Parallel Operation of Transformers

Transformer Compatibility

When connecting three-phase transformers in parallel, the two transformers should have the same transformation ratio, phase sequence and internal phase shifts. The two transformers are assumed to have their primary windings connected to the same supply.

Care must be taken to ensure that the transformers have compatible internal impedances or their load sharing will change over the load range and one may ‘hog’ the load causing it to overheat even though their combined load capacity has not been exceeded.

Phase Shifts

In a transformer connected in delta primary and star secondary (delta–star), the secondary voltage is induced such that it lags the applied voltage by 30°E. In the case of star–delta the induced secondary voltage leads the applied voltage by 30°E.

Because of the phase shifts, these two transformers must not be connected in parallel since the secondary voltages are out of phase with each other by a total of 60°E. Only transformers with the same phase shift should be paralleled.

Parallel Operation Conditions (Requirements)

Equal voltages

If two unequal voltage sources are connected in parallel, a circulating current is set up between the two sources. One transformer becomes a burden on the other and they are unable to supply full power to an external load. Essentially for the same style of connection, the turns ratios must be the same.

Same phase sequence

If different phase sequences are connected in parallel, the least that can occur is a short-circuit between the lines. Heavy circulating currents flow and damage will almost certainly occur to both transformers and perhaps the installation.

Phase angle shift

The change in phase angle from primary to secondary on both transformers must be identical or once again dangerous circulating currents will be generated.

Satisfactory parallel operation can occur only when the two transformers belong to the same group and have the same phase shift.

Compatible internal impedance

The two transformers must have a compatible range of internal impedance to allow effective load sharing across the load range. Parallel operation involves two or more transformers connected to a common source of supply and their secondaries connected to a common load. Only when the two transformers match in all important characteristics can they be expected to share the load evenly, or according to design.

Phasing Transformer Windings

Single-Phase Transformers

To determine the polarities of transformer windings, two terminals are connected together (one terminal of each winding) as shown in Figure 1.

If the voltmeter reading across the other two connections is greater than the supply voltage, the two voltages are aiding each other and the transformer is said to have dissimilar ends joined. Therefore the ‘open end’ of the primary is marked as the ‘start’ of the primary and the joined end of the secondary is also marked as a ‘start’ end.

Phasing Transformer Windings

Figure 1 Determining phasing

If the voltmeter reads less than the supply voltage, then the voltages are opposing each other and the windings have similar ends bridged, in which case both open ends should be marked as starts signified by a dot (•).

Three-Phase Transformers

The same method can be applied to the windings of a three-phase motor or transformer. It must be remembered, however, that these windings are 120°E apart and that the induced voltages must be at the same 120°E in the same sequential order.

As the transformer has three legs, each with a separate primary and secondary winding, and perhaps several secondaries, great care must be taken to ensure that only the cores of each phase are tested against one another.

Testing Final Connections

Transformer connections should always be checked before loading. Serious damage can be caused by an improperly connected transformer and operators risk injury or electric shock.


Transformer secondaries connected in star configuration have three line connections and a fourth for the neutral. All voltages should be tested without load applied. Each phase voltage should be tested between line and neutral: A–N, B–N and C–N. All voltages should be equal and meet the nameplate specifications.

The three line voltages should also be tested: A–B, B–C and C–A. These should all be identical and 1.73 times greater than the phase voltages. This is shown in the phasor diagram in Figure 2(a). A reversed phase winding will be indicated if two of the line voltages are equal to the phase voltages (see Figure 2(b)).

Testing secondary connections

Figure 2 Testing secondary connections


Delta-connected transformer secondaries should be tested by leaving one of the three bridges open and connecting a voltmeter in place of the bridge. If the connections are correct, the voltmeter will indicate zero voltage (or very small voltage) (see Figure 2(c)).

If the voltmeter reads about double the expected line voltage, do shown in Figure 2(d), one of the windings has been reversed, requiring all connections to be rechecked. If the delta was complete in such a condition, a high current would circulate.

Open-Delta Connection

The open-delta is an asymmetrical connection for three phases. It is seldom used other than for small loads or in an emergency when one phase winding of a three-phase transformer has failed. The connections are shown in Figure 3.

It is a method for providing a three-phase supply from two transformers. It can also be used to provide two single-phase supplies for small consumers. Due to the unbalanced and out-of-phase currents, a transformer is reduced to 58 per cent of its normal capacity when working in open-delta connection and therefore the connection is only suitable for small loads or emergencies, or sometimes for short duration tasks such as for autotransformer starting of three-phase motors. The transformer can be overloaded just for the starting period of the motor.

Open–delta connection

Figure 3 Open–delta connection