The parallel-operated transformers are connected to the same HV or LV busbars, especially on the load side. Transformers are operated in parallel to achieve the following objectives.
- Increased reliability through redundancy.
- Sharing of load among multiple transformers.
- Consideration for future load requirements
This practice is widely adopted in power stations and continuous process plants to ensure that in the event of transformer failures, redundancy is built into the power system to maintain the power supply through the healthy transformer.
While paralleling transformers, it is vital to remember that incorrect connections of the primary or secondary winding would either result in the parallel combination not functioning correctly or a short circuit.
For transformers to be operated in parallel the following conditions must be met.
- Same voltage ratio of both transformers.
- Same phase sequence.
- Same polarity.
- Same percentage impedance (10% tolerance variation is acceptable)
- Capacity difference between the transformers not exceeding 1:3 ratio
I addition to above, following point is to be kept in mind during operation of transformers in parallel
If transformers are provided with the tap changers, the tap settings of both the transformers should be changed uniformly at the same time to prevent undesirable high circulating currents between the transformers.