Measurement of Dielectric Properties
The ageing of the insulation of HV apparatus is not only caused by the high electric field strength but also by thermal and mechanical stresses that evolve during normal operation condition. This may lead to chemical processes associated with a gradual deterioration of the integral insulation properties. Finally, weak spots and, in extreme case, an ultimate breakdown might occur, which causes not only an unexpected outage of HV equipment but also physical, environmental and financial damages. To ensure a reliable operation of HV assets encourages high standards of quality assurance tests after manufacturing as well as advanced tools for preventive diagnostics in service. As treated already in Chap. 4, PD measurements have become an indispensable tool to trace local dielectric imperfections since the 1960s, while the measurement of integral dielectric properties, such as capacitance and loss factor measurements, became of interest for insulation condition assessment of HV equipment already since the beginning of the last century when the first HV transmission systems above 100 kV were erected. In this context, it should be noted that the dielectric properties are often determined at test frequencies different from the service frequency (50/60 Hz). So valuable information on the insulation condition may also be gathered by measuring the dielectric response under DC voltage after this is switched on and even off, as will also be treated in the following.