Comparison of the dielectric performance of polluted composite insulators under AC and DC stress using the rapid flash over method

A reduction of the asset classes will lead to leaner asset health management. In this paper a possible reduction for overhead line insulators, by combining AC and DC insulators in a common asset class, is investigated.

The main factor for determination of the asset health index of an insulator as a component of the overhead line is its dielectric performance. To ensure effective performance and ageing forecasts, the asset health models must be adapted to the actual conditions of specific lines by testing the pollution flashover characteristics of insulators polluted in service.

The access to asset data is essential for successful asset management. The choice of maintenance strategy is influenced by the information at hand. When there are limited data and insight into an asset's condition, only preventive or corrective maintenance strategies can be utilized.

Classical up-and-down tests to determine U50 have the disadvantage that they require multiple test objects to archive statistic relevance. Multiple identical test objects with the same natural pollution and ageing situation are not available.

This study utilizes the rapid flash over method specified by IEC TS 63414 CD to assess pollution flashover characteristics under both AC and DC voltage stress. This method, with its repetitive voltage stress applied to one test object, is suitability to address the requirements outlined. The resulting flashover voltage U50,RFO is not identical to U50 but is at least comparable among itself.

Basis for the rapid flashover test is a polluted insulator – in service or artificially – which is subject to a stepwise raising voltage test while being kept in a defined fog environment.

The research involves testing model composite insulators with two designs, an alternating shed insulator and an underrib shed insulator, both having a creepage distance of approximately 650 mm. The dimensions were chosen to fit the existing fog-chamber. Both insulators are subjected to multiple pollution levels after receiving an anti-hydrophobic treatment with kaolin.

The pollution flashover process is characterized by breakdown current and applied voltage and further analysed through leakage current measurements and optical analysis using a high-speed video camera.

At completion of the measurements, the results are analysed, discussed and requirements for the comparability of AC and DC pollution flashover performance and the repeatability of the test, especially concerning DC voltage, are outlined. First measurement results show a difference in flashover voltage behaviour between AC and DC voltage stress.