Aging phenomena of paper-oil insulating system under different voltage stress

Until now not much attention was paid to behavior of insulating materials under different voltage stresses. For decades, the applied research was done generally under AC voltage. As power electronics was involved in drives and dominated instead of DC drives, new troubles with lifetime occurred. The great influence of higher harmonics does not affect operating parameters only but also it limits the lifetime of the insulation system. Steep voltage rises are generated by power electronics, significant overvoltage occur. As economic aspects play its role, often not sufficient filters are utilized to reduce switching overvoltage, which leads to additive voltage stressing. Recently also DC transmission is planned or even build. DC transmission is especially interesting at very high voltage levels, which brings the space charge phenomena together. Space charge is causing electric field distribution with unexpected stresses affecting the insulation material. There is also the effect of voltage polarity, which has significant influence on material endurance. The paper describes the behavior of paper oil insulation system, which is widely used in field and affected by all voltage mechanisms. For smooth and trouble-free operation a precise lifetime test must be done to determine material limits and recommend the range of voltage stress for reliable operation.