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Space Charge Behaviors of Ester Liquid and Kraft Paper Double-Layer Insulation Systems at Polarity Reversal

The liquid-solid insulation system of converter transformers used in line commutated converter based HVDC systems is subjected to combined AC and DC stresses. In addition, the insulation system is subjected to voltage polarity reversal when the direction of the power flow of the HVDC system is reversed. Under DC voltage, space charges are injected and generated in the insulating materials and then accumulate at the liquid-solid interfaces. Thus, during polarity reversal, the insulating liquid is subjected to a high electric field that may result in electrical breakdown. This paper studies the space charge behaviors of ester liquids as a potential alternative to mineral oil in HVDC converter transformers. Space charge behaviors in ester liquid and Kraft paper double-layer samples were studied under polarity reversal. A synthetic ester and a natural ester were used and benchmarked by a mineral oil. Pulsed Electro-Acoustic technique was used to measure the space charge distribution. Before polarity reversal, it was observed that ester-Kraft paper samples have more negative charges appeared at the interfacial region, leading to a higher field in the paper layer. After polarity reversal, ester-Kraft paper samples show a faster establishment of new space charge distribution. Therefore, the liquid layer in these samples experiences a shorter duration of electric field distortion compared to the samples with mineral oil.