PURPOSE: To attain a furthermore exact diagnosis of an water tree deterioration by rectifying and integrating a pulsating current contained in a grounding line current, and converting it to a sub-DC shaped waveform.
CONSTITUTION: The grounding line current (ie) detected by a current transformer 3 is inputted to an LPF 11 to eliminate AC charged current components, and after the amplification 15, a single frequency signal is taken out by a BPF 12 from the pulsating current components and subjected to narrow-band amplification by an amplifier 16. That is, the pulsating current components are constituted of extremely low frequency components such as lower than several Hz, therefore, the frequency for narrow band amplification, which is preferably lower than 10 Hz or more preferably as low as 1 Hz, attains the detection for minute pulsating current components to improve the measuring accuracy. Also the influence of noise components is reduced by the narrow band amplification. Then, the minute movement part of current waveform required for the deterioration diagnosis is continuously converted to the smooth stable part of low level by means of rectifying 13 and integrating 14 the output of amplifier 16, so this part is detected as the DC output corresponding to the size of pulsating current.
TSUNODA YOSHIO
SAKAI SHIGEO
