Information within the dielectric response of power transformers for wide frequency ranges

Dielectric response analysis of power transformers in the frequency domain is being used widely to diagnose its ageing condition and moisture content. The Frequency Domain Spectroscopy (FDS) measurements are normally carried out between 1 kHz and 100 μHz. As low frequency measurements result in long measuring times, measurements in a higher frequency range were investigated to determine their suitability for ageing diagnostics. Unfortunately, winding resonance effects were noticed at higher frequencies and it appears that the response at higher frequencies is not purely of dielectric nature. Higher frequencies are therefore unsuitable to determine the insulation ageing of power transformers. For low frequencies, the dielectric response reflects the properties of the oil, the interfacial polarization effect and the cellulose material. Conducting the measurement over a wide frequency range enables for discriminating between these different properties and increases the accuracy of moisture calculation. However, the frequency range depends on the insulation condition so that new, dry insulations require a very low stop frequency (e.g. 100 μHz) while for conductive (wet, hot) insulations 10 mHz can be sufficient.