141 / 2018-08-25 11:58:37

Study of Correlation between Voltage Endurance and Insulation Properties for Stator Coils of High Voltage Motors

HV coils, voltage endurance, dissipation factor, partial discharge, correlation

Vacuum pressure impregnation (VPI) technique is now widely used in the insulation system of high voltage (HV) motors. It has the advantage of reducing air bubbles and pinholes within the insulation layers, therefore realizing favorable electrical properties and long service life for stator coils of high voltage motors. The dissipation factor, partial discharge and voltage endurance are useful and effective parameters to evaluate the electrical performance of HV coils. The voltage endurance test is a destructive method to determine the time-to-failure of the groundwall insulation under a high electrical stress and give direct lifetime estimation of HV coils. On the other hand, the dissipation factor and partial discharge are nondestructive measurement methods, which can give more detailed information on the quality of material and processing, and contribute to diagnose and estimate the insulation properties of coils. In this paper, the voltage endurance and insulation properties (dissipation factor and partial discharge) of stator coils with the rated voltage of 10kV are measured, and the correlation between these parameters is studied as well.
The coil specimens are divided into 6 groups according to the materials and processes, namely G1-G6, with 5 or 6 ones in each group. The voltage endurance test was carried out at 3Un 50Hz. (“Un” is used to denote the line-to-line voltage of the coil.) The dissipation factor tan δ measurement was performed over the voltage range of 0.2Un-1.2Un with intervals of 0.2Un. Partial discharge activity was measured to obtain PD inception voltage (PDIV), PD extinction voltage (PDEV) and QIEC value in operating voltage (Un/√3) and rated voltage (Un). These tests and measurements were all conducted at room temperature.
It has been found that the coils of G5 have the longest voltage endurance, which is over 400 hours; whereas the G1 and G3 coils have the shortest voltage endurance, which is less than 150 hours. Moreover, the G5 coils show lowest dissipation factor, relatively higher PDIV and lower QIEC values, while coils of G1 and G3 show largest dissipation factor, lowest PDIV and highest QIEC values. However, the PDEV values of G5, G1 and G3 show no obvious difference. These results indicate that coils with smaller dielectric loss, higher PD inception voltage and less apparent charge tend to have longer voltage endurance.
It can be concluded that coil insulation systems with various materials and processes exhibit very distinct performance in accelerated voltage endurance test, dielectric loss characteristics as well as partial discharge activities. It shows that G5 coils have the longer voltage endurance, lower dissipation factor, higher PDIV and lower QIEC values compared with G1 and G3. It suggests that coils with better insulation properties will have longer electrical lifetime. Hence, this study has revealed a close correlation between the destructive voltage endurance test and the nondestructive insulation properties diagnostic methods for stator coils of high voltage motors.