交流重畳法の水トリー劣化診断原理に関する理論的考察

抄録

AC superposition method we investigated is a new technique for hot line diagnosis of 6.6kV XLPE power cables, and mechanisms for the diagnosis are based on unique phenomenon of water trees under AC electric field. For the phenomenon we have already reported that the slight current at Δ f [Hz] was observed and recognized as a deterioration signal from water trees when AC voltage, whose frequency was selected at commercial frequency multiplied by an even number and added Δ f [Hz], was superposed on to the shielding layer of power cable with water trees. This phenomebon is probably caused by a kind of modulation effect due to some non-linear characteristics of water trees because the frequency Δ f [Hz] is not included in all frequency components of applied and superposed voltage. However, there is no theoretical explanation.
So we analyzed theoretically the time dependence of non-linear conductance of water tree under AC electric field and indicated that the conductance was able to be expanded to Fourier series composed of commercial frequency multiplied by even numbers. Also we formulated the current at Δ f [Hz] and the AC loss current including harmonic waves by using the Fourier series expansion and equivalent circuit of water tree. From the results, the characteristics of these currents confirmed by various experiments were successfully explained and effectiveness of the AC superposition method became apparent.