Electrical Technology The windings of three phase transformers The windings of three phase transformers may be connected in by Y or Δ in the same manner as for three single phase transformers. Since the secondaries may be connected either in Y or Δ regardless of which connection is used on the primaries, there must be four ways of connecting the windings of a 3-phase transformer for transformation of 3-phase voltages, namely Y-Y,Δ -Δ, Y-Δ, and Δ -Y. The inter-connections are made inside of the case so that only the terminal leads need to be brought outside the case. 1. Star-Star(Y-Y) Connections. I gives line voltage √3 times phase voltage. It is economical for small high voltage transformers as the number of turns per phase and the amount of insulation required is less. There is no phase shift between primary and secondary voltages. The drawback is that the neutral is unstable because of third harmonic component in the exciting current. 2. Delta-Delta (Δ - Δ ) Connections. The arrangement is generally used in systems which carry large currents on low voltages and specially when continuity of service is to be maintained even though one of the phases develops fault. There is no phase displacement between primary and secondary voltages. There is no distortion of flux as the third harmonic component of magnetizing current can flow in the delta connected primary windings without flowing in the line wires. The conductor is required of smaller x-section as the phase current is 1/√3 times of the line current. No difficulty is experienced due to unbalancing of loads on the secondary side. The disadvantages are that more insulation is required and the voltage appearing between windings and core will be equal to full line voltage in case of earth fault on one phase. 3. Star-Delta(Y-Δ) Connections. Such connections are used principally where the voltage is to be stepped down, as for example, at the end of a transmission line. The neutral of the primary winding is earthed. In this system, line voltage ratio is 1/√3 Times of transformer turn-ratio and secondary voltage lags behind primary voltage by 30° . Also third harmonic currents flow in the to give a sinusoidal flux. 4. Delta-Star (Δ - Y) Connections. Such connections are used where it is necessary to step-up the voltage, as for example, at the beginning of a ht transmission line. In this case neutral point is stable and will not float in case of unbalanced loading. There is no distortion of flux because existence of a Δ -connection allows a path for the third-harmonic components. The line voltage ratio is √3 times of transformer turn-ratio and the secondary voltage leads the primary one by 30°. In recent years, this arrangement has become very popular for distribution system as it provides 3- Ø, 4-wire system Join Our Blog: electricaltechnology.org/ Like & Share: https://facebook/ElectricaTechnology.org
Posted on: Thu, 14 Nov 2013 22:10:24 +0000
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