Radial stability evaluation and cumulative test of transformer windings under short-circuit condition

The radial instability of transformer winding under short-circuit condition is one of the vital factors which lead to failures. There have been many analytic and simulative investigations in the past. However, few contributions show experiments on transformers for radial stability due to the limitation of test conditions and high costs. This paper introduces the calculation of short-circuit force and puts forward the evaluation method of radial stability. Experiments are performed on real 110 kV transformers in order to validate the proposed method. To obtain the radial critical instability state and short-circuit accumulation characteristics, dozens of short-circuit and accumulation tests have been carried out under high voltage - medium voltage (HV-MV) short-circuit conditions. The experimental results prove that the accuracy of the evaluation method of radial stability and the current deviation is less than 5%. The tests show that the critical radial instability of the transformer can be obtained accurately by accumulating and increasing the value of short-circuit current. The sudden change of reactance during the test can reflect the characteristics of sudden deformation and instability of windings, which can be used as a judgment basis for critical instability. Moreover, the tests confirm the existence of short-circuit cumulative effect in a transformer.