Monday, January 12, 2015

Transformer Connections for Three-Phase Circuits: T-connected transformers , Scott connection , Zigzag connection and Three-phase-to-six-phase connections.

T-CONNECTED TRANSFORMERS

Another connection involving the use of two transformers to supply three-phase power is the T connection (Figure 14–21). 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 winding, and it is preferred that the teaser transformer contain an 86.6% voltage tap for both the primary and secondary winding. 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 (Figure 14–22). 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.

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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 power line. 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 power line. The same basic connection is made for the secondary. A vector diagram illustrating the voltage relationships of the T connection is shown in Figure 14–23. The greatest advantage of the T connection over the open-delta connection is that it maintains a better phase balance, and the T-connected transformer can be connected to provide a three-phase, four-wire output similar to that of a four-wire wye connection. T-connected transformers used to provide three-phase, four-wire power generally have voltages of 480/277 or 208/120 V. The greatest disadvantage of the T connection is that one transformer must contain a center tap of both its primary and secondary windings.

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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 (Figure 14–24). In the Scott connection, the secondary windings of each transformer pro- vide the phases of a two-phase system. The voltages of the secondary windings are 90° out of phase with each other. The Scott connection is generally used to provide two-phase power for the operation of two-phase motors.

ZIGZAG CONNECTION

The zigzag or interconnected wye transformer is primarily used for grounding pur- poses. It is used mainly to establish a neutral point for the grounding of fault currents. The zigzag connection is basically a three-phase autotransformer whose windings are divided into six equal parts (Figure 14–25). In the event of a fault current, the zigzag 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 is shown in Figure 14–26.

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THREE-PHASE–TO–SIX-PHASE CONNECTIONS

There are some instances when it is desirable to have a power system with more than three phases. A good example of this is when it is necessary to convert or rectify alternating current into direct current with a minimum amount of ripple (pulsations of voltage). Power supplies that produce a low amount of ripple require less filtering. One of the most common three-phase-to-six-phase connections is the diametrical connection (Figure 14–27). The diametrical connection is preferred because it requires only one low-voltage winding on each transformer. If these windings are center-tapped, a neutral conductor can be provided for the six-phase output, permitting half-wave rectification to be used. The high-voltage windings can be connected in wye or delta, but the delta is preferred because it helps reduce harmonics in the secondary winding. A schematic diagram of a diametrical connection with a delta-connected primary and three-phase half-wave rectifier is shown in Figure 14–28.

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