Abstract
Natural ester is a good substitute for mineral oil. It has been successfully applied in power transformers, and its application scale will continue to increase. With the aging of oil-paper insulation in transformers, moisture and impurity particles will inevitably be generated. In this paper, dry and wet mineral oil and natural ester samples, as well as dry and wet cellulose particles, were prepared. The accumulation behavior of the cellulose particles and the conductivity current in mineral oil and natural ester under a non-uniform dc electrical field were compared by using eight combinations of dry or wet cellulose particles and dry or wet insulation oils. The results show that the speed of the cellulose particles and the growth of impurity bridges are faster in wet mineral oil and natural ester, especially when the cellulose particles are also wet. The increase of electrical field strength accelerates the appearance of this phenomenon. The cellulose particle bridging phenomenon in the mineral oil is more significant than that in the natural ester. Under the same dc voltage, the saturated conductivity current in every mineral oil sample decreases as the electrode distance increases; however, the opposite phenomenon is observed in the natural ester samples. Moreover, the oil moisture content increase has a more significant influence on the saturated current and the dc breakdown voltage than the particles' moisture content increase. For the same particle concentration level, the natural ester has a lower conductivity current and a larger dc breakdown voltage than the mineral oil. The natural ester presents better resistance to particle pollution and better insulation properties than the mineral oil.
Highlights
Converter transformers play an important role in high voltage direct current (HVDC) transmission systems
The moisture content of the oilimpregnated insulation paper/pressboard may increase to 4%-8% in extreme cases, e.g., moisture ingress from the outside caused by leaking gaskets [8], [9]
Compared with the dry oil + dry particles for both an increased water content in the oil and an increased water content in the cellulose particles, moisture has a significant effect on the accumulation of fiber particles
Summary
Converter transformers play an important role in high voltage direct current (HVDC) transmission systems. The main insulation structure in converter transformers is oil-paper insulation. Because of its poor biodegradability, non-renewability and low flash point, there are an increasing number of studies on the insulation properties of natural ester and its applications [1]–[2]. Many researchers have studied the insulation properties of different kinds of natural esters. Statistical results show that the moisture content of oilimpregnated insulation paper/pressboard is usually < 0.5% for new transformers. The moisture content of the oilimpregnated insulation paper/pressboard may increase to 4%-8% in extreme cases, e.g., moisture ingress from the outside caused by leaking gaskets [8], [9]. Moisture can reduce the mechanical strength and electrical breakdown strength of oil-paper insulation and is regarded as ‘‘the first enemy’’ after temperature [8]. Under the action of an electrical field, impurity particles with high
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