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POLYPHASE CIRCUITS

[CHAP. 11

The voltage-current phasor diagram is shown in Fig. 11-11. Note that with one line current calculated, the other two can be obtained through the symmetry of the phasor diagram. All three line currents return through the neutral. Therefore, the neutral current is the negative sum of the line currents:

Fig. 11-11

Since the neutral current of a balanced, Y-connected, three-phase load is always zero, the neutral conductor may, for computation purposes, be removed, with no change in the results. In actual power circuits, it must not be physically removed, since it carries the (small) unbalance of the currents, carries short-circuit or fault currents for operation of protective devices, and prevents overvoltages on the phases of the load. Since the computation in Example 11.3 proceeded without difficulty, the neutral will be included when calculating line currents in balanced loads, even when the system is actually threewire.

11.8

EQUIVALENT Y- AND -CONNECTIONS

Figure 11-12 shows three impedances connected in a  (delta) configuration, and three impedances connected in a Y (wye) configuration. Let the terminals of the two connections be identified in pairs as indicated by the labels , , . Then Z1 is the impedance ‘‘adjoining’’ terminal  in the Y-connection, and ZC is the impedance ‘‘opposite’’ terminal  in the -connection, and so on. Looking into any two terminals, the two connections will be equivalent if corresponding input, output, and transfer impedances are equal. The criteria for equivalence are as follows:

Fig. 11-12

Mahmood_Nahvi_eBook_Schaum_s_Outlines_Theory_An  
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