There are certain definite conditions which must be satisfied in order to avoid any local circulating currents and to ensure that the transformers share the common load in proportion to their rated KVA. These conditions are:
- The transformer should have same transformation ratio e. the voltage ratings of the primaries and secondaries must be identical.
- The transformer should be properly connected with regard to polarities. If this condition is not fulfilled, e. the e.m.f. in the secondary windings of the transformers which are parallel with incorrect polarity will act together in the local secondary circuits and produce the effect equivalent to a dead short circuit.
- Primary windings of the transformers should be suitable for the supply system voltage and frequency.
- In case of 3-phase transformers, the two transformers must have the same phase-sequence, i.e. the transformers must be properly connected with regard to their phase-sequence.
- In case of three phase transformers, the two transformers must have the connections so that there should not be any phase difference between the secondary line voltages i.e., a delta star connected transformer should not be connected with a delta-delta or star-star connected transformer.