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Design Short Circuit Level Change

08/05/2014 11:18 PM

I have a.transformer of 25,000 kVA feeding 34.5/13.8kV bus at 40 kA. The out going 8 no of 13.8 kV,40 kA,SF-6, ABB circuit breakers feed to 4 x 1700 kW motors. I am now required to design an off-load 13.8 kV isolator in series with each motor feeder, say--20 meters away from 13.8 kV bus section.

I am being adviced to select 25 kA 13.8 kV circuit breaker since it is readily available at plant. Please suggest approach to decide on the Short ckt rating of the isolator which I am proposing or whether it is a must that i should introduce the similar 40 kA isolator in series for all Motors?

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Guru
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#1

Re: Design Short circuit level change

08/05/2014 11:26 PM

MV isolators are a common bit of kit.

Which manufacturer have you contacted?

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Guru

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#2

Re: Design Short circuit level change

08/05/2014 11:38 PM

Assuming 10 % percentage impedance of 25 MVA transformer, short circuit current on the LV side will be less than 15kA. Therefore you can go in for 25 kA isolator on the motor circuit.

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#3

Re: Design Short Circuit Level Change

08/07/2014 12:52 AM

I suppose you have two transformer incomers to the 13.8kV switchboard. If the transformers are operated in parallel (or if there is no fail proof to prevent paralleling the transformers at 13.8kV bus), you need to consider the fault contribution of both the transformers combined as well as the contribution of motors for any equipment in the outgoing circuits.

20meters distance is not going to reduce the fault levels in case MV systems (it is so in case of LV).

It is always safer to go for 40kA rated isolators. There are cases where the transformer incomers are upgraded (as the load grows) but the same switchboard is used as the fault levels and incomer breaker ratings can allow the same.

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#4
In reply to #3

Re: Design Short Circuit Level Change

08/07/2014 1:14 AM

Please do not confuse. The post says " a transformer....". Where is the question of parallel operation ? Further distance matters on each path for short current flow. More the distance , more cable impedance and less short circuit current.

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#8
In reply to #3

Re: Design Short Circuit Level Change

08/08/2014 3:06 AM

20m is not going to reduce the fault level - I think you find it does.

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Guru

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#5

Re: Design Short Circuit Level Change

08/07/2014 1:29 AM

The advise you have requested requires Power engineers design knowledge and ability to analyse. One has to prepare SLD and calculate the flow of short circuit currents on each path. For this you should furnish all the relevant parameters of each device like transformer , motor , cable etc. Therefore it will be advisable for you to consult a professional consulting power engineer, instead of requesting from forum like this.

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#6

Re: Design Short Circuit Level Change

08/07/2014 5:43 AM

If you subject your 25kA off line isolator to a 40kA fault you will have an incident.

The fact you are 20m away from the switchboard and likely further away from your transformer your fault level will have dropped. Only a proper study will tell you whether it has dropped below 25kA.

You make no mention of how long the fault might be on for - i.e. how long the protection is set to clear.

Suggest either fit 40kA units or carry out a study.

To install 25kA units without verfication is mad.

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#7

Re: Design Short Circuit Level Change

08/08/2014 12:16 AM

I agree with raghun. I don't think one may use a switch breaker instead an isolator since the isolator job is to separate the energized part from the working part of a circuit, so this has to be visible and to assure good distance between disconnected parts in order to avoid -in an overvoltage case- a dangerous potential reaches the working not energized part.

Never-the-less the isolator disconnects the circuit only at no-load, in a short-circuit case it has to withstand thermal and dynamic effect of the short-circuit current, so it has to be suitable to the maximum short-circuit current [rms, peak and duration].

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#9

Re: Design Short Circuit Level Change

08/08/2014 7:03 AM

I don't think the short-circuit [three-phases, solid short, neglecting the high voltage and taking into consideration the contribution of 8 induction motors of 1700 kW ] it will be more than 17 kA.

If the motor cable shall withstand 25 kA 1 sec then 185 sqr.mm copper will be suitable.

In this case the short-circuit current at motor terminal will decrease with only 0.65% [less than calculation error].

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#10

Re: Design Short Circuit Level Change

08/11/2014 3:31 AM

For just clarification...What is the Primary Voltage of Transformer?...what is meant by 34.5/13.8 kV bus?

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