Joined: 25 July 2008
Can anyone help me with a problem I have in calculating the fault level on the secondary side of a 3 winding transformer where both secondaries feed a VFD switchboard (both secondary windings are of equal rating - 2500kVA)
Each secondary feeds the VFD switchboard via an incoming ACB on to a rectifier and from there on to a common DC bus bar, so they are not directly interconnected. AC induction motors are fed via Inverters that take their supply from the DC bus. It is assumed that there will be no motor contribution to a fault occuring on either of the AC incomer ACBs.
The IEC annotates the transformers as follows:
Primary winding - side "A"
Secondary winding - side "B"
Secondary winding - side "C"
My query is, for a fault on the incomer from side "B" can I regard the transformer as a 2 winding unit as the incomer from side "C" is not in parallel and there is no motor contribution or (as I suspect) is there contribution from the side "C" directly to side "B"?
Thanks in advance
Joined: 07 October 2004
To confirm, Is each transformer secondary supplying a separate switchboard which in turn are supply VSD driven motors? I'll assume they are.
If they were simple induction motors supplied from the downstream switchboards then you would get some contribution from the induction motors located on the unfaulted switchboard via the transformer (i.e. C to B winding) to the faulted switchboard.
However, if the motors are supplied from a VSD (rectifier/inverters etc.) then you would get very little to no contribution from the motors for a fault upstream of the VSD, i.e. for a fault on your transformer secondary switchboards.
In short I don't think you can treat the three winding transformer as a two winding transformer, however if all your motors are supplied via VSD's then the only contribution to a secondary transformer fault would be from the primary winding.
Hope that helps.