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Title: Ageing of liquid insulation systems including nanoparticle suspensions
Author: Chen, Guanduo
ISNI:       0000 0004 6062 0201
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2016
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Liquid based insulation systems in the Electrical Power Industry will suffer a reduction in their dielectric performance over time because of ageing. Changes due to ageing effects include reduction in breakdown strength and increases in the conductivity of the liquid leading to larger losses in the system. In recent years, adding nanoparticles have been discovered to be a possible way of improving the properties of the liquid insulating material by increasing breakdown strength and thermal conductivity. It is therefore important to investigate whether the presence of nanoparticles can affect the mechanisms or rates at which ageing occurs. The research presented in this thesis focuses on the influence of ageing on the conductivity of Shell Diala S3 ZX-IG mineral insulation oil and the number and mobility of charge carriers present in the system. The conductivity and mobility have been derived from current transients produced under bipolar conditions at relatively low electric fields (0.3 to 0.7 kV/cm). Background material on ageing processes and reaction kinetics and possible diagnostic measurements; conductivity, mobility and how they can be measured and the nature properties of nanoparticles are provided. The techniques used to accelerate the ageing of the insulating liquids are then discussed with some broad estimates of the degree of acceleration. The development of the measurement system used to record the current transients and the interpretation of these transients in terms of mobility and conductivity is then described. Three sets of aged samples have been considered: samples that were thermally aged without nanoparticles (to act as a reference); samples that were thermally aged then had EFH1 magnetite nanoparticles added and samples that had Sigma magnetite nanoparticles added prior to thermal ageing. Current transients were measured for each sample over a range of voltages and these measurements were repeated over 5 successive days. Changes were observed in the current transients over this 5 day period. Possible reasons for these changes are discussed and the fitting techniques developed to determine the initial values and the equilibrium values of the parameters are described. For oil samples without nanoparticles added, thermal ageing resulted in a monotonic increase of the conductivity. Changes were also observed in the mobility of charge carriers. The mobility of charge carriers initially decreases in the first 48 hours of ageing and then increases with further ageing time. Increase of the density of charge carriers was observed as a result of thermal ageing. The samples with EFH with ageing time. The higher conductivities appear to be due to a higher mobility of charge carriers in the samples with EFH1 nanoparticles added. Again there is no simple relationship between mobility and ageing time. The study using the Sigma nanoparticles indicates that adding a relatively low concentration of nanoparticles to an unaged sample or to a sample after ageing does not change the conductivity of the liquid and the mobility of charge carriers significantly. However if the same concentration is added prior to thermal ageing significant changes in both the conductivity and the mobility of charge carriers is observed. This suggests that the rate of ageing or the ageing mechanism is affected by the presence of the nanoparticles. Although the addition of nanoparticles provides an approach of improving the dielectric properties of insulating oil, special attention needs to be paid on the drawbacks caused by the presence of nanoparticles. Using the insulating oil modified by nanoparticles can lead to an increase of the energy losses. In addition, the thermal ageing rate of the insulating oil could also be increased by the presence of nanoparticles. Further investigation of the effect of nanoparticles on ageing is required before the nanofluid can be used as alternative of the traditional insulating oil. Key words: Aging, liquid insulating material, nanoparticles, time-of-flight measurement, conductivity, mobility.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral