Transformers in Parallel

From the data given the transformers are identical with same turns ratio, same capacity and of same vector group and fulfil the conditions for parallel operation. They should operate safely in parallel operation.The phase sequence must be checked before paralleling. The only important point to be remembered is the tap changing switches must be at same position for all the three transformers and should check and confirm that the secondary voltages are same. When the voltage tap need change all three tap changing switches should be operated identical for all transformers. The OL settings of the SF6 also should be identical. If the substation is operating on full load condition, tripping of one transformer can cause cascade tripping of all three transformers.

thank you for your reply.

Just I want to be sure

When the transformers are operated in parallel, the fault current would be very high on the secondary side.Supposing percentage impedance of one transformer is say 6.25 %, the short circuit MVA would be 25.6 MVA and short circuit current would be 35 kA.

If the transformers are of same rating and same percentage impedance, then the downstream short circuit current would be 3 times (since 3 tansformers are in Parallel) approximately 105 kA. this means all the devices like ACBs, MCCBs, switch boards should withstand the shorcircuit current of 105 kA.This is the maximum current.This current will get reduced depending on the location of the switch boards, cables and cable length etc.How ever this aspect has to be taken into consideration.

Another point is ,there should be Directional relays on the secondary side of the transformers.

Thank you Ramvinod,

What do you mean by DIRECTIONAL RELAY?

Is it a negative KVAR relay?

Directional relays are Reverse current protection relays.

Perhaps he means differential relays, such as an 87T? In such a scenario, this relay would isolate a transformer fault to the unit in which it occurred.

Take note that is very important to use high capacy circuit breakers protection when they are located near to the transformers.

Best regards.

francisco.sanchez@hp.com

I have an installation including three identical power transformers 1600KVA 20/0.4 KV, 50 HZ oil cooled, DYN11 coupling, powered by the same MV feeder, installed in the same room, distance between transformers less than 10meters( 30 feet).

What parameters should be taken in consideration if transformers will be parallelled? and what type of protection device should be added?

actual protection includes SF6 switchgear with 51P, 51G, 52P, 52G relay .on MV side, and an air circuit breaker LV side.

There shall be reverse power relays at LV side of all transformers. Tap position and tap changing shall be identical. There shall be an inter tripping arrangement between LV side breakers and MV side breakers such that when MV side breaker of any transformer trips corresponding LV side breaker shall also trip. cables, switchgears etc. shall have S.C. rating of minimum fault current of 3 transformers.

Thanks to all