EQUIVALENT CIRCUIT OF A THREEWINDING TRANSFORMER BASICS AND TUTORIALS

EQUIVALENT CIRCUIT OF A THREE WINDING TRANSFORMER BASIC INFORMATION
What Is The Equivalent Circuit Of A Three Winding Transformer?


Various forms of a three-winding transformer equivalent circuit have been proposed, but the simplest and most useful is the so-called T equivalent circuit, shown in Figure 4.9.

The magnetizing branch is omitted in the T equivalent since the magnetizing impedance is normally much greater than the series impedances. If voltages and impedances are expressed in per unit values, then the ideal transformers can sometimes be omitted also; however, in some cases 1:1 ideal transformers are retained so that the connections to the primary, secondary and tertiary circuits can be properly represented by the equivalent circuit.

In a three-winding transformer, eddy-current losses occur in each winding from stray flux produced by the other two windings, even if the third winding is not carrying any load. Therefore, each series resistance element in the T equivalent circuit of a three-winding transformer represent eddy-current losses produced by currents in other windings.

Hence, a series resistance does not belong to any particular winding but is distributed among all three widings. To derive the series impedance values in the T equivalent circuit, impedance measurements are made of each pair of windings taken two at a time.

One winding is short-circuited with voltage applied to the other winding while the third winding is open-circuited. The current is measured through the winding with the applied voltage. The impedance is equal to the applied voltage divided by that current.

The test setup to measure the impedance between the H and X windings of a single-phase three-winding transformer is shown in Figure 4.10.

The test for a three-phase, three-winding transformer is similar except that three-phase voltages are used. There are three sets of measurements taken. 

The first set of measurements applies a three-phase voltage to the H1, H2, and H3 terminals with the X1, X2, and X3 terminals shorted together and the Y1, Y2, and Y3 terminals open. 

The second set of measurements applies a three-phase voltage to the H1, H2, and H3 terminals with the Y1, Y2, and Y3 terminals shorted together and the X1, X2, and X3 terminals open. 

Finally, a three-phase voltage is applied to the X1, X2 and X3 terminals with the Y1, Y2, and Y3 terminals shorted together with the H1, H2, and H3 terminals open. The ZHX, ZHY, and ZXY impedance values are determined by dividing the voltages by the currents in each test. 

THREE WINDING TRANSFORMER BASIC INFORMATION
What Is Three Winding Transformer?


The three-winding transformer is a subset of multiwinding transformers. In addition to the usual primary and secondary windings, a third tertiary winding is added to each phase. Having three winding can serve several purposes:

• Three windings allow connecting three systems together where each system has a different operating voltage.

• Three windings provide electrical isolation between dual input circuits or dual output circuits having the same operating voltage.

• If the third winding is Δ-connected, this can stabilize voltages, supply third harmonic currents to magnetize the transformer core, filter third harmonics from the system, and provide grounding bank action when the primary and secondary windings are both Y-connected.

Sometimes a tertiary winding may serve more than one function at the same time. For example, a 13.8 kV Δ-connected tertiary winding on a 230 kV– 69 kV Grd.Y-Grd.Y transformer helps to stabilize the primary and secondary voltages, provides grounding bank action to partially shield the primary circuit from secondary ground currents, and provides 13.8 kV supply voltage to a station-service auxiliary transformer.

(Note: When a group of windings are connected in parallel to increase the current capability of a secondary winding, the parallel group is considered one winding and not several separate windings. Using multiple sets of low-voltage windings in parallel is common in large generator step-up transformers; however, these are still considered two winding transformers.)

Sometimes a tertiary winding is intended only to magnetically interact with the primary and secondary windings so it may not have any external terminal connections. In these cases, the tertiary winding is said to be an imbedded tertiary.

Imbedded tertiary windings are found only in three-phase transformers and are always Δ-connected. One corner of the Δ-connected imbedded tertiary winding is sometimes grounded internally to limit capacitively coupled voltages.

For single-phase transformers, the standard labels for the tertiary bushings are Y1 and Y2. For three-phase transformers, the standard labels for the tertiary bushings are (Y0), Y1, Y2, Y3.