Moisture diagnostics of power transformers by a fast and reliable dielectric response method

This contribution introduces a newly developed dielectric response method to measure the dielectric response and calculate the moisture concentration in oil-paper insulated power transformers. This method combines the measurement of charging and discharging currents in time domain (Polarization and Depolarization Currents PDC) and the dielectric spectroscopy in frequency domain (Frequency Domain Spectroscopy FDS). The frequency range from 5 kHz down to 0.1 Hz is measured in frequency domain, whereas the range from 0.1 Hz down to 0.1 mHz or below is measured in time domain. During the measurement the time domain data are continuously transformed into frequency domain. This results into a reduction of the time need of 47-75 % compared to the conventional measurement in frequency domain. Two novel features improve moisture analysis: First, the influence of conductive aging byproducts is compensated, and next, the low frequency data are weighted. Moisture analysis is based on the comparison of transformer measurements to a modeled dielectric response as derived from a data pool. This data pool consists of dielectric responses of new pressboard samples measured at various temperatures and moisture contents. The time dependent properties of oil conductivity are also considered. This follows into a reliable moisture analysis even of aged oil-paper insulations. Examples of onsite moisture determinations compare this new method to other measurement methods like oil and paper sampling with subsequent Karl Fischer Titration.