VCB RATING?

[mnnc]

Hi guys, I would like to refresh myself on how to compute for the vcb rating, if you have am 2MVA transformer rated 34.5kv(pri)/ 380V/220V(sec). Is this formula correct? I= kva(1000)/ 1.732(34500). I came up with the 33.47amps. I have been given a munufacturer's spec with a vcb rating of 630A. Can you explain this to me how'd they came up with this figure. also how do we compute for the busbar rating from VCB to 2mva transformer? I would like o hear your insights on this. Thank you very much.

 

[unclebob]

I believe the VCP will be located on the primary side of the xfo. You need a short circuit study. The best way is to ask
your local utility for fault currents values (3 phases and ground faults)at your location.

The current you have calculated is good but it is the nominal primary current at 100% load.

If the breaker is to be used on the secondary side, an approximate 3 phases fault current can be found:

1- calculate the nominal secondary current (Inom)
2- use the impedence of the xfo in PU (IZ)

I3pf = Inom / IZ

Use a software for more precise values.

Hope it helps a bit.

 

[mnnc]

unclebob,

what software do you recommend in obtaining the value of the VCB rating.

the VCB rating will be located at the primary size of the transformer.

 

[PQ]

Try "EDR" Im not so sure of the web page, but you can search in at google. The other way is to get the faulth current from your utility or the size ot transformer the supply your 34.5kV distribution from there use unclebob formula...

 

[unclebob]

mnnc,

We use SKM Power Tools but other softwares are available.
One thing, these are pretty expensive.

 

[mc5w]

For this high of a ratio transformer the primary system should be considered to have infinite short circuit capacity and the secondary short circuit current is limited only by traansformer impedance.

The 630 amp rating could be the smallest circuit breaker available for 34.5 KV.

 

[davidbeach]

For this high of a ratio transformer the primary system should be considered to have infinite short circuit capacity and the secondary short circuit current is limited only by traansformer impedance.

What is your basis for that statement? Certainly there is nothing in circuit analysis that would support such a generalization. What may work to produce a quick, approximate, result for small transformers may seriously overstate the available fault current on the secondary of larger transformers.

 

[mc5w]

The primary and secondary short circuit currents for a 5% impedance transformer connected to an infinite bus would be:

669 amps at 43.5 KV, 60,606 amps at 220Y380 volts

This amount of current is indistinguishable from a primary system overload. If lets say that the primary voltage droop from zero to 600 amps is 2% then at 1,269 amps we would have about 4.23% droop and a secondary short circuit current maximum of 58,030 at 220Y380 volts. Considering that a 3,000 amp circuit breaker typically has a minimum AIC of 100,000 amps it does not matter what the calculation is. For an ITE Siemens 1,200 amp circuit breaker 65,000 AIC is the same price as 100,000 AIC.

If the transformer impedance is 2% on an infinite bus then the short circuit currents are:

1,673 at 43.5 KV and 151,515 at 22Y380 volts

For the same stiff actual primary system we would have 7.58% voltage droop and a secondary short circuit current of about 140,000 at 220Y380 volts. Again, not likely to mean a hill of beans difference.