Fractional derivative formalism for non-destructive insulation diagnosis by polarization–depolarization current measurements

The fractional derivative method is considered as a possible formalism for the non-destructive insulation diagnosis by polarization and depolarization current measurements. The underlying motive is based on non-Debye relaxation processes clearly observed in long-time relaxation experiments with oil–paper, electrolytic and ultra-capacitors, transformers and other systems. Modes of relaxation at different time scales depend on charging prehistory and are sensitive to the state of a system. Fractional formalism provides useful parameters characterizing the state of the insulation system, which can be extracted from polarization–depolarization current measurements. Adequacy of the technique is examined by comparison of theoretical and experimental results for oil–paper, and ultra-capacitors.