VCB RATING?
VCB RATING?
(OP)
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.






RE: VCB RATING?
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.
RE: VCB RATING?
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.
RE: VCB RATING?
RE: VCB RATING?
We use SKM Power Tools but other softwares are available.
One thing, these are pretty expensive.
RE: VCB RATING?
The 630 amp rating could be the smallest circuit breaker available for 34.5 KV.
RE: VCB RATING?
RE: VCB RATING?
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.