Frequency response analysis for exact power transformer impedance

The frequency response analysis (FRA) of the transformer impedance is known as a diagnosis method for the damages in a transformer. It is reported the transfer function as FRA. However, the impedance of the transformer cannot be measured correctly in the high-frequency range when the influence by the stray capacitance becomes large. Also, it is not obvious the influences of residual flux to the transfer function or the impedance. Here, the exact measurement method for the FRA of transformer impedance is described. The authors proposed the measurement circuit diagram in which the influence of the stray capacitance to the earth can be included. In the circuit diagram, the resistance for the current measurement is connected between the power supply and the transformer terminal. Therefore, it can measure the exact transformer impedance. The magnetic flux remains in the iron core after the transformer was interrupted. In the FRA measurement, the residual flux has an effect on the impedance. The residual flux can be removed by supplying attenuated rectangle voltage waveform. It is cleared the difference of the impedance at the frequency range from DC to resonance point. The exact transformer impedance will be derived by this proposed measurement method and de-magnetising.