FiGURE 64.6 Transformer circuit model Positive/Negative Sequence Zero Sequence FIGURE 64.7 Sequence equivalent transformer circuits. Magnetic(core) loss P=Pe+ Ph=vi/R (6421) Core eddy current loss = P Core hysterisis loss Ph Hence. 2l=fiRe vi/R (64.2) A second concern is fluctuation of secondary voltage with load. A measure of this situation is called voltage regulation, which is defined as follows: Voltage Regulation(VR) (64.23) where V2FL =rated secondary voltage, with the transformer supplying rated load at a user-specified power factor, and VINL secondary voltage with the load removed(set to zero), holding the primary voltage at the full load value A complete performance analysis of a 100 kVA 2400/240 V single-phase transformer is shown in Table 64.1 64.4 Transformers in Three-Phase Connections Transfoldings t be accounted for. The three sets of windings may be individually connected in wye or delta rmers are frequently used in three-phase connections. For three identical three-winding transformers, in any combination. The symmetrical component transformation can be used to produce the sequence equiv alent circuits shown in Fig. 64.7 which are essentially the circuits of Fig 64.3 with R, and Xm neglected The positive and negative sequence circuits are valid for both wye and delta connections. However, Y-A connections will produce a phase shift which is not accounted for in these circuits c 1999 by CRC Press LLC© 1999 by CRC Press LLC Magnetic (core) loss: Pc = Pe + Ph = V1 2/Rc (64.21) Core eddy current loss = Pe Core hysterisis loss = Ph Hence: SL = I¢ 1 2 Req + V1 2/Rc (64.22) A second concern is fluctuation of secondary voltage with load. A measure of this situation is called voltage regulation, which is defined as follows: (64.23) where V2FL = rated secondary voltage, with the transformer supplying rated load at a user-specified power factor, and V2NL = secondary voltage with the load removed (set to zero), holding the primary voltage at the full load value. A complete performance analysis of a 100 kVA 2400/240 V single-phase transformer is shown in Table 64.1. 64.4 Transformers in Three-Phase Connections Transformers are frequently used in three-phase connections. For three identical three-winding transformers, nine windings must be accounted for. The three sets of windings may be individually connected in wye or delta in any combination. The symmetrical component transformation can be used to produce the sequence equiv￾alent circuits shown in Fig. 64.7 which are essentially the circuits of Fig. 64.3 with Rc and Xm neglected. The positive and negative sequence circuits are valid for both wye and delta connections. However, Y–D connections will produce a phase shift which is not accounted for in these circuits. FIGURE 64.6 Transformer circuit model. FIGURE 64.7 Sequence equivalent transformer circuits. Voltage Regulation (VR) = V V - V NL FL FL 2 2 2