About the dimensioning of a needle plane electrode arrangement for comparative investigations of partial discharges in air

Needle plane electrode arrangements are often used to perform basic investigations of insulation discharge physics. The main reasons and advantages are that any kind of parameters influencing the measurement are well known and also limited in number, if compared to solid insulation. However, taking into account the existing parameters of this arrangement will lead to the assessment that commonly an important part is neglected. Usually only the needle-tip-radius, the gap distance and the magnitude of the voltage-stress are used for specification of the arrangement. This is due to the fact that, based on those data, the electrical field is calculated by a simplified equation and the values are used to evaluate the measurement results. However, in practice, the needle has to be mounted and connected within a conductive part. This may lead to a strong influence on the field distribution. By the use of finite element method (FEM), the performance of the needle-plane arrangement, as used in practice, is investigated. The result is that the calculation of the electrical field is easily possible by this equation, if some requirements are fulfilled.