Influence of cellulose contamination level on electrical conductivity of natural and synthetic ester liquids compared to mineral oil

The design of feasible insulation for converter transformers requires the understanding of field distributions in insulating materials, which are normally a combination of oil and cellulose-based materials. The AC field distributions within the insulation system depend on permittivities of materials. Contrary to AC field distribution, DC field stresses are determined by conductivities of the insulating materials. Therefore, knowledge about the conductivity of the insulating liquid is of tremendous importance for design and safe operation in the HVDC transmission technology. The conductivities of insulating liquids depend on several parameters, e.g. time, temperature, particle type and contamination level, moisture content of liquid and electrical field strengths. In this contribution, the effect of different cellulose contamination levels on electrical conductivities of insulating liquids is presented for natural and synthetic ester liquids compared to mineral oil. The time behavior of electrical conductivity is studied adding different amounts of particles in a defined volume of liquids. The aim of this comparative study is to illustrate the electrical conductivity behavior of mentioned alternative liquids and their applicability as HVDC insulation. The measurements are performed using different types and amounts of dried and wet cellulose particles. In addition, the impact of moisture content of various particle levels on the liquids conductivity is measured. The results indicate an increase of electrical conductivity for higher contamination levels in the liquid samples. Moreover, the water content of cellulose contamination also leads to differences in electrical conductivities in the various insulating liquids.