Three-Phase Transformers -- part 3

Closed Delta with Center Tap

Another three-phase transformer configuration used to supply power to single phase and three-phase loads. This circuit is virtually identical to the circuit shown with the exception that a third transformer has been added to close the delta. Closing the delta permits more power to be supplied for the operation of three-phase loads. In this circuit, it is assumed that the three-phase load has a power requirement of 75 kilovolt amperes and the single-phase load requires an additional 50 kilovolt-amperes.

Three 25-kilovolt-ampere transformers could be used to supply the three-phase power needed (25 kVA x 3 = 75 kVA). The addition of the single-phase load, however, requires one of the transformers to be larger. This transformer must supply both the three-phase and single-phase load, which requires it to have a rating of 75 kilovolt-amperes (25 kVA 1 50 kVA = 75 kVA).

In this circuit, the primary is connected in a delta configuration. Because the secondary side of the transformer bank is a delta connection, either a wye or a delta primary could have been used. This, however, is not true of all three phase transformer connections supplying single-phase loads.

+++++20 Single-phase loads supplied by a wye-delta transformer connection.

Closed Delta without Center Tap

+++++21 Three-phase four-wire connection.

+++++19 Closed-delta connection with high leg.

In the circuit shown, the transformer bank has been connected in a wye-delta configuration. Notice that there is no transformer secondary with a center-tapped winding. In this circuit, there is no neutral conductor. The three loads have been connected directly across the three-phase lines. Because these three loads are connected directly across the lines, they form a delta connected load. If these three loads are intended to be used as single-phase loads, they will in all likelihood have changing resistance values. The result of this connection is a three-phase delta-connected load that can be unbalanced in different ways. The amount of current flow in each phase is determined by the impedance of the load and the vectorial relationships of each phase. Each time one of the single-phase loads is altered, the vector relationship changes also. No one phase will become overloaded, however, if the transformer bank has been properly sized for the maximum connected load.

+++++22 Neutral conductor is supplied by the incoming power.

Delta-Wye Connection with Neutral

The circuit shown is a three-phase transformer connection with a delta-connected primary and wye-connected secondary. The secondary has been center-tapped to form a neutral conductor. This is one of the most common connections used to provide power for single-phase loads. Typical voltages for this type of connection are 208/120 and 480/277. The neutral conductor carries the vector sum of the unbalanced current. In this circuit, however, the sum of the unbalanced current is not the difference between two phases. In the delta connection, where one transformer was center-tapped to form a neutral conductor, the two lines were 180 degree out of phase when compared with the center tap. In the wye connection, the lines are 120 degree out of phase. When all three lines are carrying the same amount of amperage, the neutral current is zero.

A wye-connected secondary with center tap can, under the right conditions, experience extreme unbalance problems. If this transformer connection is powered by a three-phase three-wire system, the primary winding must be connected in a delta configuration. If the primary is connected as a wye connection, the circuit will become exceedingly unbalanced when load is added to the circuit. Connecting the center tap of the primary to the center tap of the secondary will not solve the unbalance problem if a wye primary is used on a three-wire system.

If the incoming power is a three-phase four-wire system as shown however, a wye-connected primary can be used without problem.

The neutral conductor connected to the center tap of the primary prevents the unbalance problems. It is a common practice with this type of connection to tie the neutral conductor of both primary and secondary together as shown. When this is done, however, line isolation between the primary and secondary windings is lost.

+++++23 T-connected transformers.

+++++24 T-connected transformers with same voltage rating.

+++++25 Voltage vector relationships of a T connection.

+++++26 T-connected transformers provide a three-phase four-wire connection. X0 is used as a center tap to the other phases. Main primary Main secondary; Teaser primary; Teaser secondary.

T-Connected Transformers

Another connection involving the use of two transformers to supply three-phase power is the T connection. In this connection, one transformer is generally referred to as the main transformer and the other is called the teaser transformer. The main transformer must contain a center or 50% tap for both the primary and secondary windings, and it is preferred that the teaser transformer contain an 86.6% voltage tap for both the primary and secondary windings. Although the 86.6% tap is preferred, the connection can be made with a teaser transformer that has the same voltage rating as the main transformer. In this instance, the teaser transformer is operated at reduced flux. This connection permits two transformers to be connected T instead of open delta in the event that one transformer of a delta-delta bank should fail.

Transformers intended for use as T-connected transformers are often specially wound for the purpose, and both transformers are often contained in the same case. When making the T connection, the main transformer is connected directly across the powerline. One primary lead of the teaser transformer is connected to the center tap of the main transformer, and the 86.6% tap is connected to the powerline. The same basic connection is made for the secondary. A vector diagram illustrating the voltage relationships of the T connection is shown. The advantages of the T connection over the open-delta connection is that it maintains a better phase balance.

This permits the T connection to be operated as a three-phase four-wire wye connection like that of a wye connection with center tap. When connected in this manner, the T-connected transformers can provide voltages of 480/277 or 208/120. The greatest disadvantage of the T connection is that one transformer must contain a center tap of both its primary and secondary windings.

+++++27 Scott connection.

Scott Connection

The Scott connection is used to convert three-phase power into two-phase power using two single-phase transformers. The Scott connection is very similar to the T connection in that one transformer, called the main transformer, must have a center, or 50% tap, and the second, or teaser transformer, must have an 86.6% tap on the primary side. The difference between the Scott and T connections lies in the connection of the secondary windings. In the Scott connection, the secondary windings of each transformer provide the phases of a two-phase system. The voltages of the secondary windings are 90 degree out of phase with each other. The Scott connection is generally used to provide two-phase power for the operation of two-phase motors.

Zig-Zag Connection

The zig-zag or interconnected-wye transformer is primarily used for grounding purposes. It is mainly used is to establish a neutral point for the grounding of fault currents. The zig-zag connection is basically a three-phase autotransformer whose windings are divided into six equal parts. In the event of a fault current, the zig-zag connection forces the current to flow equally in the three legs of the autotransformer, offering minimum impedance to the flow of fault current. A schematic diagram of the zig-zag connection.

+++++28 Zig-zag connection.

+++++29 Schematic diagram of a zig-zag connection.