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Why is It Important to Know Transformer Labels?

When testing a turns ratio of a transformer using DV Power TRT instruments, three labels are shown on their display: A, B, C.

These labels are extremely important since their misunderstanding and misuse can lead to wrong conclusions. They can, but don’t have to be always an indication of the phase denomination or core leg denomination (as in “A” for the first one, “B” the middle one and “C” for the third one). There are situations where one label is associated with two phases or legs of the magnetic core, such as “A” being an indication for phases U and V in Example 1 below.

This is very important when determining in which leg of the three-phase transformer the instrument has detected the problem.

The labels A, B, and C are associated with:

  • The ratio of two voltages measured (excited and induced)
  • Percentage difference from the nameplate value
  • Magnetizing current through the primary (excited) winding
  • Phase displacement between excited and induced voltages.

These values are shown in the single-phase test, where a single-phase voltage is applied for the test, as well as in the true three-phase test when applying the three-phase test voltage.

When applying the true three-phase test voltage the voltages at the associated terminals are being compared. In this case labels A, B, C represent:

  • Phase to ground values of these voltages: A – U1U-1N, B – U1V-1N, C – U1W-1N
  • Phase to phase values: A – U1U-1V, B – U1V-1W, C – U1U-1W.

When applying the single-phase test to a three-phase transformer, then A, B, C, labels indicate the same values as in the three-phase test:

  • Phase to ground values of these voltages: A – U1U-1N, B – U1V-1N, C – U1W-1N
  • Phase to phase values: A – U1U-1V, B – U1V-1W, C – U1U-1W.

Please note that here labels A, B and C don’t indicate the leg of the magnetic circuit: A first, B middle and C the third (this is where the confusion comes to play – mixing the phase or leg of the transformer with the labels).

Depending on the configuration of a transformer, there are situations where two phases on two different legs are designated with one label. They are compared with the associated two-phase induced voltage (on the secondary winding). This example is a typical comparison of two phases (line to line voltage) on both sides of the transformer. The two phases are on two different legs of the magnetic circuit.

Transformer labels without accessible neutral
Example 1 – Transformer without accessible neutral

It would be incorrect to interpret the result of a measurement designated as A or B or C as values associated with the phases U, V or W.

Depending on the winding configuration, winding under test can be located on one or more magnetic legs. In Example 1, the tested windings are located on two legs, while in Example 2 with zig-zag configuration, they are on all three legs. Here the flux induces voltages measured on all three legs of the transformer magnetic core. Furthermore, voltages induced by the half-windings on the same leg are canceling each other (third leg).

Example 2 – Transformer with zig-zag configuration

Let’s look now at one detected defect and analyze the location of the problem. In Example 3, incorrect analysis of the labels associated with the phases can lead to pinpointing the problem to the wrong winding.

Example 3 – Transformer with a defect

At this transformer with configuration Dyn1, a problem exists at the third leg of the magnetic core. The true three-phase test showed that the excitation current of the B label is 7 times higher than the other two. If this result was associated with the middle leg of the magnetic core, labeled as B, a big mistake would be made.

The label B is associated with the test applied between terminals 1V-1W. This B value of the excitation current is associated with the third leg of the magnetic core, as per the diagram is shown above.

Table 1 – Measurement results

Thus, the result labeled as B shows in fact a problem with the third leg of this transformer. Obtained values are shown in Table 1.

Note: At the end of the TRT user guide you can find the table of all transformer configurations which should be used when diagnosing the problem area of various transformer connections.

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August 13, 2019

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