Conditions for Satisfactory Parallel Operation of Transformers

Posted on October 8, 2017 in Transformers by With 0 Comment
Parallel Operation of Transformers

Parallel Operation of Transformers (Credit:

For the satisfactory parallel operation of transformers there are certain conditions that must be satisfied. Satisfactory parallel operation of the transformers implies that the transformers connected in parallel share common load approximately in proportional to their ratings. The parallel operation is advantages that the spare parts can be used interchangeably and their storage is easy.

The most satisfactory condition is achieved when the load shared by the transformers is in exact proportional with their ratings.

Conditions for Satisfactory Parallel Operation of Transformers

The conditions that must be followed for satisfactory parallel operation of transformers are as follows:

  1. The supply system voltage and frequency must suit the primary windings of the transformers.
  2. The transformers that are connected must have same polarity. In case of three phase transformers, the transformers should have same angular displacement and same phase sequence.
  3. The voltage ratio of primaries and secondaries of the transformers must be same.
  4. The percentage impedances should be equal in magnitude and have same X/R ratio in order to avoid circulating currents and operating at different power factors.
  5. If the transformers have the different KVA ratings, the equivalent impedances should be inversely proportional to individual KVA rating to avoid circulating currents.

Explanation of Each Conditions in Brief

The condition – 1 is common and easily satisfied.

The condition – 2 is an important condition for the faithful parallel operation of transformers. The secondary windings which are connected in parallel are either in phase with each other having zero-degree phase displacement between the voltages or the voltages are in opposite time-phase relationship with a phase difference of 180° between them. A closed series circuit is formed by this parallel connection.

If two voltages are having same phase relationship, the induced voltages from either common connection to the other are in the same direction. Thus there is no current flow in the series circuit. Thus the parallel operation works satisfactory with no load connected. The windings so connected will have same polarity.

If the two voltages are in phase opposition, the induced voltages from either common connection to the other are in opposite directions, hence adding to one another in the series circuit. Thus a large current will flow in the series circuit which is equivalent to a short circuit on both transformers. The two windings will have opposite polarity and parallel operation will not work properly.

This polarity designation should not be confused with the terms additive polarity and subtractive polarity as applied to individual units which are used only to give the relative direction of induced voltages and are helpful in proper paralleling of the transformers.

For parallel operation of three phase transformers having same voltage ratio, polarity may not be required to be considered. For satisfactory operation, the angular displacement and phase rotation between the two units to be paralleled must be the same. Paralleling of three phase transformers is simplified by standardizing the lead markings and various three phase connections have been placed in three different groups depending upon their angular displacement.

If condition – 3 i.e. voltage ratios of primaries and secondaries of the transformers must be same is not satisfied the difference in voltage between the windings will cause a current to flow in the circuit at all time. This circulating current will be limited only by the sum of the impedances of the two transformers. Though this condition is not perfectly met still parallel operation is possible at the cost of the circulating current.

For satisfactory parallel operation, the circulating current should not exceed 10 percent of normal load current.

With violence of condition – 4 i.e. if the percentage impedances are equal in magnitude then also parallel operation is possible. Under this case impedance triangle are not identical in shape and size. Also the two transformers will operate at different power factors from the power factor of the combined load.

The ratio of resistance to reactance must also be same for each of the transformers. So that the currents in each of the transformers are in phase. If these ratios are not same, the currents in the transformers are not in phase and the sum of the windings current will be greater than the line current.

If the percentage impedance of all the units connected in parallel is same then the load will divide in the individual units in the ration of their capacities even if the error is introduced due to unequal ration of resistance and reactance. The total permissible load will be equal to the sum of capacities of the individual units connected in parallel.


  1. Transformers & Induction Machines by M. V. Bakshi & U. A. Bakshi

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Hello, I'm Kalpesh, An Electrical Engineer and the founder of Substation System.

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