Apparatus for detecting degradation of an arrester

An apparatus for detecting degradation of an arrester grounded by a grounding conductor, comprising:

• a wave conversion circuit for producing a total leakage current signal corresponding to the total leakage current flowing through the arrester, said total leakage current signal including a resistive component superimposed on a capacitive component;
• means coupled to said wave conversion circuit for generating a cacelling wave signal Ic based on the total leakage current signal, including a phase shifter circuit for shifting the phase of the cancelling wave signal Ic;
• a differential amplifier for adding said cancelling wave signal Ic to the total leakage current signal and thereby extract a resistive-component leakage current signal Ir at an output thereof;
• a multiplier for multiplying the output signal Ir from said differential amplifier by said cancelling wave signal Ic;
• a time-averaging circuit for averaging an output signal from said multiplier over time;
• and
• means for adjusting the phase shift amount in said phase shifter circuit to produce a 90.degree. phase difference between the signals Ir and Ic so that the output signal from said time-averaging circuit may become zero, thereby bringing the phase of the cancelling wave signal Ic in agreement with the phase of the capacitive component of the total leakage current signal to detect at the output of said differential amplifier the resistive component of the total leakage current signal indicative of degradation of said arrester.