On-line monitoring of transformers by means of the transfer function method

High-voltage impulse tests are part of the acceptance tests of power transformers. During the impulse test impulse voltages and currents (e.g. the neutral current) are digitally recorded. The transfer function (transadmittance) of the test object can be calculated as quotient of the spectra of voltage and current. The evaluation of the transfer function offers new possibilities to indicate internal insulation faults of the test object or displacements in the winding arrangement. The transfer function method can also be used for on-line monitoring of transformers in electric power systems. Thereby, the transfer function is calculated from transients caused by switching operations. Measurements of transient signals in substations confirm the suitability of the transfer function method for on-line monitoring of power transformers. But to interpret the measuring results on a voltage transformer and a power transformer in a correct way further research is necessary. Some practical aspects are discussed