# Parallel Operation of Transformers

For supplying a load in excess of the rating of an existing transformer, another transformer is usually connected in parallel with it since replacing it with a single larger unit is a costly alternative. The cost of a spare smaller rating transformer is also lower.

Also, it is preferable to have a parallel transformer in case of emergencies; at least half the load can be supplied when one of the transformers is taken out of the service.

# Parallel Operation of Transformers

For paralleling of transformers, their primary windings are connected to source bus-bars and secondary windings are connected to the load bus-bars. Various conditions that must be fulfilled for paralleling of transformers are given below.

1. The line voltage ratios of the transformers must be equal (at each tap): If the transformers connected in parallel have slightly different voltage ratios, then due to the inequality of the induced emfs in the secondary windings, a circulating current flows in the loop formed by the secondary windings under the no-load condition; the circulating current may be much higher than a normal no-load current.

When the secondary windings are loaded, this circulating current will tend to produce unequal loadings on the two transformers, and it may not be possible to supply the full load (one of the transformers may get overloaded).

2. The transformers should have equal per-unit leakage impedances and the same ratio of the equivalent leakage reactance to the equivalent resistance (X/R): If the ratings of both transformers are the same, their per-unit leakage impedances should also be the same for equal loading. If the ratings are unequal, their per-unit leakage impedances based on their own ratings should be equal so that the currents carried by them will be proportional to their ratings.

In other words, for unequal ratings, the numerical (ohmic) values of their impedances should be in inverse proportion to their ratings to have the currents in them in line with their ratings.

A difference in the X/R ratio of the two components of their per-unit impedances results in different phase angles of the currents carried by them; one of the transformers works with a higher power factor and the other with a lower power factor than that of the combined output, and the real power will not be proportionally shared by them.

3. The transformers should have the same polarity: The transformers should be properly connected with regard to their polarity. If they are connected with incorrect polarities then the two emfs, induced in the secondary windings that are in parallel, will act together in the local secondary circuit and produce a short circuit.

## Parallel Operation of Transformers | 3 Phase

The previous three conditions are applicable to both single-phase as well as three-phase transformers. In addition to these three conditions, two more conditions need to be fulfilled for the parallel operation of three-phase transformers:

4. The transformers should have the same phase sequence: The phase sequence of the line voltages of both the transformers must be identical. If the phase sequence is incorrect, pairs of phase windings will be short-circuited in every cycle.

5. The transformers should have the zero relative phase displacement between the secondary line voltages: Windings can be connected in a variety of ways that produce different phase displacements of the secondary voltages with respect to the primary voltages.

The winding connections can be classified into distinct vector groups. Each vector group notation consists of an uppercase letter denoting the connection of the three phases of the HV winding, followed by a lowercase letter denoting the connection of the three phases of the LV winding, and lastly a clock number representing the displacement of the line-to-ground voltage phasor of any phase of the LV winding with respect to the corresponding HV winding phasor placed at 12 O’clock.

Commonly used three-phase connections can be classified into four groups:

Group 1: Zero phase displacement (Yy0, Dd0, Dz0)

Group 2: 180° phase displacement (Yy6, Dd6, Dz6)

Group 3: –30° phase displacement (Yd1, Dy1, Yz1)

Group 4: +30° phase displacement (Yd11, Dy11, Yz11)

Letters y (or Y), d (or D), and z represent star, delta, and zigzag connections respectively.