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Title: Pre-breakdown and breakdown phenomena of alternative transformer liquids under lightning impulse voltages
Author: Lu, Wu
ISNI:       0000 0004 6497 906X
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2016
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In the past decade, alternative liquids such as synthetic ester liquid, natural ester liquid and gas-to-liquid (GTL) based oil are increasingly considered as alternatives to mineral oils in large high voltage power transformers to achieve better environmental, thermal and electrical performances. In regard to applications in power transformers, any alternative liquid needs to have sufficient lightning impulse strength. Lightning impulse strength as basic insulation level is essential for the insulation design of power transformers. In addition, tests under lightning impulse voltages tend to reveal the intrinsic properties of liquids. Therefore this thesis focuses on the pre-breakdown and breakdown performances of a number of alternative transformer liquids under lightning impulse voltages. A comprehensive study on streamer characteristics and breakdown voltages of a synthetic ester, a natural ester and a mineral oil with the presence of cellulose particle contaminations under both positive and negative standard lightning impulse voltages was investigated. The results indicated that in uniform electric fields, the lightning impulse breakdown voltages of the ester liquids and the mineral oil reduce gradually with the increase of cellulose particles. The detrimental effect is further enhanced when the particles are wet and when the effective stressed area/volume of electrode configurations is increased. At the same conditions, 50% lightning impulse breakdown voltages of the ester liquids are always comparable to that of the mineral oil. Further study of particle effect on streamer initiation and propagation showed that the presence of cellulose particles leading to reduced breakdown voltages in uniform electric field is due to the promotion of streamer initiation in liquids. This conclusion is valid for both the ester liquids and the mineral oil. The streamer and breakdown phenomena of an inhibited GTL based oil in a 50 mm point-plane gap were investigated under standard lightning impulse voltages. A mineral oil was also tested as a benchmark. The results of streamer stopping length and average propagation velocity showed that under positive polarity, the GTL oil has a higher breakdown voltage but a lower acceleration voltage than the mineral oil. Under negative polarity, the results showed that the GTL oil has a higher breakdown voltage and a higher acceleration voltage than the mineral oil. The difference of breakdown voltages and acceleration voltages between the GTL oil and the mineral oil can be explained by the fact that the GTL oil has a much lower concentration of aromatic compounds than the mineral oil. To quantitatively describe the streamer structure in transformer liquids, structure parameters including fractal dimension, branch number and length distribution of segments were obtained from the 2-D projected streamer images based on a self-developed MATLAB programme. The results showed that the fractal dimension, branch number and area of both slow and fast streamers in liquids follow the same relationship with applied voltage: the higher the voltage, the larger the value of these three parameters. Furthermore, it was also found that below the breakdown voltage, streamers in ester liquids have more branches and larger fractal dimension compared with mineral and GTL oils, which is especially prominent under negative polarity.
Supervisor: Liu, Qiang Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available