Abstract
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.
Highlights
Mineral oil is commonly used, in liquid-filled transformers, as an insulating and cooling medium [1,2,3]
The delay time (∆t = tBSIT − tPDPIT ) is defined as the duration measured between pre-inception time (PDPIT) and inception time of the streamers leading to the breakdown, the PD pre-inception time (PDPIT) and inception time of the streamers leading to the i.e., breakdown streamer inception time (BSIT)
The duration of the delay time is high in mineral insulating oil and is generally more than 8 s for all the liquids regardless of the tip radius and age factor
Summary
Mineral oil is commonly used, in liquid-filled transformers, as an insulating and cooling medium [1,2,3]. Natural and synthetic ester fluids are polar, as opposed to traditional mineral oils [6]. In the past few decades, the emphasis has been laid on the comparative study of some characteristics of streamers, in ester liquids and mineral oil, under lightning impulse voltage [7,8]. The condition of the liquid has a noticeable influence on the breakdown behavior This is because the degradation of transformer liquids is accompanied by the generation of decay products, water molecules, and other decay particles (generally conductive). The digital technology and use of ultrahigh-speed cameras and schlieren techniques have improved the understanding of these phenomena [2] Some characteristics, such as shape, propagation velocity, and acceleration voltage, were studied by many researchers [2,9]. Typical streamer velocities are 0.1 km/s for the 1st mode,
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