Partial discharge inception voltage in liquid dielectrics and its definition

Insulating liquids represent the largest part of the insulating system in HV transformers and similar components; they are used for their excellent insulating and cooling performances and, for these reasons, they must also have high dielectric strength, high thermal conductivity and low viscosity. An important test able to give useful information on the properties of the same insulating liquids is the Partial Discharge Inception Voltage (PDIV) determination. PDIV measurement of liquid dielectrics is presently standardized by IEC TR 61294 (Determination of the partial discharge inception voltage-Test procedure), issued in 1993. This test is more sensitive than that used for the determination of the breakdown voltage (IEC 60156 or ASTM D1816 test methods) but requires a more complex experimental setup and, for this reason, it is less used. In IEC TR 61294 is also defined the Partial Discharge Inception Voltage (PDIV) as the lowest value at which at least only one partial discharge occurs of an apparent charge equal to or exceeding 100 pC, when the oil sample is tested under some specified conditions. This definition is slightly different from that given in the IEC 60270 (High-voltage test techniques-Partial discharge measurements) which reports that the PDIV is the applied voltage at which 'repetitive partial discharges are first observed in the test object, when the voltage applied is gradually increased from a lower value at which no partial discharges are observed'. The present paper reports an experimental investigation for comparing PDIVs which may be found in different insulating liquids adopting different definitions, considering that in the nearest future the IEC TR 61294 will be revised and the ongoing project (WK65707) for preparing an ASTM Standard with the same scope.