Temperature dependence of the pre-breakdown and breakdown phenomena in natural esters under AC stress

Abstract

Natural ester dielectric fluids are being used as alternatives for mineral insulating oils in oil‐filled power apparatuses. Although partial discharges (PDs) are recognised as one of the main causes of degradation of dielectrics used in power equipment, their characteristic voltages appear to be temperature‐dependent. In this contribution, the pre‐breakdown and breakdown behaviour of two natural ester fluids, extracted from canola oil (NE1) and soybean oil (NE2) is reported. To assess the effectiveness of the temperature dependence, −5, 0, and + 20°C have been identified as test temperatures. It was found that the different pre‐breakdown and breakdown characteristic voltages increase with increasing temperature for both types of esters. The delay time is always longer for canola oil when compared to soybean oil regardless of the test temperature. The analysis of the average propagation velocity of NE1 and NE2 indicates that PDs propagate faster in NE2 than in NE1. The field strength at the tip of the needle electrode during the inception of the streamers leading to breakdown indicates that streamers in both oils are propagating in the slow mode. It is found that the temperature has a significant influence on the PDs’ parameters for both fluids.

First‐mode of negative streamers: Inception at liquid/solid interfaces

“…After the streamer propagation-expansion stops, it starts to implode and collapse [11]. The collapsing dynamics of the gaseous phase of the streamer has been also observed for other authors in [12,13], describing the apparent collapse of the first mode negative streamer in long time periods.…”

On the Feasibility of Monitoring Power Transformer’s Winding Vibration and Temperature along with Moisture in Oil Using Optical Sensors

“…Any change in the temperature is reflected in the central wavelength, which is shifted based on Equation (5). With the assumption that the fiber grating is only affected by the temperature, the central wavelength shifting value of the FBG can be obtained [ 25 ]: where the grating pitch variation with temperature is described by , which depicts for the fiber’s thermal expansion coefficient.…”

“…This is translated into linear intensity changes that accurately reflect the actual vibration. To discriminate the influence of temperature, a self-calibration algorithm was introduced to re-establish the operating point, and such compensation delivers an indirect method to monitor temperature [ 25 ].…”

“…Moisture particularly affects the dielectric characteristics (conductivity, breakdown strength, dissipation factor, etc. ), along with the resistance to ageing, of the oil-paper insulation [ 23 , 24 , 25 ]. The insulation ageing is catalyzed by heat, through pyrolysis, ageing by-products, dirt, vibrations/overload, and electrical stress/voltage waves, etc.…”