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Calculations for Short Circuit Withstand Capability of a Distribution Transformer

Geno Peter .P

During normal lifetime, transformers are submitted to a variety of electrical, mechanical and thermal stresses. One of the most critical situations is that caused by external short circuits, which produces high currents in the transformer windings and hence high internal forces in the windings. These forces are potential sources for damaging transformers. In this paper the different parameters for short circuit withstand capability of a transformer is calculated considering a 1500KVA, 22KV/415V, Dyn11 Distribution Transformer .The short circuit withstand test of a Transformer cannot be performed in the Test Laboratory of any manufacturer. There is a need for a high test field power, especially for large units with ratings of 100MVA and above. The tests can only be performed at a few powerful test stations, such as those operated in KEMA (Netherlands), EDF (France), CESI (Italy) or IREQ (Canada) [1]. Short circuit tests are expensive. The cost of the test itself is high. The cost also increases due to activities such as, transporting the transformer under test from the factory, local installation at the test laboratory and again at the factory, untanking and inspection, repetition of dielectric tests etc. For this reason the IEC standards also permits demonstration of the short circuit ability using calculation and design considerations.