Three Phase Transformers Most three phase transformers use a three legged

# Three phase transformers most three phase

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Three-Phase Transformers Most three- phase transformers use a three- legged laminated iron core Large steel bars with insulated bolts pressing the laminated core together Three-Phase Transformers Each transformer is protected by fuse cutout and surge arrester In the distribution network: Three single-phase transformers can form a three-phase transformer bank as shown in this photograph Three-Phase Transformers Figure 7.35 Very large three-phase network transformer (courtesy Siemens, Erlangen Germany). Three-Phase Transformers Figure 7.36 A three-phase wye-delta transformer with three-legged iron core. The Y connected primary current is The delta winding current is Delta connected line current is rate rate rate V V I 3 3 3 Y S S rate rate rate V V I 3 3 S S rate rate rate line V V I I 3 3 3 3 S S Three-Phase Transformers The flux in the legs induces voltage in the coils. When the transformer is loaded, a current will flow in both the primary and secondary windings. The voltages and currents are in phase in the windings placed on the same leg. This implies that in the case of a wye-delta connection, the line-to-ground voltages V AN , V BN and V CN induce line-to-line voltages V ab , V bc and V ca that are in phase, respectively
Transformers Karady & Holbert 13 EEE 360 Three-Phase Transformers Operation principles The iron core has three legs with equal cross-sections . Each leg of the core carries primary and secondary windings. In Figure 7.36, the primary (A,B,C) is wye and the secondary (a,b,c) is delta connected. However, transformers can be connected in any of the connections listed on the next slide. The transformer is usually supplied by a balanced three- phase voltage, which produces a flux in each leg of the iron core. The sum of the fluxes is zero, which eliminates the need for a return path. Three-Phase Transformers The table shows the usual connections for three-phase transformers wye-wye (Y-Y) seldom used, imbalance and 3 rd harmonics problems wye-wye-delta (Y-Y- ) frequently used to interconnect high voltage networks (240 kV/345 kV). The delta winding filters the 3 rd harmonics, equalizes the unbalanced current, and provides a path for ground current wye-delta (Y- ) frequently used as step down (345 kV/69 kV) delta-delta ( - ) used for medium voltage (15 kV), one of the transformers can be removed (open delta) delta-wye ( -Y) step-up transformer in a generating station Three-Phase Transformers Operation Analysis Wye–Wye Connection Wye-Wye Connection Figure 7.37 Wye connected three- phase transformer In this transformer, the primary and secondary currents and voltages are in phase. The neutral points are grounded.

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