Partial discharge investigation under humidity conditions via dissipation factor and insulation capacitance tip-up test

The humidity role in the partial discharge (PD) inception mechanism is quite complex and there is no widely accepted theory that can describe the humidity effect on PD. In this paper, the PD activity of inter-turn insulation of electric motor winding is experimentally investigated for different relative humidity conditions at 30°C ambient temperature. Since the inception of PD leads to a boost in the dissipation factor (i.e., tanδ) and insulation capacitance (IC), tanδ and IC tip-up test can be adopted as an indirect measurement of PD. The test campaign is carried out employing twisted pairs as samples and both tanδ and IC are measured under a sinusoidal voltage regime. Apart from being indirectly detected, PD activities are also directly measured through a photomultiplier while the tip-up test is performed. The adoption of direct PD measurement is to discriminate the PD from other ionization processes (e.g., water film ionization) which cannot be distinguished by indirect measurement. The experimental findings allow stating that the change of partial discharge inception voltage (PDIV) at higher relative humidity is due to the different places of water film formation on the insulation surface and Paschen’s law combined with finite element analysis is used to interpret the effect of water film on PDIV. In addition, it is verified that the tanδ and IC tip-up test is not suitable for indirect measurement of PD in high humidity conditions. In fact, the higher leakage current, due to the water condensation on the insulation surface, influences the tanδ and IC measurements causing an increment in their values regardless of the PD inception.