Measurement and evaluation of partial discharge inception voltage for enameled rectangular wires under AC voltage

Recently, the market for EVs (Electric vehicles) and HEVs (Hybrid electric vehicles) has been expanding. In these vehicles, rectangular wires have started to be used for the traction motor coil to increase the motor performance. As a result, the insulation techniques become important because the applied voltage between the wires is increasing due to a change in the motor structure and the increase in the driving voltage. In order to safeguard the motor insulation, partial discharge (PD) between coil wires must be controlled, and to do this, the coil wires should be coated with an insulator of a suitable thickness. For this purpose, the PD measurement technique is important in design and inspection on the manufacturing lines. However, the PD measurement between wires is unstable, and is especially affected by the measurement method and environmental factors. In this paper, we clarify the factors as well as the mechanism affecting the PD inception voltage (PDIV) between enameled rectangular wires under AC voltage. The effects of the ambient humidity, the pre-discharge conditions and the measuring frequency on the PDIV were analyzed. As a result, it was confirmed that when there was pre-discharge in a high humidity environment, the PDIV displayed fluctuating characteristics in which it momentarily decreased and then increased. We observed the discharges and analyzed the electric field distribution. The results showed that in high humidity environment, pre-discharge causes moisture contained in the air to adhere to the coating surfaces, resulting in an increase in the surface conductivity, which influences the electric field strength in the air gap between coil wires. These analyses indicated that the PDIV can be measured stably and reliably at high frequencies in a high humidity environment.