Dielectric response of power transformers and the negative dissipation factor

This paper presents the appropriate measurement circuit and the interpretation scheme of the dielectric response of oil-paper insulated Power Transformers. The traditional three terminal measurement technique (voltage source, current input, guard wire) occasionally leads to the appearance of a negative dissipation factor. An equivalent circuit diagram along with practical measurements was used to investigate the reasons for this problem. These are guard resistance, capacitances to tank, instrument impedance and inductance of windings. Particularly the guard resistance as well as the instruments input resistance should be low. The dielectric measurement is conducted across a wide frequency range in order to discriminate between the influences of oil, interfacial polarization (transformer construction geometry) and water content. By a novel combination of time and frequency domain measurement, a very fast measurement of a wide data range can be accomplished. This is fully detailed in conjunction with measurement examples for power transformers, to illustrate the impacts of connection technique and frequency range on the accuracy of moisture determination.