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Participant

Join Date: Jul 2014
Posts: 3

Upstream And Downstream Short Circuit Current

07/28/2014 5:33 AM

Dear Sirs,

I have just got a doubt and seeking your valuable advices. I have a 11kV ring in 3 substations.In two substations the short circuit rating is 25kA for 3 sec and the other substation is 25kA for 1 sec. Is this a correct methodology. Substations are 1-2 kilo meters apart. Connecting with 2 runs of 1Cx500sq.mm Cu/XLPE/AWA/PVC per phase.

One substayion is not yet constructed and Switchgear fabrication is ongoing.

Please advice.

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Commentator

Join Date: Mar 2014
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#1

Re: Upstream And Downstream Short Circuit Current

07/28/2014 7:33 AM

Who designed this?

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Guru

Join Date: Oct 2012
Location: 100 miles North from the World Center
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#2

Re: Upstream And Downstream Short Circuit Current

07/28/2014 9:40 AM

Do you mean you have 3 Ring [Main Units] of 11[12] kV 25 kA/1-3 sec.? Usually the rated breaker current could be 630 A [See Schneider Electric Medium Voltage Distribution Catalogue RMU 24 kV].

If the 3 RMU are connected in series and the ends are connected to the same source, then the maximum load current would be 630 A.

According to Nexans Cables Catalogue [for BS 6622 6.35/11 kV cable Cu/XLPE/AWA/PVC], maximum current for single-way duct [underground] could be 570 A.

[120 RHO, 15oC Earth temperature, 0.8 m depth].

See: Nexans 6-36kV Medium Voltage Underground Power Cables XLPE insulated cables.

The short-circuit current [one cable per phase] may withstand is 40 kA/3 sec. [from rated 85 oC up to 250 oC]

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#4
In reply to #2

Re: Upstream And Downstream Short Circuit Current

07/28/2014 12:15 PM

Thanks for your responce.

What i mean is i have three switchgears.One will be considered as upstream substations having two feeders going to two downstream substations. Then this two downstream substations are connected together.This will be a ring circuit. However downstream interconnection will be open in both places and will be keeping as a redundancy source.

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Guru
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#3

Re: Upstream And Downstream Short Circuit Current

07/28/2014 10:06 AM

Is this run as a ring with one point of supply or an open ring with two points of supply?

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#5
In reply to #3

Re: Upstream And Downstream Short Circuit Current

07/28/2014 12:18 PM

Open ring with two points of supply.

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Guru

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#6
In reply to #5

Re: Upstream And Downstream Short Circuit Current

07/28/2014 2:12 PM

In Electrical Engineering power distribution it is always better to draw sketch when you request for any clarification, so that we can understand better to guide you better.

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Ramesh,Freelance Electrical/automation Consultant
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#7

Re: Upstream And Downstream Short Circuit Current

08/12/2014 11:37 AM

There is mistake in your question. Short circuit rating is same for 3 substation but the withstanding time is different. Nobody design time based protection nowadays. It shall be energy based protection . Calculate the fault level for 3 substations and the Equipment/switchgear manufacturer to confirm the withstand time. There is nothing wrong to consider 3 seconds. But it will increase the cost of switchgear.Normally, short circuits shall be protected within milli seconds. No need to withstand for 3 seconds.
Better to re-check and discuss it with consultant engineer for your project.

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