Transformer Configurations

Transformers are built in both single-phase and three-phase power. Up to this point, we have focused on single-phase power. You will recall from earlier modules that single-phase means two power lines as an input source. This means that a voltage transformation is accomplished with one primary and one secondary winding.

You also know from previous modules that most power distributed today is three-phase AC. Because power companies distribute generated power on three lines as three-phase power, three-phase transformers with three windings are integral to the electrical power system. The three windings of the Three-Phase Transformer are connected in the proper sequence to match the utility's incoming power.

Configuration for Single Phase Power
There are different configurations for both single-phase and three-phase systems. Let's look at single-phase first. Single-phase transformers are often used to supply power for residential lighting, receptacle, air-conditioning, and heating needs.

A transformer with a 120 volt AC secondary can take care of the lighting and receptacles. But, a transformer with a 240 volt AC secondary could handle all the residential needs mentioned. A 240 volt AC secondary could handle the heavier 240 volt power requirements of air conditioning and heating. The same 240 volt AC secondary could handle the 120 volt AC needs by tapping the secondary in the center.

Single phase transformers can be made even more versatile by having both the primary winding and secondary winding made in two equal parts. The two parts of either winding can then be reconnected in series or parallel configurations.

Series Configuration Parallel Configuration

Single-phase transformers usually have their windings divided into two or more sections. When the two secondary windings are connected in series, their voltage is added. When the secondary windings are connected in parallel, their currents are added.

For example, assume each secondary winding is rated at 120 volts and 100 amps. When series-connected, it would be 240 volts at 100 amps, or 24KVA. When parallel-connected, it would be 120 volts at 200 amps, or 24KVA.

In series connections, care must be taken to connect the coils so that their voltages add. If the reverse happens, a short-circuit current would flow in the secondary, causing the primary to draw a short circuit from the source. This would damage the transformer, as well as the source, and perhaps the connector.

Configuration for Three Phase Power
Power may be supplied through a three-phase circuit containing transformers in which a set of three single-phase transformers is used, or one three-phase transformer is used. Using three single-phase transformers to accomplish the task is cumbersome, but it can be done. When used in this way, the arrangement is called a Transformer Bank

When a considerable amount of power is involved in the transformation of three-phase power, it is more economical to use a three-phase transformer. The unique arrangement of the windings and core saves a lot of iron, losses, space and money.

Delta and Wye Defined
There are two connection configurations for three-phase power:
Delta and Wye.

Delta and wye are Greek letters that represent the way the conductors on the transformers are configured. In a delta connection, the three conductors are connected end to end in a triangle or delta shape. For a wye, all the conductors radiate from the center, meaning they are connected at one common point.

Both the primary and secondary can have either of these configurations. The four possible connection configurations are:

They can be used with either three single-phase transformers or one three-phase transformer. Figure 16 shows three single-phase transformers in a wye-to-wye configuration. Figures 17 and 18 show three-phase transformers, in a wye-to-delta configuration, and a delta-to-delta configuration respectively.

Recalling the electrical symbol for a transformer from Module 3, Fundamentals of Electrical Distribution, the delta and the wye symbols are often used to indicate the primary and secondary winding connections in a one-line diagram.

Many high schools use a wye-to-wye step-down transformer bank, as shown here. The versatility of the power is the key to its popularity.

The system provides three-phase, 208-volt power for three-phase motor loads, such as heavy equipment in the Industrial Education department. It also provides single-phase, 208-volt power for small single-phase motor loads, such as science lab equipment. Of course, it can also produce single-phase, 120-volt power for lighting loads, used throughout the building.