Parallel operation of electrical power transformers

It is common to use several smaller rated transformers connected in parallel instead of one bigger rated transformer in electrical power systems.Advantages of parallel operation of transformers are:

  • Increased reliability and maximized electrical power system availability. This is because it is not necessary to disconnect the whole system if fault occurs with one of the transformers. What is more, system operators can disconnect any transformer for maintenance purpose, while other transformers will serve the load, therefore, this ensures continuous electric power supply.
  • Maximized electrical power system efficiency. Sufficiently good efficiency of a power transformer is within a wide loading range, from 50% to 100% of rated transformer power. However, if loading is smaller, efficiency of a transformer decreases, therefore, in case several transformers are installed in parallel, it is possible to switch on only those transformers which will give the total demand by running only on efficient loading.

However, in order to connect two or more power transformers in parallel, several technical conditions have to be satisfied.

Same voltage ratio and identical position of tap changer

If two transformers of different voltage ratio (or tap changer position) are connected in parallel, there will be a difference in secondary voltages. Because of this, a circulating current between secondary and primary windings will appear. Internal impedance of a transformer is small, therefore, only a small voltage difference may cause a considerably high circulating current and additional losses. Vector diagram of two parallel transformers with different secondary voltages is depicted in Fig.1, circulating current is “I”.


Fig. 1 Vector diagram of parallel transformers secondary voltages. Here “I” is  circulating current, “Udif” – voltage difference.


The presence of circulating current is also apparent in ESMOGrid calculations. In Fig. 2 between bus 4 and bus 5 current is not flowing because system is symmetrical and transformers T1 and T2 have identical parameters.

Fig. 2 Power flow calculation results of symmetrical system with two parallel transformers.

Fig. 2 Power flow calculation results of symmetrical system with two parallel transformers.

However, if we change, for example, a tap changer position in one of the transformers, the circulating current will flow between bus 4 and bus 5. This case is shown in Fig. 3 where T2 secondary winding voltage is increased by 10% and over 30 A current is present between bus 4 and bus 5.

Fig. 3

Fig. 3 Power flow calculation results of unsymmetrical system. Here transformer T2 tap position is changed by 10%.


Same short circuit voltages

Power flows carried by parallel transformers are proportional to their MVA ratings if short circuit voltages of both transformers are equal. This relationship is shown by an equation:


Where UkI and UkII are short circuit voltages, PI and PII power flows through transformers, SNI and SNII nominal powers (MVA) of transformers. In Fig. 4 transformer T1 has a lower short circuit voltage, however, as both transformers have the same MVA rating, power distribution between transformers is not equal and T1 is loaded more than T2.


Fig.4 Transformer T1 is loaded more than T2, because its  short circuit voltage is smaller.

Same phase sequence

The phase sequence of parallel transformers must be equal. Otherwise, during the cycle, each pair of phases will be short circuited.

Same phase angle shift (vector group)

Vector group of a transformer is determined in accordance with the phase displacement between line voltages of secondary windings. If two transformers with different vector groups are connected, a circulating current occurs as shown in a vector diagram in Fig. 5, what is more, if transformers with opposite voltage vectors are connected, this will produce short circuit conditions. Therefore, only transformers with the same vector groups can be connected. In ESMOGrid parallel connection of transformers with different vector groups is considered impossible.

Fig. 5

Fig. 5  Vector diagram of parallel connected transformers with different phase angle shift.


Different system impedance

It is important to notice that if impedance of parallel system branches are not equal, loop power flows will occur despite other requirements are satisfied.

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