Space Charge Behaviours of Mineral Oil and Ester Liquids Impregnated Cellulose Paper Insulations under DC Stress

Abstract

Space charges could accumulate in the oil paper insulation system unfavourably under the DC electric stress, which increasingly attracts academic and industrial attentions due to the fast growth of HVDC transmission networks. Previous research has focused on conventional mineral oil impregnated cellulose paper insulation. Ester liquids, including synthetic esters and natural esters, were introduced as alternative transformer liquids due to their high biodegradability and low fire risk. There have been increasing interests in applying ester liquids in HVDC converter transformers. Therefore, this thesis studies space charge behaviours in both mineral oil and ester liquid impregnated cellulose paper samples using Pulse Electro-Acoustic technique under various insulation conditions. The first part of the study focuses on 400 μm thick single-layer paper samples impregnated with a mineral oil. The effects of test temperature, degree of polymerisation (DP) of paper, low molecular weight acid (LMA) of paper and water content of paper on space charge behaviours were investigated under an electric field of 20 kV/mm. The results showed that high temperature strengthens the homo-charge injection and accelerates the charge decaying process. The reduction of DP of paper due to thermal ageing strengthens the homo-charge injection but greatly slows the charge dissipation. Similarly, the addition of LMA in the paper causes strong homo-charge accumulations and slows the charge dissipation. The increase of water content of paper leads to strong homo-charge injection and accelerates the charge decaying process. In addition, a newly-found negative charge shifting phenomenon for paper samples with high water contents (>4%) was characterised in detail. The study was then conducted on 400 μm thick single-layer paper samples impregnated with a synthetic ester and a natural ester under similar conditions. The homo-charge injection and accumulation are stronger in the samples impregnated with the ester liquids, compared to the mineral oil impregnation. Faster charge dissipation was also observed in the ester liquid impregnations. In addition, influence of temperature on the space charge behaviours is more significant in the case of ester liquid impregnation than the mineral oil impregnation. However, the ester liquid impregnations illustrate less sensitivity against the reduction of DP of paper. The effects of water content of paper on the space charge behaviours are similar in general between the ester liquid impregnations and the mineral oil impregnation. The third part of the study focuses on mineral oil-paper double-layer samples of which the oil layer has a thickness of 220 μm and paper layer of 200 μm. The effects of test temperature, voltage polarity reversal and water content on space charge behaviours were investigated under an average electric field of 20 kV/mm. It was found that the negative charges accumulate at the interfacial region which results in enhanced electric field with the maximum of 50 kV/mm in the paper layer. The negative charge peak and its effect on the field distortion decrease with the increasing temperature. At the voltage polarity reversal moment, high electric field of 43.0 kV/mm temporarily appears in the oil layer, which poses a threat to liquid insulation failure. The last part of the study was conducted on ester-liquid-paper double-layer samples under similar conditions. Comparing to the mineral oil-paper sample, much more negative charges appear at the interface region in the ester liquid-paper samples, which leads to much stronger field enhancement with the maximum of about 90 kV/mm in the paper layer. The interfacial charge peak and its effect on field distortion also decrease with the increase of temperature, which is, however, more profound in the natural ester than in the synthetic ester. At the voltage polarity reversal moment, high electric fields (43.5 kV/mm for synthetic ester and 36.0 kV/mm for natural

Date of Award	31 Dec 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Zhongdong Wang (Supervisor) & Qiang Liu (Supervisor)