Leakage current and flashover onset with various conductivities on HVDC glass insulators

Insulator leakage current monitoring is of interest to power utilities since it is usually a good indicator of the performance of insulators. Currently, the South African power utility, Eskom, does not have a device that can clamp around in-service energised HVDC insulators. This device would be required for continuous real time monitoring of the line insulators and secondly, for use by live line workers to determine if it is safe to work on energised HVDC insulators. A prototype device consisting of magnetic field sensors was developed and initial leakage current (LC) measurements have been conducted at Eskom’s Corona Cage High Voltage Test Facility. A shunt resistor was used as a control measurement. Two experiments are performed on the HVDC glass insulators. The first experiment determines the relationship between current and increasing pollution layer conductivity. This experiment shows that flashovers can occur for current of 1.4 mA and higher. At each pollution layer conductivity, 5 insulator energisations are performed. The current measured increases with an increase in pollution but a sharp decay for current is observed in between the 5 consecutive measurements. The second experiment was performed to determine the probability of a flashover occurring due to a certain current with an equivalent salt deposit density of 0.03 mg/cm2. The insulator string had a unified specific creepage distance of 9.6 mm/kV. The second experiment showed that there was a chance of a flashover occurring at a current level of 0.6 mA with a salt deposit density of 0.03 mg/cm2. The next step in the research regarding leakage current on HVDC will be to place the developed sensor and shunt resistor on Eskom’s high voltage transmission line to perform in-situ measurements around glass and composite insulators.