Prebreakdown phenomena in organic ester transformer fluid

Prebreakdown current versus voltage characteristics have been recorded for di-iso-octyl phthalate transformer fluid using the ramp voltage application technique. The object was to compare the results with similar data obtained from dimethyl siloxane and Voltesso 35 dielectric fluids. The characteristics of the ester oil were similar to those of the silicone oil and transformer oil, insofar as they obeyed the equation I ∝V^s. The value of s for ester oil is a constant through all the experimental conditions and has the value 1.040±0.035. For transformer oil and silicone oil, s is changed when the maximum applied voltage and gap separation are varied. The response current for the ester oil is about two orders of magnitude stronger than that for the other two oils. Plotting the current vs. voltage in log-log scale, there is no threshold point for ester oil. The current on this scale increased very close to linearly with the ramped applied voltage. The direction of progression around the current vs. voltage loop reverses compared to that of silicone oil, which only happens at very high stress in transformer oil