Application of a simple method for determining the equilibrium resistivity of dielectric luiquids

For the design of the electrical insulation of an UHVDC (Ultra High Voltage Direct Current) converter transformer the resistivity of the pressboard and mineral oil has to be known. For solid insulation the resistivity is virtually constant, however, in liquids it is dependent on, e.g., applied electric stress and time of electrification. Several methods are discussed in the standards. One way to characterize the oil is the measurement of the equilibrium resistivity, in contrast to an apparent resistivity for oil under high electric stress. Measurements using a low voltage triangular shaped voltage form allow calculation of equilibrium resistivity. The oils investigated show equilibrium resistivity in the range 5E11 Ohm·m to 5.96E12 Ohm·m, which are typical values for transformer oil. There is good agreement between these results and reference measurements using dielectric response