Improvement of the return voltage method for water tree detection in XLPE cables

The return voltage method is a well established diagnostic technique to determine the condition of insulation systems e.g. transformers, mass impregnated cables or XLPE cables. Commonly used parameters to assess the insulation condition from a return voltage measurement are the maximum value, the initial slope and the time of the return voltage peak. Compared to other dielectric diagnostic techniques in the time domain, e.g. relaxation current measurements, a return voltage measurement is less noise sensitive, which is important for on-site measurements. However, the wave shape of the return voltage is not only determined by the relaxation function of the dielectric polarisation. Especially for low loss materials, such as XLPE with its very low conductivity, the wave shape is strongly influenced by leakage currents, which result from e.g. polluted terminations, damaged cable joints or even a comparatively low input resistance of the voltage measuring device. The paper describes a modification of the classical return voltage method to eliminate the influence of unknown parasitic resistances. This is documented by return voltage measurements of XLPE cables with a definite resistance in parallel to the insulation. During the return voltage measurement the specimens were repeatedly short circuited. With the introduction of these intermediate short circuits, the shape of the `genuine´ return voltage without any parallel resistances can be reconstructed