Correlation between Voltage Endurance and Dielectric Loss Tangent/Partial Discharge for Stator Coils of HV Motors

Vacuum pressure impregnation (VPI) technique is now widely used in the insulation systems of high voltage (HV) motors and generators due to good electrical, mechanical and thermal performances. The voltage endurance, dielectric loss tangent and partial discharge are the most common and effective means to assess the electrical properties of full-size stator coils. In this paper, the relationship between voltage endurance and tanδ as well as the partial discharge is investigated for stator coils with the rated voltage (Un) of 10kV. The voltage endurance test is carried out at 3U
_n
50Hz. The tanδ measurement is performed over the voltage range of 0.2 U
_n
- 1.2 U
_n
with intervals of 0.2U
_n
. Partial discharge is measured to obtain the inception voltage, extinction voltage and Q
_IEC
values in the operating voltage (U
_n
/√3) and the rated voltage (Un). It has been found that the coil group with longer voltage endurance tends to show lower tanδ and tip-up, higher inception and extinction voltages and smaller apparent partial charges. It manifests that the destructive voltage endurance test and non-destructive diagnostic measurements of tanδ and partial discharge have good consistency in assessing the insulation properties for HV coils. Moreover, calculations of correlation coefficients between voltage endurance and tanδ or partial discharge parameters show that the tanδ and partial discharge magnitude at the operating voltage can be more effective for evaluating the quality and estimating the lifetime of insulation system for HV coils, compared with such parameters measured at the rated voltage.