15kV switchgear high interrupting ratings

[rockman7892]

I have a situation where the interrupting rating for the breakers on a 15kV metal clad switchgear lineup is aprox 64kA which exceeds the 63kA interrupting rating of the breakers.

The lineup is double ended lineup fed from (2) 70 MVA transformers with the breaker NC and a good bit of motor contribution fed from the switchgear.

I’m looking at a couple of different options to address this issue. One option is replacing the switchgear/breakers with adequately rated breakers however I’m not aware of any breakers with a higher than 63kA interrupting rating. A couple of questions I had

1) is there an option for a metal clad switchgear breaker that has a higher than 63kA interrupting rating or is 63kA the largest based on ANSI C37?

2) Would it make sense in this case to explore inserting a CLR in the tie breaker which would be perhaps a more economical solution for reducing SC duty?

3) Any other modeling techniques to perhaps eliminate any conservativeness in model impedance? Some things that come to mind are confirming cable lengths, looking at motor impedance model, and clarifying redundant motors. When two feeders from the gear are opened and motor contribution removed the fault duty drops below 63kA so to me that indicates looking at the motor modeling and feeders.

 

[RRaghunath]

I agree with your conclusion. The estimated fault current is slightly above the breaker rating and this means it is worth checking the estimation thoroughly.
Also, check tolerances on the impedances of incoming transformers and connected motors you considered. You may be aware that if the transformer tested %Z is available (name plate value), there is no need to consider any negative tolerance on the transformer %Z. Same way, for motors if you consider actual information from the data sheets or from the name plates, it could also have a good impact.
If you still want to go for reducing the fault level, 'Buscoupler open' operation could be the simpler option. For maintaining the reliability of power supplies to downstream motors, you may consider high speed bus transfer scheme to close the bus coupler in case of tripping of one of the transformer incomers.

 

[wbd]

Are any of the motors fed by a VFD? It may be a non regen type so you could eliminate motor contribution from these. If you can't eliminate any conservative inputs and you can't find higher rated equipment, you may be left a CLR or replacing transformers with a higher impedance value. None of which is inexpensive.

 

[rockman7892]

Thanks for the responses.

Is anyone aware of switchgear/breakers that have a higher interrupting than 63kA that would fit a MV distribution application where metalclad gear would be applied? Is there metalclad gear that has higher than 63kA interrupting?

 

[davidbeach]

Not a great option, but...

How are the breaker withstand (momentary, not interrupting) ratings compared to the available fault current? If the withstand is high enough, and you can withstand a bit of a delay in clearing the fault you could go with an instantaneous trip of the tie for faults on either side. The tie won't be overdutied because it never needs to interrupt the contribution of both transformers.

You could also use a partial differential and trip the tie and one of the mains. A bigger outage than just tripping the tie, but not as bad as blowing up the gear. Tripping both the tie and one main provides a degree of redundancy in that you've accomplished what you need to do if only one of them opens and you're not having to wait for breaker failure tripping.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[dpc]

If you're that close, you really should do a rigorous equipment duty evaluation per ANSI C37. Keep in mind that the issue is the fault current through the breakers, not necessarily the total bus fault current. Many times, the individual breakers never see the maximum possible bus fault current.

As mentioned, you also have to look at the 1/2 cycle ratings of the breakers, not just the interrupting ratings.

 

[Kiribanda]

It looks to me that 64kA is your calculated symmetrical sc duty at the location with closed tie.
1)Since ANSI & IEC stds relevant to bkrs are harmonized, there is no bkr listed at the next higher rating of 80kA (R10 series) unless it is a "CUSTOM BUILT" one.
2)Even if "custom built", the manufactuer has to derive & supply "custom built" TRV curves at 80kA for you to verify its TRV capability. Lot of $$$ for you.
3)If the sym duty is 64kA, you have to check the close & latch duty of the bkr and also the peak withstand capability of the swgr.
3)Therefore, you may have re-calculate your sc duty by fine tuning your input data/parameters.
4)If there is no other option, my suggestion is to use a set of CL fuse (similar to IS-LIMITER)at the TIE, so that during a fault
4.1 the peak current is reduced heavily by the CL fuses
4.2 the main-tie-main with closed tie, will be de-coupled by the CL fuses within 4ms approximately and run as a main-tie-main with open tie
until the fault is cleared and restore normal operation.

 

[davidbeach]

Could also put a current limiting reactor on the tie, leave it inserted when both transformers are in service and bypass it when one of the mains is open.

I’ll see your silver lining and raise you two black clouds. - Protection Operations