Parallel Operation of Transformers

If they were fed from the same 33kV source and not connected to the same downstream 15kV load, then they would not be operating in parallel; each transformer would not know of the other's existence, so to speak.

To make the question clear, the transformers are fed from a common 66KV busbar and our plan is to make them operate parallel only in the 33Kv side, but isolate the 15Kv bubar so that they will operate separately in the 15Kv side.

As far as I understand you a two transformers, 3 windings each, 66K k/33 kV/15 kV and you want to paralell them on the 66/33 kV but no parallel on the 15 kV side because of the impedance.

Overall, if the conditions are good for connecting them in parallel only between 66 kV and 33 kV then there is no problem. You can go ahead with it, just be careful not to accidentaly parallel them on the 15 kV side.

cheers

That does make sense.

Intriguing Question! What is the vector group of the transformer? What are the loads connected to the 33kV Side and to each of the 15kV Side? Are they feeding semiconductor loads by any chance?

the transformers have the same vector group of Ynd11d11 and the connected loads with each of the lines vary from household lighting to cement factories.

If the loading on the unparalleled 15kV sides is unequal, then it could cause differential voltage drops in the two 15kV secondaries and thus would cause circulating currents between them. The transformer differential protection could spuriously trip.

On my experiences previously, it was the same issued; where vector Tx is same, different input of Tx, and output need to be merge out together without disturbing the transmission line. Base on simulation programme we'd identified the output is fluctuated base on load demand, what we suggesting is the process behaviour (output) should auto synchronised with impedance generated with load demand.

What's the worst thing that could happen?

poof ,boom!