Evaluation of insulating materials during DC voltage charging by DCIC method

We have developed a new method “direct current integrated charge (DCIC) method” for evaluating a deterioration of insulating materials in power equipments. The three important dielectric parameters, that is, conductivity, relative permittivity, and accumulated charge amounts are evaluated simultaneously by a relatively simple treatment of the method. Since the total amount of charges is found as an integration of currents, charging voltage of a measuring capacitor which capacitance is known and which is connected in series to the sample under dc voltage, the time dependence of charges Q(t) can be estimated as an integration of currents I(t) in the method. For understanding of the dielectric properties, we should investigate a current flowing behavior of the materials. Capacitance and permittivity are estimated from the momentary charging current, conductivity from the leakage current and a charge accumulation behavior from the absorption current respectively. The influences of the absorption current to charge accumulation can be estimated to calculate the ratio of the integrated charge at a certain time, which is 300 sec here, and that of initial state of charging as Q₃₀₀/Q₀. When the value Q₃₀₀/Q₀ is 1, there is no charge accumulation in the bulk of the sample. However when the ratio is increasing from 1, some charges may accumulate in the bulk. Investigating the electric stress dependence of the ratio, if the charge accumulation occurs under lower dc stress than in the previous measurement, we can assume the sample has deteriorated something else. As a first step of studies, we have tried to measure some generally used polymers, polystyrene (PS), polypropylene (PP), low-density and high-density polyethylene (LDPE and HDPE) by the method, and have compared the obtained results to the space charge distributions by PEA (pulsed electro-acoustic) method here. As a result, the good correspondence between both measurements was recognized.