Parallel operation of transformers is required in cases such as the power to be delivered is more than the individual transformer rating. In such cases operating two or more transformers to facilitate the power flow is possible, but certain conditions to be followed while operating transformers in parallel condition
Conditions for parallel operation of Transformers:
- Polarities of the transformers must be same (wrong polarity leads to dead short)
- The voltage rating of both primary and secondaries are identical. This means the that the transformation or turns ratio must be same for the transformers which are operated to be parallel. (Voltage ratio is to be maintained to avoid circulation current)
- Percent (or per unit ) impedance of the transformers are to be same in magnitude and should have the same phase angle (X/R ratio should be same for transformers operating in parallel else division of load will not be proportional to the kVA ratings of transformers)
- Phase displacement between the primary and secondary line voltages of the transformers should be same (transformers of star/star and delta/star cannot be paralleled because of the phase difference of -30o making paralleling impossible (cannot be compensated))
- Phase sequence of the transformers should be same. (Phase sequence is the order in which the terminal voltage attains their maximum value. Therefore in paralleling the two three phase transformers those terminals whose voltage attains maximum values simultaneously must be paired up.
Conditions 1,4 and 5 are absolutely essential and must be fulfilled, condition 2 must be satisfied to a close degree and condition 3 (X/R ratio) must be satisfied in order to have equal loading on the transformers.