DC testing has been accepted for many years as the standard field method for performing high-voltage tests on cable insulation systems.
Whenever DC testing is performed, full consideration should be given to the fact that steady-state direct voltage creates within the insulation systems an electrical field determined by the geometry and conductance of the insulation, whereas under service conditions, alternating voltage creates an electric field determined chiefly by the geometry and dielectric constant (or capacitance) of the insulation.
Under ideal, homogeneously uniform insulation conditions, the mathematical formulas governing the steady-state stress distribution within the cable insulation are of the same form for DC and for AC, resulting incomparable relative values; however, should the cable insulation contain defects in which either the conductivity or the dielectric constant assume values significantly different from those in the bulk of the insulation,the electric stress distribution obtained with direct voltage will no longer correspond to that obtained with alternating voltage.
As conductivity is generally influenced by temperature to a greater extent than the dielectric constant, the comparative electric stress distribution under DC and AC voltage application will be affected differently by changes in temperature or temperature distribution within the insulation. Further-more, the failure mechanisms triggered by insulation defects vary from one type of defect to another. These failure mechanisms respond differently to the type of test voltage utilized.
For instance, if the defect is a void where the mechanism of failure under service ac conditions is most likely to be triggered by partial discharge, application of direct voltage would not produce the high partial discharge repetition rate that exists with alternating voltage. Under these conditions, dc testing would not be useful.
However, if the defect triggers failure by a thermal mechanism, DC testing may prove to be effective. For example, DC can detect the presence of contaminants along a creepage interface. In the case of joints and accessories, their dielectric properties may differ from that of the cable with regard to conductivity. This may result in a DC stress distribution at the interfaces between the cable and the accessory that is very different from the stress under AC voltage. A careful examination of the system is necessary prior to a DC test in order to avoid difficulties.
Testing of cables that have been service aged in a wet environment (specifically, XLPE) with dc at the currently recommended DC voltage levels (see IEEE P400.1™) may cause the cables to fail after they arereturned to service. The failures would not have occurred at that point in time if the cables had remained in service and not been tested with DC.
Furthermore, from the work of Bach, et al.[B7], we know that even massive insulation defects in extruded dielectric insulation cannot be detected with DC at the recommended voltage levels. After engineering evaluation of the effectiveness of a test voltage and the risks to the cable system, high direct voltage may be considered appropriate for a particular application. If so, DC testing has the considerable advantage of being the simplest and most convenient to use.
The value of the test for diagnostic purposes is limited when applied to extruded insulations, but it has been proven to yield excellent results on laminated insulation systems.
SOURCE: IEEE Std 400-2001 – Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems