Transformers in parallel 1

[seanmcc]

I need to work out how a transformer arrangement is going to share load. The situation is that there is two 25MVA 66/33KV transformer suppling load to a 33KV bus. The load on the 33KV bus is minimal and does not need back up plan. The transformers also have 7.6KV tertiary windings that supply 7.6KV bus, which is well loaded and needs a backup plan, in the event of a failure. Plan is to use a 33/7.6 transformer, phasing is not a problem, but not sure about load sharing, and do not know how much of the 66/33 impedance will need to be added to the 33/7.6 impedance to work out load share. Simple diagram attached, assume failure of TX2 and operating on TX1 & TX3 7.6KV load share is where I'm not sure, no load share at 33 as only one TX suppling

Not sure where my source is in each case for working out sharing 33kv bus or 66kv bus?

Thanks for you help

Sean

 

[prc]

Attached enclosure explains the loading condition. In case 33Kv circuit is loaded, then there is a chance of circulating current due to voltgae drop in X1

 

[prc]

One more point.Why do you want the 5 MVA transformer to be Dy connected? I think it should be Yd to avoid a star 7.6 kV getting connected to delta 7.6 kV.

 

[seanmcc]

PRC
It is not a case of wanting it to be connected this way, more a case of temporary back up using what we have availible until substation is refurbished in 4 years time. We have the two 66/33/7.6 transformers on site, had three there until one failed a couple of weeks ago. 33KV load is not an issue but the 7.6 load is, so looking at our options with out spending large $s. Wanted to understand how the pictured option would work if one of the existing 66 transformers failed and we connected the 33/7.6 as shown.

Seanmcc