The specific aspects of degradation of electrical insulation under DC voltage

A significant development in high voltage DC transmission technology can be observed, especially in last decade. In addition to design and construction issue, it is necessary to focus also on diagnostic tests during the operation. Electrical insulation systems under DC voltage are affected by degradation stresses, which have a negative impact on their reliability and proper operation. DC voltage is stressing insulation elements and systems in different manner than AC voltage. The DC degradation processes has to be further studied and described particularly because of future development and prognosis of insulation material state. Aging phenomena under AC voltage is well known and precisely described over years. In addition accelerated laboratory aging has been standardized in various standards. However the degradation phenomena under DC voltage behave differently. This paper is concerned on specific aspects of degradation under DC voltage. To describe this phenomenon it is essential to study the structure of material in microscopic scale. Polar covalent bonds and hence the molecules with dipole moment are not rotating under DC voltage, they only take the appropriate position during a voltage change eg. switching the polarity. Another negative phenomenon taking place in the DC field is the gathering of space charge on non-homogeneities in the material structure. The space charge locally distorts the electrostatic field and thus can it significantly threaten the insulation system, respectively the whole device.