Due to the low biodegradability of mineral oil, intense research is conducted to define alternative liquids with comparable dielectric properties. Natural ester liquids are an alternative in focus; they are used increasingly as insulating liquid in distribution and power transformers. The main advantages of natural ester liquids compared to mineral oil are their good biodegradability and mainly high flash and fire points providing better fire safety. The dielectric strength of natural ester liquids is comparable to conventional mineral oil for homogeneous field arrangements. However, many studies showed a reduced dielectric strength for highly inhomogeneous field arrangements. This study investigates at which degree of inhomogeneity differences in breakdown voltage between the two insulating liquids occur. Investigations use lightning impulses with different electrode arrangements representing different field inhomogeneity factors and different gap distances. To ensure comparisons with existing transformer geometries, investigations are application-oriented using a transformer conductor model, which is compared to other studies. Results show significant differences in breakdown voltage from an inhomogeneity factor of 0.1 (highly inhomogeneous field) depending on the gap distance. Larger electrode gaps provide a larger inhomogeneity at which differences in breakdown voltages occur.
The insulation system of power transformers is a key topic which has to be taken into account in order to achieve an optimized design and safe longterm operation. The design of feasible insulation for high voltage direct current transformers requires the understanding of field distributions in insulating materials in both AC and DC operation because the transient stresses are dependent on both conductivities and permittivities and geometry. The steady state DC field stresses are determined by conductivity of insulating material which is normally a combination of oil and paper. Therefore knowledge about the conductivity of the oil is of great importance for design and safe operation in DC case. Moreover the space and surface charges will occur in field space and also will influence the electric field. If such charges are present in a noteworthy amount, models depending on material properties only will probably be an approximation. The conductivity of insulating material is dependent on several parameters, e.g. time, temperature and electrical field strength and the geometry of the arrangement. In this contribution, a new measuring cell with plate type electrodes is presented for analysis of oil conductivity. The measurement accuracy of the set-up is investigated both analytically and experimentally. Consequently, measurements are performed for investigating the time behavior of conductivity. The results show that the determination of ion mobility using polarity reversal test represents to a suitable approach to investigate the transit times more precisely.
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