PD measurements on low voltage busbars at operating temperatures

Inhalt:
A lot of low voltage products are continuously in the process of miniaturization. This leads to smaller insulation distances between voltage carrying electrical conductors of opposite polarity. Thus the electrical stress of the insulation material is increased to a level formally attributed only to high voltage apparatus. Additionally, some systems are cast into a solid dielectric, whereas alternatively they could be insulated with a combined air/solid system. So far, partial discharge (PD) measurements for low voltage applications were not deemed to be generally necessary. Even if they are carried out, it is usually done at room temperature. But, systems under operating conditions always exhibit higher temperatures than room temperature due to ohmic losses especially when mounted in enclosures. Here conducted exemplary PD measurements of low voltage busbars showed that the inception voltage (defined in IEC 60270), among others, is dependent on the temperature of the system. A small dependency is to be expected if air is part of the system due to the temperature induced change of the density of air. But the change is higher than the density change would yield and it was also observed with solid insulation. Another possible influence is the relative permittivity, which is temperature dependent as well. The reduction of the inception voltage might pose a risk for safe and reliable operation of the system since those devices are expected to stay in service for several decades. In this paper PD investigations on low voltage compact busbars are presented. The testing procedure will be explained, the boundary conditions will be given and the results of the low voltage busbars are shown. Additionally, the relative permittivity of the investigated insulation materials is presented. The results of the PD measurements are compared to the measurements of the relative permittivity and as a conclusion a recommendation concerning PD testing for low voltage systems is given.