Comparison of Different Methods for Characterization of DC Conductivity of Insulating Polymers

Knowledge of the electrical conductivity of polymer insulation materials is important for designing cable components for high-voltage direct-current (HVDC) applications. The aim of this study is to compare three measuring techniques, namely, charging current, surface potential decay (SPD) and dielectric response measurements. The first method is time consuming because it requires that the current through a material is recorded while a direct-current (DC) voltage is applied for a prolonged period of time. The DC conductivity is calculated from the steady state current, which is typically only approached after several hours to days. In SPD experiments, the decay of a potential, induced by corona charging, is monitored and utilized to estimate the field-dependent conductivity from one single measurement. Frequency domain dielectric spectroscopy (FDS) measurements allow to estimate the DC conductivity from the low frequency response. Two materials, low density polyethylene (LDPE) and cross-linked polyethylene (XLPE), were used. The SPD measurements overestimated the conductivities of both materials compared to those obtained by the charging current method for the entire studied range of temperature (30-70° C) and electric field (10-50 kV/mm). At the same, FDS measurements underestimated the temperature dependency of the conductivity for both LDPE and XLPE. It was noticed that the SPD technique demonstrated a very good repeatability and required significantly shorter measuring time compared to other methods.