Characteristics and Winding Arrangement of Three-Winding Transformers

Characteristics of Three-Winding Transformers

The three-winding transformers are used in substations in cases where power plants must feed two voltage levels or when multiple voltage levels are connected at substations. This solution is more economical, taking less space and being easier to maintain instead of using two standard transformers.

Generally, a three-phase three-winding transformer uses Y-Y-Δ connection where primary and secondary windings are Y-connected and the third is Δ-connected. This Δ-connected third provides a closed loop for third harmonic currents to flow, hence there is no third harmonic voltage in the Y-connected windings. Therefore, it provides neutral point for both sides. Further, the third winding may be connected to synchronous condensers in long-distance transmission systems in order to improve the power factor of the line.

The rated capacity of each winding is specified as needed; the rated capacity is defined by the size of the largest windings, usually the primary, taken as 100%. Common configurations include 100/100/50, 100/50/100, and 100/100/100.

Winding Arrangement of Three-Winding Transformers

Three-winding transformers have high-voltage, medium-voltage, and low-voltage windings all wound on the same core. The connection to an AC power source from any one winding would induce different voltages on the other two windings for loads requiring two distinct voltage levels.

Winding Arrangement for Step-Up Transformers:

It goes in an arrangement of medium-voltage winding, low-voltage winding, to high-voltage winding from innermost to outermost.

For this reason, insulation can be assured if the high-voltage winding is on the outermost layer. In such situations, the low-voltage winding has been placed between high-voltage and medium-voltage windings. It does help to distribute the leakage magnetic field evenly and ensures a reasonable leakage reactance, with no undue leakage flux and other additional losses while ensuring good voltage regulation and operational performance.

Winding Arrangement for Step-Down Transformers:

Low-voltage winding, medium voltage winding, high-voltage winding, in that order, from innermost to outermost.

In this respect, the high-voltage winding has to be on the outside for proper insulation. The medium voltage windings are between the high and low simply because of considerations for insulation.

Conclusion

Three-winding transformers are one of the most common applications of the transformer in power systems. They provide an efficient solution in connecting various levels of voltage. Many applications are required from these kinds of arrangements due to their special characteristics; therefore, electrical distribution greatly depends on them.