Parallel Operation of 3-phase Transformers-Conditions to Satisfy
The various conditions that must be fulfilled, for the successful parallel operation of 3-phase transformers, are as follows:
- The line voltage ratios of the transformers must be same.
- The transformers should have equal per unit leakage impedances.
- The ratio of equivalent leakage reactance to equivalent resistance should be same for all the transformers.
- The transformers should have the same polarity.
In addition to these four conditions, two more essential conditions that must be fulfilled for parallel operation of three-phase transformers are as follows:
Relative-phase displacement:
The relative phase displacement between the secondary line voltages of all the transformers must be zero i.e. the transformers to be connected in parallel, must belong to the same group number.
YY with ΔΔ transformers cannot be connected in parallel.
Note: Transformers of group numbers 3 and 4 can be successfully operated in parallel.