Gas generation in natural ester and mineral oil under partial discharge and sparking faults

摘要

Mineral oil has been widely used in power transformers for more than a century because of its excellent dielectric and thermal characteristics. However, it is nonbiodegradable and could cause serious environmental problems if leakages or spills occurred. With increasingly strict environment rules and regulations, esters are sometimes used instead of mineral oil in transformers rated up to 245 kV [1]. Esters have different molecular structures than do mineral oils and thus different physical, dielectric, and chemical properties. They also have higher viscosities, and so higher temperatures are required for the impregnation of cellulose insulation [2]; higher viscosity also lowers cooling efficiency and increases the operating temperature of the transformer [3]. Esters are more hygroscopic than mineral oil and thus have higher absolute water content (ppm); their dielectric strength decreases with increasing relative humidity in a way similar to mineral oil [4]. In a quasi-uniform electric field, esters generally show breakdown strengths comparable to those of mineral oils. In a divergent field, discharges in the esters propagate faster and further than in mineral oils at the same voltage level, and hence the esters exhibit higher partial discharge (PD) repetition rates under AC stress [5], [6] and lower lightning impulse breakdown voltages, especially at large gaps [7], [8]. Accordingly, the design and operation of ester-filled power transformers need to be carefully considered. Dissolved gas analysis (DGA) is a common technique used to diagnose incipient faults in mineral-oil-filled transformers [9]?????????[11]. Before the introduction of online monitors, DGA involved periodic oil sampling followed by laboratory analysis of the gas components dissolved in the oil. In the past decade, an increasing number of multi-gas online monitors have become commercially available, enabling almost continuous monitoring of gassing in service [12]. In recent years, some studies of- fault gas generation in esters have been reported [13]?????????[21]. It was found that, under thermal faults, the fault gases generated in esters are similar to those generated in mineral oils, although the molecular structures of esters are different [13]. The generation rate of dissolved gases in natural esters is slightly lower than in mineral oils, and natural esters are particularly stable under medium-temperature thermal faults [14], [22]. Ethane is generated in natural esters if exposed to oxygen under normal operating temperatures (<140??????C) [23]; under thermal faults up to 600??????C, ethane and carbon oxides account for more than 70% of fault gas generation [15], [22]. When subjected to PD, esters generate a higher level of dissolved gases (mostly hydrogen) than do mineral oils [18]. Under breakdown, similar concentrations of dissolved fault gases (mainly acetylene) are generated in esters and in mineral oils [16]. The interpretation of DGA data for esters needs to be improved, particularly the classification boundaries between medium-temperature and high-temperature thermal faults, and between low-energy and high-energy discharges [13], [20].