The Effect of Electrical and Moisture Degradation of Metal Oxide Varistor with Insulation Diagnosis

Condition based monitoring of surge arresters (SA) has become a primary concern to ensure power supply quality on medium and high voltage networks. Understanding of SA behavior helps forecast their failure. This paper presents the analysis of dielectric response and conductivity of Metal Oxide Varistor (MOV) under degradation factors (normal ageing, impulse ageing and moisture ageing) using Polarization and Depolarization techniques. Normal ageing is represented by stressing the MOV with 115°C temperature. For impulse ageing, 20, 40 ,60 and 100 shots of positive and negative polarity of standard lightning impulses current were applied. Moisture ageing was performed by soaking MOV under 115°C temperature for 12 hours representing wet condition and the soaked blocks were left at room temperature for 24 hours representing dry condition. Polarization and Depolarization Current (PDC) was measured after each ageing step. Resistivity, conductivity and polarization index were interpreted for insulation diagnosis. The experiments were taken on the MOV with rated voltage 3 kV and 5kA-nominal discharge. The importance of nondestructive electrical diagnostics based on polarization methods is validated by test results and is highlighted in detail in this paper. Finally, the results of different ageing factors were compared and discussed.