Conditions for Parallel Operation of Transformers

Conditions for Parallel Operation of Transformers

When two or more transformers are run in parallel their primary windings are connected to supply bus bars and secondaries are connected to load bus bar.When connecting two or more transformers in parallel certain  conditions has to be met for satisfactory performance. These are the conditions for parallel operation of transformers.
a) same polarity.
b) same voltage ratio of transformer
c) same percentage impedance.
d)Same Phase Sequence

Same Polarity
                            Transformers should be properly connected with regarding to its polarity.The polarity of the transformers connected in parallel should be same otherwise it may lead to dead short circuit.When connecting two or more transformers in parallel it is essential that their terminals of similar polarities should joined to same bus bar.bar. Otherwise emfs induced in the secondaries will be in opposite polarities and they acts as a short circuit even for no load.Polarity of transformer means the direction of induced emf in secondary. If the directions of induced secondary emf in two transformers are opposite to each other when same input power is fed to the both of the transformers, the transformers are said to be in opposite polarity. If the directions of induced secondary emf in two transformers are same when same input power is fed to the both of the transformers, the transformers are said to be in same polarity.

Same Voltage Ratio
                               The voltage ratio of the  transformers*( turns ratio) which are connected in parallel should be kept equal in order to avoid losses occurring in transformers due to load circulating currents.If two transformers connected in parallel have a slight different in their voltage ratio ,the parallel operation is possible.But due to the difference in volt ratio even the primary supply voltage is identical there will be a difference in secondary voltages even in no load condition.So there will be a circulating current between secondaries and therefore between primaries also.This circulating current makes unequal loading conditions,so transformers in parallel system may be overloaded. As the internal impedance of transformer is small, a small voltage difference may cause sufficiently high circulating current causing unnecessary extra I2R loss.

Same Percentage Impedance
                              If the impedance of transformers which are connected in parallel are not same, then the parallel operation is possible but the power factor at which two transformer operates will be different from the common load .So the transformers will not share load in proportional to their KVA ratings.
                             In other words,The current shared by two transformers running in parallel is strictly depends on  their KVA ratings. As we know electric current carried by these transformers are inversely proportional to their internal impedance. From these two statements it can be said that impedance of transformers running in parallel are inversely proportional to their KVA ratings.So if the impedance are not identical ,load sharing will also be different.
                      Identical percentage impedance of two different transformers is difficult to achieve practically that is why the transformers run in parallel may not have exactly same percentage impedance but the values would be as nearer as possible.

Same Phase Sequence
                           The phase sequence or the order in which the phases reach their maximum positive voltage, must be identical for two parallel transformers. Otherwise, during the cycle, each pair of phases will be short circuited.