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The online monitoring is becoming increasingly important for operators of high voltage assets. One of the most important assets are transformers, especially power transformers, which are mainly insulated with paper together with an insulating liquid for impregnation. An important parameter is the aging condition of the insulating paper, which allows an estimation of the operational safety. This variable is determined by the degree of polymerization (DP-value) and provides information about the mechanical stability of the paper. However, paper samples are currently required for the measurement, which is carried out in a laboratory in accordance with IEC 60450, whereby the paper is destroyed. This study is conducted to determine the DP-value of a paper insulation using a novel, patented optical sensor. The investigation shows that oil-free paper samples have very strong linear correlation between the value measured with the sensor and the DP-value. Therefore, a condition assessment is possible within seconds, as the measurement by means of the optical method is significantly faster and more precise than the conventional viscometric method. It also shows that the assessment is independent of the insulating oil, in which the paper is aged. The sensor could be used for an improved condition assessment of transformers to increase the operational safety.

The assessment of the aging condition is becoming increasingly important for operators of high voltage assets. Power transformers are especially one of the most important units in the electrical power grid. Due to the fact that oil-paper insulation is still the most economical form of insulation, and is therefore found in the majority of transformers, the condition assessment of the paper insulation is decisive for estimating the remaining service life. Since the determination of the condition via furans is associated with certain fluctuations, several other aging parameters are investigated. Therefore, this study intends to determine how those aging markers of the insulating liquid behave in the absence of insulating paper and how the values change with insulating paper; thus, from the difference between important parameters, indicating the paper aging might be possible. For the investigation, a non-inhibited and an inhibited insulating oil as well as a further inhibited oil with a different composition, are tested as the most common insulating liquids. For this purpose, an open system is reproduced, which has permanent contact with the environment air. Accordingly, the study is carried out for long-term thermal stress to obtain samples with a wide spectrum of aging status. The entire study extended over a period of 14–15 weeks at a temperature of 130 °C with performing the measurements of the parameters at various aging times. The study reveals that the results can be used for an improved condition assessment of transformers.

This research presents a comparative study of the ageing process of an oil-paper-insulation using Kraft paper. The main goal of this research is the comparison of the ageing behaviour of the insulation with different weight ratios and the influence on pertinent ageing markers, which are used to estimate the remaining lifetime of an oil-paper insulated transformer. Therefore, an investigation has been conducted to measure different parameters of a non-inhibited insulating oil during a long-term thermal stress over 15 weeks at a temperature of 130 ℃ to examine the influence of the ratio on the formation of these markers. The higher temperature compared to the normal operation temperature of a transformer is used to accelerate the thermal ageing of the insulation material in order to simulate the lifetime of a transformer. In addition to the ageing of pure paper and pure insulating oil, the paper-oil weight ratios 1:5, 1:10 and 1:20 will be investigated. The aim is to study how the respective insulating medium ages individually and to determine the ageing behaviour of the insulating materials when used together as a mixed dielectric, as it can be found in real transformers. Therefore, a typically used uninhibited liquid based on mineral oil is used as insulating liquid. Ageing markers such as water content, acidity as well as DGA analyses and other parameters are investigated. Furthermore, the DP-value of the paper samples is also measured. For this, an open system is reproduced, so that a permanent contact with the ambient air is given to simulate a free breathing transformer. The main aim of this study is to investigate the influence of the various weight ratios and how it influences the possibility of indirectly determining the condition of the insulating paper. For this purpose, ageing markers are to be investigated. The obtained results showed that the weight ratio has no influence on the ageing rate of paper but on some ageing markers.

Even today, oil-paper insulation is one of the most important insulation systems for high-voltage components, which is mainly used in transformers. Due to an increase of regulations and obligations for transformer operators, alternative insulating liquids are becoming more and more important, especially esters, as these liquids are characterized as an environmentally friendly and low flammability substitute. So far there is only little experience on the ageing behaviour of ester-paper insulations and on the formation of ageing products with this mixed dielectric. The aim of this study is to investigate the influence of a synthetic based ester, a rapeseed-based and a soya-based natural ester on the ageing behaviour of a Kraft paper insulation. In this context, the correlation of ageing markers, such as the acid value, the moisture content etc., is measured in the liquid and the current condition of the paper insulation is investigated. It could be observed that the ageing behaviour of studied samples is different to mineral oil based liquids and also differences between synthetic and natural esters could be seen.