Minimizing dielectric testing time

The water content in the paper insulation is one of the most important parameters for the remaining lifetime of power transformers. Dielectric frequency response (DFR) measurements in a large frequency range allow users to measure this moisture content without the disadvantages of conventional techniques like oil samples. However, up to now, the measurement time of those measurements was quite long and could take many hours, depending on the condition of the asset. Additionally, it was difficult to estimate the required frequency range and therefore the required measurement time. Measuring a too small frequency range leads to shorter measurement times but is likely to cause inaccurate results for the water content whereas measuring a too large frequency range causes unnecessary long measurement and outage times. The current paper shows two methods to decrease the required measurement time significantly. At first, a method is shown how the required frequency range can be reliably detected. The presented technique is based on the individual physical properties of the measured asset and applicable to all oil-paper insulated assets. It can be easily automated so no expert knowledge is required. Additionally, a second method is shown where the required frequency range can be measured faster using an advanced, time domain based technique named PDC+. A case study of 19 dielectric power transformer measurements is used to prove both techniques are working reliably on real power transformers in real test situations on site. The combination of both methods enables even inexperienced users to conduct accurate DFRbased moisture assessments on power transformers within the shortest possible measurement time.