Matching transformers for parallel operation

3 rules for parallel transformers

The following rules must be obeyed in order to successfully connect two or more transformers in parallel with each other:

1. The turns ratios of all of the transformers must be nearly equal.
2. The phase angle displacements of all of the transformers must be identical.
3. The series impedances of all transformers must be nearly equal, when expressed as ‘%Z‘ using the transformer impedance base.

The first two rules are required so that the open-circuit secondary voltages of the transformers are closely matched in order to avoid excessive circulating currents when the parallel connections are made. The last rule is based on the fact that for a given voltage rating and %Z, the ohmic impedance of a transformer is inversely proportional to its KVA rating.

Therefore, transformers with different KVA ratings can be successfully operated in parallel as long as their %Z values are all approximately the same.

Example of parallel transformers and loud BANG!

Two three-phase 10,000 KVA 66,000∆ – 12,470Y volt transformers were in parallel operation in a substation. The primaries of the two transformers are connected to a 66 kV transmission line through a single air break switch. This switch is designed to interrupt magnetizing current only, which is less than 1 A.

The transformers were being removed from service and the secondary loads had been removed. A switchman then started to open the air break switch, expecting to see a small arc as the magnetizing current was interrupted.

Instead, there was a loud ‘BANG’ and there was a ball of flame where the air break switch contacts had vaporized. Something was obviously wrong. Upon closer inspection, it was revealed that the two transformers had been set on widely different taps:

The first transformer was on the 62,700 V primary tap and the second transformer was on the 69,300 V primary tap. Both transformers had a 7% impedance. Because the turns ratios were unequal, a circulating current was set up even without any secondary load. The open-circuit secondary voltage difference, assuming 66 kV at the transformer primaries, is calculated below.

The per-unit circulating current in the secondary loop is equal to ∆E_s divided by the sum of the per-unit impedances of the two transformers //

Converting I_c into amperes //

Since I_c flows in a loop in the secondary circuit, the current out of the secondary of the first transformer equals the current into the secondary of the second transformer. But since the turns ratios are not equal, I_c does not get transformed into equal and opposite currents at the primaries.

The net current through the air break switch, I_AB, is the difference in the primary currents //

Reference // Power Transformers Principles and Applications by John J. Winders, Jr.