Parallel operation of transformers – Connection & 5 conditions for perfect operation

In order to supply loads exceeding the rating of an existing transformer, two or more transformers are connected in parallel to the existing one. The parallel operation of transformers facilitates load sharing between the transformers. When a connected load at a load centre exceeds the MVA of the existing transformer, it is easier and economical to install additional transformers in parallel to the existing one than to replace the existing one with a single larger transformer.

In addition to the above mentioned, the parallel operation of transformers improves the reliability of the system. For example, if any one of the transformers operating in parallel fails, at least the other transformers can supply the connected loads. But if a larger transformer is used instead of a set of parallel-connected ones, any failure in it will result in a complete blackout.

Parallel operation of transformer
The above picture shows six transformers in parallel operation. Image source: Mirage Electronic devices

The primary of parallel operating transformers are connected to the same source busbar whereas their secondary windings are connected to a common load busbar. In order to successfully connect two or more transformers in parallel certain conditions needs to be satisfied. In the following sections we will discuss in detail on those conditions.

Conditions for successful parallel operation of transformers

The following five principal characteristics needs to be satisfied for a successful parallel operation of transformers:

  1. The line voltage ratios of all transformers must be equal.
  2. All transformers must have equal per-unit impedances and the same ratio of equivalent leakage reactance to equivalent resistance.
  3. The polarity of all transformers must be equal
  4. All transformers must have the same phase sequence.
  5. The phase displacement between the primary and secondary terminals must be the same.

1. Equal line voltage ratios.

The line voltage ratio of all transformers operating in parallel must be equal, otherwise, it will result in improper loading between the transformers. For example, if two transformers connected in parallel have slightly different voltage ratios, under no-load conditions, the inequality in the secondary induced EMFs will result in circulating currents in the loop formed by the secondary windings.

Due to the low impedance of the windings, the magnitude of the circulating currents will be quite high. When the transformers are loaded, the circulating currents cause unequal loading. It is possible that one of the transformers may be fully loaded or even overloaded while the other is no-loaded. In order to avoid such situations during the parallel operation, the turns ratios of all the transformers must be equal.

2. Equal per-unit impedances and the same ratio of equivalent leakage reactance to equivalent resistance.

In order to ensure equal load sharing the per-unit impedances of the transformers must be equal. Suppose two transformers of the same rating are operating in parallel, their per-unit leakage impedances should be equal. If the ratings are unequal, their per-unit leakage impedances based on their rating should be equal.

If two transformers with different ratio of equivalent reactance to equivalent resistance of per unit impedance are paralleled, the current carried by those transformers will not be equal. The one with the lower power factor carries more current while the one with the higher power factor carries a lesser current. This will result in improper load sharing. Hence, for proper parallel operation of transformers, per-unit impedances and the same ratio of equivalent leakage reactance to equivalent resistance must be equal.

3. The polarity of all transformers must be equal

The polarity of the primary and secondary windings of the transformer must be taken into account while connecting transformers in parallel. If the polarity of the windings is not the same, it will result in a short circuit.

4. All transformers must have the same phase sequence.

The phase sequence of transformers in parallel operation must be identical. Mismatch in the phase sequence will result in short circuits.

5. The phase displacement between the primary and secondary terminals must be the same.

In order to ensure successful parallel operation of transformers, The phase displacement between the primary and secondary terminals must be the same. The primary and the secondary of a three-phase transformer can be connected in a variety of ways. Each connection produces a different magnitude and phase displacement of secondary voltages. The winding connection of the two transformed connected in parallel must have the same displacement. Refer to the following table for details.

Operative Parallel Connections of Three-Phase Transformers – (connecting two transformers in parallel)

The following table shows all operative combinations of transformer windings that can be followed while connecting transformers in parallel.

Transformer-1Transformer-2
1L.V. Side: Delta
H.V. Side: Delta
L.V. Side: Delta
H.V. Side: Delta
2L.V. Side: Star
H.V. Side: Star
L.V. Side: Star
H.V. Side: Star
3L.V. Side: Delta
H.V. Side: Delta
L.V. Side: Star
H.V. Side: Star
4L.V. Side: Star
H.V. Side: Star
L.V. Side: Delta
H.V. Side: Delta
5L.V. Side: Delta
H.V. Side: Star
L.V. Side: Delta
H.V. Side: Star
6L.V. Side: Delta
H.V. Side: Star
L.V. Side: Star
H.V. Side: Delta
7L.V. Side: Star
H.V. Side: Delta
L.V. Side: Star
H.V. Side: Delta
8L.V. Side: Star
H.V. Side: Delta
L.V. Side: Delta
H.V. Side: Star

Parallel operation of single phase transformers

For parallel operation of single-phase transformers, the first three condition of a those mentioned above have to be satisfied. For single-phase transformers, phase sequence and phase displacements never come into the picture.

Advantages of parallel operation of transformers

Following are the merits of parallel connected transformers instead of singular transformer.

  • The availability of transformers for service can be increased by connecting two or more of them in parallel. During maintenance, only those transformers that requires maintenance can be taken out of service while the other can still supply the loads.
  • The parallel operation of transformers improves the reliability of the system. Even if any one of the transformers fails due to some internal faults, the other will be available for the service.
  • The efficiency of a transformer is always maximum at full load conditions. By operating transformers in parallel, maximum efficiency of power distribution can be achieved by switching only the number of transformers required to supply the total demand.
  • As transformers can operate in parallel, the power system can be easily scaled for increasing load demands.

Summary

There are certain conditions that needs to be satisfied during parallel operation of transformers. Voltage ratios, polarity markings, phase sequence, resistance reactance ratio and phase displacements needs to be taken care of during the parallel connection.

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