Parallel operation of 3-phase Transformer

The following conditions are to be fulfilled before paralleling two three phase Transformers:-

1. The voltage transformation ratio of the both transformer must be same.
2. The polarity must be same.
3. The percentage impedance should be same.
4. The vector diagram and the phase displacement must be same.
5. The phase sequence must be same.

Effects of percentage impedance on load sharing in parallel operation of :-

Voltage drop between no load to full load of transformer is determined the percentage impedance of the transformer. Now, we describe the effect of percentage impedance on load sharing in parallel operation. Let, Percentage impedance of Transformer 1 = Z1 % And Percentage impedance of Transformer 2 = Z2 %

1. If the percentage impedances of the transformers are equal ( i.e Z 1 % = Z 2 % ) running in parallel , both the transformers will share the load in proportion to their capacities i.e Ratting of transformer ( MVA/KVA rating).
2. If percentage impedance of Transformer-1 is greater than the percentage impedance of Transformer-2 ( I.e Z 1 % > Z 2 % ) and both are running in parallel operation. That time, lower percentage transformer, i.e Transformer-2 will have less voltage drop and therefore it will take greater share of the load. Quite the opposite the transformer -2 ( i.e higher percentage impedance) will have higher voltage drop and therefore it will take lower share of the load.

Qun: In parallel operation of two transformers if their ratings are equal but percentage impedance are different how will they share a load?

Ans: The loads will be shared inversely as the impedances of the transformers expressed in percent of normal voltage.

Qun: What will happen if the voltage ratio of the transformers running in parallel are not identical?

Ans:Due to unequal voltage ratio, secondary voltage of the both transformer will be different resulting a circulating current will flow through the secondary winding of the both transformer. This circulating current will act as extra load of the transformer and cause heating of transformers even on no load condition of the transformers. And due to over heating of transformer , transformer may be damaged.

Qun:In parallel operation of two transformers if their ratings and also percentage impedances are different how do you calculate their load sharing?

Ans:- In case of different percentage impedance for different ratings of transformers the percentage impedance have to be converted to the same basic kVA.

Qun:Is it necessary for the ratings of two transformers to be the same for parallel operation?

Ans:Not necessary but they should share the load in proportion to their ratings.

Note That:-

Do not connect the transformers at incorrect polarity in parallel, incorrect polarity results a dead short circuit. Hence, both the transformer will be damaged.