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Three Phase Transformer: Connection and Configuration

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Document pages: 19 pages

Abstract: 3- phase distribution transformers generally have a Delta (Triangular) connected primary winding and a Star (Wye) connected secondary winding. The secondary Star configuration allows for a 3-phase and Neutral, Dy1 supply providing line and phase voltages and currents. Commonly the Neutral and Earth connections which provide a 5-wire (3-phases + Neutral + Earth) TN-S (separate Neutral and Earth) supply to homes and buildings are connected together at the star point of the three secondary winding, thus the Neutral and Earth are effectively bonded together and remain at the same zero or Earth potential. However, with a TN-C (combined Neutral and Earth) 4- wire (3-phases + Neutral) supply the Neutral wire also acts as the Earth wire, but the neutral wire is still bonded along with a separate Earth electrode to the star point of the secondary winding to provide the required phase voltages and currents. TT and IT configurations have different Neutral and Earthing arrangements. Being a Delta connected Primary winding and therefore a balanced load for the supplying network, there is no Neutral or Earth connection to the three primary winding, (Earthing is done at the generator source) but it is possible that the physical metal casing of the transformer has been bonded to the secondaries Earthing bar for safety.

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