sec. is just a rule of thumb and is called the "practical inrush" because it is what usually gives you problems with fuses, breakers etc. But the peak at the exact moment of energization can theoretically be a lot higher, i.e. 25 - 30X depending on what the phase angle of the supply was at that moment. But that would only last for around .01 sec. and usually doesn't affect the circuit protective device.

 

[jghrist]

Whoever calculated the 11 kA inrush must have assumed 25 times rating for transformer inrush. As indicated by jraef, this is possible for 0.01 sec, but only if the system impedance is low enough. The inrush cannot be higher than the available fault, and the breaker has to be rated for the available fault. I don't see how a breaker that is properly sized can be destroyed by inrush current.

What is the momentary rating of the breaker? According to the OP, the rating is 3.2 kA, but I'd agree that this is unusually low for a 480 volt (or any voltage) breaker. The OP says the breaker failed but did not trip. It didn't say if it failed while trying to trip. I don't know the relationship between momentary and interrupting ratings of low voltage breakers.

 

[davidbeach]

For a 400A breaker, 3200A sounds an awful lot like the magnetic instantaneous trip setting.

 

[Laplacian]

3200A would be the instantaneous pickup, not the interrupting rating (usually 25KAIC minimum).

You say there are 2 each 3KW motor loads, but the remaining ~355KVA load is small transformers (8 or more 45KVA or smaller)? Are they 3 phase, 1 phase, or a combination of both?

How exactly is the breaker failing? Are the contacts welding? If so, your problem is DC offset due to remnant flux in the transformers. See this paper for more detail:

http://pm.geindustrial.com/faq/Documents/T35/GER-3989A.pdf

The simple solution is to disable the instantaneous function. If this is not an option, you need to implement one of the mitigation steps mentioned in the paper (harmonic restraint, etc.).

Additional comment for statements made by other posters: The connected load may or may not be an issue depending on what loads are energized at any given time. There is not enough information in the post to make this determination. I would seriously doubt that all those transformers would be fully loaded. Many industrial applications have connected loads greater than 200% of incoming breaker ratings due to sparing or batch operations. It all depends.

 

[nightfox1925]

I got new developments. We will be scheduling a visual check on the breaker early next week and we will get nameplate details on the deffective 400A main normal supply breaker. We will also get more details of the 45kVA rated transformers. I got a feed back though that these transformers are explosion proof types and may have been scott Tee connected cheap ones. Is there any way I can compute the in-rush by calculation as an estimate?

I have read an old journal that it is recommended to provide synchro check relay or a phase monitor on the ATS switch to ensure that the emergency generator is in phase with the normal supply before initiating an open transition transfer to relieve the in-rush currents...I don't kinda understand this part...maybe some folks can clear some picture here.

Lastly, is there a electronic trip molded case circuit breakers available with a defeated instantaneous setting to allow a time delay. I am always under the impression that molded case circuit breakers (adjustable or not) will have a fixed built in instantaneous element that actuates at high faults to protect themselves.

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[nightfox1925]

Breaker information is as follows; Cutler Hammer
Series C KDC400amp, 3 Pole, 600Vac/250Vdc, Cat KDC3400F
Digitrip Trip 310, Cat 4KES400T
In=Ir=400amps
STpk=8 x Ir
Stdelay=300ms

If the in-rush is supposed to last for only a few cycles and the breaker is already set at 300ms, what is causing the breaker trip? is there a possibility that the in-rush may last for more than 300ms? Although the breaker is set at 300ms time delay...is the breaker having an undefeated instantaneous element? requesting for your opinions.


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[waross]

A transformer inrush is normally associated with transformer energization.
It is not always so.
Any abrupt change in applied voltage and or phase angle may trigger a transient current spike.
There are several ways that an ATS may transfer.

Slow open transition; This has enough delay between opening and closing that the residual voltages may decay. The inrush can be expected to be the same as the normal energization inrush.

Closed transition: The supplies are synchronized and then paralleled, similar to bringing a generator online with other generators. There is no inrush if done properly.

Fast open transition; The transition is completed in less time than it takes the residual voltages to decay.
the instantaneous currents may be much worse than normal as the applied voltage and the decaying voltage may be 180 deg. out of phase. The voltage may be 200% and if the transformers saturate the current will be much higher than expected.

Fast open transition with phase check or synchronization: This is a compromise between closed and open transition.
The fast transition is delayed until the voltages are in phase. As the voltages are in phase there is very little "bump" or inrush. The faster the mechanical action of the transfer switch the better.
Fast closed transition; Once you go to closed transition the duration of transfer is of low importance.
Open versus closed transition:
A closed transition raises issues with the utility, and with protective relay settings. These are avoided with the fast synchronized transition and the inrush currents are reduced to much less than normal energization inrush currents.

 

[nightfox1925]

Thanks for all the inputs folks!! It is appreciated.

waross, I am just curious though, if there is a large current spike due to out of phase switching in addition to transformer in-rush...this should normally last for a few cycles right and should not trip a time-delayed circuit breaker set at 300ms (0.3 seconds). This defeats the notion of coordination. Does this transient last 300ms and above which causes the time delayed circuit breaker to trip?

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[nightfox1925]

Waross, this is my understanding with your comment. During a fast open transition, the residual voltage decay is slower and there may be a transient overvoltage of 200% which would saturate the transformers once ATS re-transfers to the normal supply. The saturation would yield a very high current of let say 10,000% or more of rated load current....Does this mean that this transient condition may have exceeded the breaker momentary and interrupting ratings that's why it gets damaged? How long will it last then? Is it enough time that it would cross the tripping zone of the 300ms time delayed circuit breaker?

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[davidbeach]

All molded case and insulated case breakers have a magnetic instantaneous element, even those which claim not to, called an instantaneous override. It may be that the instantaneous override is set at the short time withstand rating of the breaker - common in insulated case breakers. It may also be considerably lower, not shown on the TCC and buried somewhere is the fine print of the notes. Many of the newer generation of low voltage power circuit breakers that are based on an insulated case design but given ANSI ratings also have these instantaneous overrides but they can not be set below the short term withstand rating of the breaker. It is only with the old iron frame air break low voltage power circuit breakers that you could be truly free of an instantaneous trip element.