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"Phasing out" is a method to either check the phase shift across a transformer due to the winding characteristics or phasing to another transformer to ensure that the transformer is connected the same way. These tests usually apply to three phase Grid Transformers.

Grid transformers can be wound in a number of ways. The simplist version is described as YY0. That is two Y wound three phase winding laid directly against each other. There is no phase shift ie 0 degrees. Other transformers can be wound YD that is a Y winding wound against a delta winding. Automaticallly there will be a phase shift across this winding.

Field test involve energising the high voltage side with a 3 phase supply. Voltages are then measured across windings. Ie measure the input voltage on Primary side Eg 230 Volts then measure the volts on secondary side between each phase to fill up a test table.

eg Rp Rs, Rp Ys, Rp Bs etc....

From these measurements it is possible by drawing a suitable vector diagram of each phase to show the phasing of each phase, the ratio and the angle of the transformation across the transformer.

An alternative (simpler) method is to use an oscilliscope from this superimpose the to voltages and measure the angle between them. It isn't so effective in the field with large voltages and not quite as accurate.

Lastly to phase the new transformer with the existing network measure the voltages of each phase to the corresponding phase on the network. The reading should be zero if youre phasing is correct and to confirm full phase voltage between phases.

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