The interpretation of return voltage curves of paper-oil-cables

The paper reports on return voltage measurements performed on different 10 kV paper-oil insulated belted-type cables used in network systems with isolated neutral and with effective earthing of the neutral. The different cable design, e.g., the increased thickness of insulation between conductor and sheath for isolated systems, has a strong influence on the return voltage curves. Thus, a diagnostic method, which is exclusively based on single parameters such as the maximum or the initial slope of the return voltage curves, may lead to a misinterpretation. A more sophisticated evaluation method, which uses the time dependence of the complete return voltage curve is proposed in this paper