Condition assessment of potential transformers

High-voltage potential transformers are important equipment in power plants and energy distribution networks. To monitor their condition, Dissolved Gas Analysis (DGA) was introduced about 30 years ago and has now become the primary method for assessing the condition of potential transformers. It has made it possible to identify different types of faults, since these can be identified by the characteristic distribution patterns of the gases dissolved in the oil.

Recently, time series of the DGA values of oil samples from some measuring transducers showed noticeable and initially inexplicable reductions in the concentration of certain gases. It was already apparent from the first laboratory tests that the distribution pattern of the gas-in-oil concentrations changes significantly when paper is added to the oil samples. Since such effects can affect the accuracy of the interpretation schemes of DGA values, a research project to extend the tests to real objects was applied for.

The investigations planned for this purpose focused on the medium-term concentration profile of characteristic decomposition gases, which were added to the insulating oil of six potential transformer models. In order to narrow down the possible causes of the changed gas patterns, the tested transformers were equipped with both porcelain and composite insulators. Voltage transformer active parts were installed in two transformers, the others were only filled with oil. Additionally, the temperature distribution in the transformers also needed to be recorded with additionally installed temperature sensors and under the influence of different installation conditions.

The primary goal of the research project in cooperation with Kärnten-Netz GmbH, Pfiffner Messwandler AG, VERBUND Hydro Power AG and the association Österreichs E-Wirtschaft was to determine the causes of the decrease in concentration of specific gases. Another goal was to find out the best seasonal time for routine oil sampling. Another objective of the study was to obtain quantitative information on the water vapor and gas tightness of composite hollow insulators.

The results of the investigation led to the unexpected conclusion that not only water and carbon dioxide are preferentially absorbed by insulating paper, but also other important error or key gases.