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Importance of the parallel operation of transformers

Transformers are placed in parallel by electric utilities when they want to provide a ‘stronger’ voltage source and will result in higher available fault current that can be delivered downstream.  Usually the utility transformer can serve its own load but two are put together to achieve with a ties primary and secondary bus to ‘stiffen’ the voltage to ride through system load changes. Because here we have more transformers connected in parallel. So even if, one of the transformers gets into a fault or is taken out for maintenance or repair, the load can continued to be serviced. Therefore by this we can reduce the spare capacity also. 
Before two or more transformers connected in parallel and to share a common load satisfactorily, there are some conditions to be satisfied. Those are the voltage ratio (turns ratio) and the per unit impedance of each machine on its own base must be the same. When two transformers are connected in parallel, the impedances of the transformers must match (within 10%) to divide the load approximately equally between the two transformers or to divide the load according to the rating of each transformer. If the transformers to be connected in parallel are equipped with load tap changing windings, then the impedances for each of the tap changer positions must match. If these conditions are not met, then one of the transformers could conceivably carry a continuous overload, resulting in overheating.
Also the polarity must be the same, so that there is no circulating current between the transformers. The polarity of connection in the case of single phase transformers can be either same or opposite. Inside the loop formed by the two secondary the resulting voltage must be zero. If wrong polarity is chosen the two voltages get added and short circuit results.     The phase sequence must be the same and no phase difference must exist between the voltages of the two transformers. When these requirements are satisfying we can have a good parallel operation of transformers which is very useful and favourable in practice.

Generally, application of parallel transformers allows achieving the following benefits:

1)      Reducing the total capacity of electrical transformers (as compared to separate their work). The decrease of total installed capacity is reached:
·         by lowering the overall  demand load to the diversity of loads connected to different transformers
·         by using a higher load rate of parallel transformers
·         less required backup in case of electrical transformer failure
      2)   Reduction of electricity losses in electrical transformers due to a possible disconnection of unloaded transformers
      3)    Improving the power quality due to the stable level of short circuit current throughout the network
      4)  Increasing the reliability of operation of protective devices in the case of phase-to-earth short circuits in the network.
     5)   Possibility of placing electrical transformers in operation phase-by-phase