Search

In the last decades, a large focus is being placed on the sustainability and safety of the power transformer spectrum. Ester liquids, which have interesting properties such as high fire point and biodegradability, are gaining needed attraction. Since in-service condition, thermal aging deteriorates the physicochemical and electrical properties of liquid dielectrics, it is important to study their long-term behavior. In this contribution, the pre-breakdown and breakdown behavior of ester fluids (synthetic and natural) under AC stress are investigated. Important characteristics, such as partial discharge pre-inception voltage, partial discharge inception voltage, breakdown voltage, average streamer velocity, and inception electric field, were assessed. The influence of the radius of curvature (of high voltage needle electrode) as well as the thermal degradation of typical ester liquids are also discussed. Mineral oil was also included in the tests loop as a benchmark for comparative purposes. It is found that the pre-inception voltage of ester liquids was, in most cases, higher than that of mineral oil. For a given radius of curvature, the streamer inception and breakdown voltages decreased with thermal aging. During the streamer initiation, the electric field at the electrode tip decreased with the increase in the radius of curvature. The velocity of the streamers seems to increase with the decrease in the radius of curvature. The period of vulnerability, the so-called “delay time”, seems to be independent of the aging or the radius of curvature for a given condition of the liquid.

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.