Influence of the initial condition of the insulation liquid on the formation of fault gases due to electrical transformer faults

Abstract

In the energy supply system, power transformers are one of the most important components with regard to the transmission and distribution of energy. Particularly at higher voltage levels, a mixed insulation of paper and insulation liquid is used in transformers. With increasing operation time of a component, the applied electric field and thermal stress lead to aging processes in the form of a decomposition of the insulation liquid, whereby fault gases are formed, and the dielectric properties increasingly deteriorate. Additionally, faults can occur within a transformer, which similar to the aging mechanisms results in the formation of fault gases. A distinction is made between electrical and thermal faults. To detect these faults, samples of the insulation liquid are taken and examined for fault gases using DGA. The gas concentrations can be used to perform a fault analysis using the existing interpretation methods. Nowadays, ester liquids are increasingly being used as insulation liquids instead of conventional mineral oil-based insulation liquids due to their better environmental compatibility and good properties in terms of burning and flash point. However, since the DGA interpretations methods were developed on the basis of mineral oil-based insulation liquids, it is not confirmed whether these are applicable for ester liquids.

Since an electrical fault is more severe than a thermal fault, early detection and countermeasures are essential to prevent the component from being critically damaged or destroyed. Therefore, electrical faults were simulated in this work and investigations were carried out with regard to conventional fault gases and higher-value C₃-C₅ fault gases in order to improve conventional interpretation methods and thus, enable earlier detection of electrical faults.

In this thesis, electrical stress in the form of partial discharges and lightning impulse voltages was simulated and the resulting fault gas formations were investigated.

The aging behavior and thus, the formation of fault gases of the natural ester liquid Envirotemp FR3 as a reaction to electrical stress in the form of partial discharges as a function of the applied energy and the water content was therefore investigated as part of this work. A significant increase in fault gas formation was observed for both, increasing applied energy and higher water contents. In addition to the conventional fault gases, higher-value C₃-C₅ hydrocarbons were detected as well.

For the investigations with lightning impulse voltage, three mineral oil-based insulation liquids and two ester liquids were compared with regard to the formation of fault gases using different influencing parameters. The applied energy, the water content, and the electrode gap were varied as part of these investigations. Additionally, the influence of voltage polarity was investigated for the natural ester liquid Envirotemp FR3.

With regard to the applied energy, the water content, and the electrode gap, it was not only possible to identify differences between the mineral oil-based insulation liquids and the ester liquids in terms of fault gas formation. Additionally, there were also clear differences within the mineral oil-based insulation liquids investigated in this work. The same applies for the fault gases formed within the ester liquids. For the variation of voltage polarity, however, a significant increase in fault gas formation was observed with positive polarity compared to negative polarity. Furthermore, it was established that a higher number of incomplete impulses leads to a significant reduction in the fault gases formed.

In the course of the investigations, possible characteristic fault gases could be identified for each of the insulation liquids on the basis of several influencing parameters. Thus, the basis of existing interpretation methods of the dissolved gas analysis was specified.