The influence of the stress level on electro isolating oil state from electric power transformers

Abstract

The paper has three parts. The first part presents the characteristics of the power transformers that make the object of the analysis and their loading characteristics (load curve, over-voltages, over-currents). The second part shows the state characteristics of the electric insulating oil for each transformer under analysis, in terms of some essential state indicators: dielectric rigidity, tangent to the angle of losses and concentration of the seven essential gases (H2, CH4, C2H6, C2H4, C2H2, CO and CO2). The third part of the paper makes an analysis of the level of correlation between the power transformer straining degree and the state characteristics of the electric insulating oil. Based on the results obtained proposals are made regarding the ways of reclaiming and replacing or the electric insulating oil in power transformers depending on the transformer degree of straining. 1. PRELIMINARIES Power transformers (PTs) are elements of utmost importance for the electricity transmission and distribution systems, as they may have the biggest consequences in case of nonavailability, are the most complex equipment and their intrinsic investment value is high. These are the reasons why the reliability of power transformers is an important concern for users, producers and researchers alike. Reliability centred maintenance [7] of a PT is a relatively new trend in research with important economic implications. If we consider the reports recently published in IEEE Transactions on Power Delivery and the Proceedings of CIRED Session only, we can see that an essential instrument of the reliability centred maintenance (RCM) of a PT is the technical diagnosis (TD). The state parameters of the insulation (electro insulating oil and winding insulation) are defining magnitudes for the diagnosis of the technical condition of a power transformer. The analysis of the contents of gases dissolved in the transformer oil is an already proven method widely used in the technical diagnosis of power transformers. Nevertheless, the research works dedicated to this method are still very topical. In [13] a deeper TD method is proposed based on the analysis of the gases dissolved in oil (DGA), using a fuzzy model of the real element (gas contents, concentration ratios). In order to increase the accuracy of PT TD methodology a map is proposed in [12] to be attached to such equipment, showing the evolution of the contents of gases dissolved in oil and their ratios. The proposed maps are