A comparison of oil-impregnated pressboard electrical conductivity behavior for two different high density pressboards

Estimation of material parameters is an essential step during design of high voltage direct current (HVDC) insulation systems. The key role of insulation coordination in power transformers is to provide the required insulation among different voltage classes to prevent undesirable phenomena like breakdown. The design of feasible insulation for transformer requires understanding field distributions in insulating materials. HVDC equipment is normally stressed with AC, DC and superimposed stresses during its operation time. As a result, deep knowledge of permittivity and conductivity of the insulation materials is necessary for construction of reliable insulation systems. In this investigation, conductivity of two oil-impregnated high density pressboard plates with a thickness of 2 mm from different manufacturers is compared. During the polarization and depolarization currents measurement, the oil-impregnated pressboard samples are located in a cylindrical cell with plate electrodes, which is filled with oil. The preparation of the pressboard plates is a very sensitive step during these measurements, because small amounts of impurities and imperfect impregnated samples cause a significant difference on the measurement results. Hence, the preparation of the samples is precisely described in different steps to insure the same treatment of the pressboard samples. Moreover, the electrical conductivity of two groups of high density pressboards is investigated depending on field strength, temperature and moisture content of the specimens. For this purpose, a step voltage is applied to the pressboard sample and the polarization current is measured over 10,000 s. Furthermore, the behavior of board conductivity is studied at three different field strengths, 0.5, 1 and 3 kV/mm, at 30 °C. Finally, a comparison takes the influence of temperature on electrical conductivity into account. In addition, the measurement results of the relative permittivity for oil-impregnated pressboard are presented depending on the moisture content of the samples.