Behavior of Natural Ester Oil under Negative and Positive Lightning Impulse Stress

Ester oils are becoming potential alternatives for mineral oil due to their better biodegradability, higher fire point, higher moisture tolerance and continuous overloading capabilities. With the increase in its popularity, it becomes important for transformer, reactor, instrument transformer manufacturer and power utilities to understand the complete insulating properties of natural esters and their impact on insulation design. In this paper, behavior of natural ester oil under positive and negative polarities of lightning impulse waveshape are analyzed with respect to mineral oil. The breakdown strength of the mineral oil is considered as a benchmark. Two types of natural esters are used for comparison of impulse waveshape of polarity results with mineral oil. The impulse tests are performed with vertical configuration test cell under completely divergent field using sphere-needle electrode configuration with various gaps between the electrodes. The impulse breakdown data obtained from all the impulse polarity tests are statistically analyzed by 3 parameter weibull distribution in order to establish the 1% breakdown voltages for natural ester oil and mineral oil. Furthermore, electrical stress analysis is performed based on 1% breakdown voltages resulting in calculation of utilization factor by using FEM technique.