A practical example of power transformer unit winding condition assessment by means of short-circuit impedance measurement

Knowledge of the short-circuit impedance variation is necessary for verifying the observance of the technical conditions (including the design conditions), certain nonconformities in the winding execution, checking the winding behavior in operation, verifying the system and short-circuit calculation (including the correct protection setup), the loading of the same type units, or more importantly, of the different type ones. The measurement of the short-circuit impedance by means of a three phase low voltage power source can be performed as the value of impedances does not depend on the applied voltage. A modern measurement system has been developed and used for performing a great amount of measurements, the results of which have been compared with those of the manufacturer´s, and with those obtained by means of high voltage measurements. At the end of the paper a case study regarding a short-circuit event on a 250 MVA power transformer unit is presented. A baby hawk landed with his wings spread on the tertiary winding bushings causing a three-phase short-circuit that damaged the power transformer unit. The short-circuit impedance measurements that have been performed clearly indicated the mechanical failure of the power transformer unit.