FIGURE 64.5 Two-winding transformer-equivalent circuits. All values in per-unit. (a)Ideal case;(b)no load current negligible;(c) precise model. R GX) Circuits(a) and(b)are appropriate when -Zm is large enough that magnetizing current and core loss is 64.3 Transformer performance There is a need to assess the quality of a particular transformer design. The most important measure for performance is the concept of efficiency, defined as follows: (64.19) where Put is output power in watts(kW, MW)and Pin is input power in watts(kw, Mw) The situation is clearest for the two-winding case where the output is clearly defined (i.e, the secondary winding), as is the input(i.e, the primary). Unless otherwise specified, the output is understood to be rated power at rated voltage at a user-specified power factor. Note that Pn P.= sum of losses The transformer is frequently modeled with the circuit shown in Fig. 64.6. Transformer losses are made up of the following component Electrical losses I'i Reg=IFRi+ IR2 (64.20a) Primary winding loss =IR, (64.20b) Secondary winding loss =BR, (6420c) c 1999 by CRC Press LLC© 1999 by CRC Press LLC (64.18) Circuits (a) and (b) are appropriate when –Zm is large enough that magnetizing current and core loss is negligible. 64.3 Transformer Performance There is a need to assess the quality of a particular transformer design. The most important measure for performance is the concept of efficiency, defined as follows: (64.19) where Pout is output power in watts (kW, MW) and Pin is input power in watts (kW, MW). The situation is clearest for the two-winding case where the output is clearly defined (i.e., the secondary winding), as is the input (i.e., the primary). Unless otherwise specified, the output is understood to be rated power at rated voltage at a user-specified power factor. Note that SL = Pin – Pout = sum of losses The transformer is frequently modeled with the circuit shown in Fig. 64.6. Transformer losses are made up of the following components: Electrical losses: I¢ 1 2 Req = I 1 2R1 + I2 2R2 (64.20a) Primary winding loss = I 1 2R1 (64.20b) Secondary winding loss = I2 2R2 (64.20c) FIGURE 64.5 Two-winding transformer-equivalent circuits. All values in per-unit. (a) Ideal case; (b) no load current negligible; (c) precise model. Z R jX R jX m c m c m = + ( ) h = P P out in