Three-phase Transformer

When this balance is disturbed, due to either an unbalance in the loads or due to a ground fault, a residual or zero phase sequence voltage in the neutral circuit will appear. When one of the phases in the secondary of a three-phase transformer is open circuited and a three-phase supply is applied to its primary windings, there will appear... 

Nominal voltage ratio e.g. 6.6 kV/110 V for two phase or three phase transformers and -transformers 4 times this for line to neutral... 

Following three phase transformations [951] (>298 K), NH C decomposition begins [915] in the solid phase at 423 K but only becomes extensive well above the melting point ( 440 K). Decomposition with the evolution of N20 and H20 from the melt is first order [952,953] (E = 153—163 kJ mole-1), the mechanism suggested involving intermediate nitramide formation. Other proposed schemes have identified NOj [954] or the radical NH2NO [955] (<473 K) as possible participants. Studies [956,957] have been made of the influence of additives on NH C decomposition. 

Three-Phase Transformations in Binary Systems. Although this chapter focuses on the equilibrium between phases in binary component systems, we have already seen that in the case of a entectic point, phase transformations that occur over minute temperature fluctuations can be represented on phase diagrams as well. These transformations are known as three-phase transformations, becanse they involve three distinct phases that coexist at the transformation temperature. Then-characteristic shapes as they occnr in binary component phase diagrams are summarized in Table 2.3. Here, the Greek letters a, f), y, and so on, designate solid phases, and L designates the liquid phase. Subscripts differentiate between immiscible phases of different compositions. For example, Lj and Ljj are immiscible liquids, and a and a are allotropic solid phases (different crystal structures). 

Table 2.3 Common Three-phase Transformations in Condensed Binary Systems...

The Cu-Zn system (see Figure 2.7) displays a number of intermediate solid solutions that arise due to limited solubility between the two elements. For example, at low wt% Zn, which incidently is the composition of alloys known as brass, the relatively pure copper a phase is able to accommodate small amounts of Zn as an impurity in the crystal structure. This is known as a terminal solid phase, and the solubility limit where intermediate solid solutions (such as a + /S) begin to occur is called the solvus line. Some of the three-phase transformations that are found in this diagram include a peritectic (5 - - L -> e) and a eutectoid (5 -> y - - e). Remember that these three-phase transformations are defined for equilibrium coohng processes, not heating or nonequihbrium conditions. 

On the top of the chamber there is a cylindrical extension C which is cooled in order to effect condensation of mercury vapour. Six iron anodes A (there may be twelve or even eighteen of them) are suspended from ring R which covers the top of the chamber. Current for the rectifier is supplied by a three-phase transformer the secondary windings of which have their middles connected in a neutral point in this way the secondary windings have six ends connected to six anodes A by choking coils D in series. 

A Two three-phase transformers, 60 cycles, two mair B Two three-phase transformers, 60 cycles, one mair i incoming supply lines i incoming supply line ... 

This formula applies to single-phase transformers, or to one phase of a three-phase transformer. 

Figure 6.6 Commonly used primary and secondary winding connections for three-phase transformers.

Three methods of arranging the windings of three-phase transformers are commonly encountered star, delta and zig-zag. Each method can be applied to either or both of the primary and secondary windings, Figure 6.6 shows the three forms. 

Phase sequence has an important application in the connection of three-phase transformers. The secondary terminals of a three-phase transformer must not be connected in parallel until the phase sequence is the same. 

A three-phase transformer bank can be easily created by using three single-phase transformers. The two sides of these three transformers can be either connected as a wye or a delta, thus allowing four possible types of connections (1) wye-wye, (2) wye-delta, (3) delta-wye, and (4) delta-delta. These possibilities are shown in Fig. 10.108 to Fig. 10.112. As can be observed in the figures, the primary and secondary phase windings are drawn in parallel. 

Another possible way of producing a three-phase transformer is to use a three-legged core with the primary and secondary windings of each phase placed on the three legs. Such a construction has an advantage over the bank construction in that a smaller amount of core material is used. However, the three-phase bank has higher versatility since it can operate even with one phase out of operation. 

Fig. 20.8 Three phase transformer coils with Y ("wye" or "star") wiring.

Some of the topics covered in the book might be difficult to find in other books, including the avoidance of measurement errors caused by excessively high or low input impedances, reading electrician s (as contrasted to electronic) symbols, understanding the shaded pole ac motor, getting 208 volts from delta or wye three-phase transformers, and optimizing a PK) furnace controller. 

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