In 1989, the low-frequency superposition method was developed as an online diagnostic method for XLPE cables for 6 kV power distribution, and its application to the field has started. However, various problems became clear, and the authors developed the AC insulation resistance measurement method in 1993 with the aim of solving these problems.
Since then, the results of online diagnostics using AC insulation resistance measurement have been accumulated, and the ability to detect deterioration of water trees has been confirmed to be excellent. However, there are unclear points such as the measurement range and the validity of the measurement results. Therefore, the validity of the measurement range and measurement results was verified by numerical calculation using an equivalent circuit.
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