| dc.description.abstract | This thesis provided insights into the performance of insulation systems of biodegradable liquids for HVDC convertor transformers through investigating space charge dynamics and conductivity properties of biodegradable liquid impregnated Kraft paper under DC fields. Space charge dynamics and conductivity properties are strongly associated with each other and they are determining factors in designing insulation systems for converter transformers.
To fulfil the experimental investigation, a Pulsed Electro-Acoustic (PEA) measurement system including a pulse generator, a test cell, a signal detection system and a signal processing algorithm, was developed in accordance with IEC 62758 for space charge measurements. In addition, a system was developed for conductivity measurements.
Space charge dynamics in Kraft papers impregnated with synthetic ester liquid and mineral oil at fresh and dry conditions were measured and compared under different electric fields. Under the same electric fields, ester liquid-impregnated Kraft paper (ELIP) exhibited higher total amount of charges and higher electric field distortion than mineral oil-impregnated Kraft paper (MOIP) after 60 minutes of electrification. In addition, charge decay rate in ELIP was higher than that in MOIP, thus total amount of remaining charges in ELIP was lower than that in MOIP after 30 minutes of decay.
Space charge dynamics in aged Kraft papers re-impregnated with fresh synthetic ester liquid and fresh mineral oil at dry conditions were investigated focusing on the impact of physical ageing of Kraft paper, which was represented as the reduced Degree of Polymerization (DP) only. For early-stage of ageing (DP ≈ 950:750), peak charge densities at both electrodes and total amount of charges in samples increased for both types of samples. This can be attributed to the increase in permittivity and the decrease in conductivity at this ageing stage. At middle-stage of ageing (DP ≈ 600), the two measured features of space charge dynamics continued to increase in MOIP but decreased in ELIP. The later observation can be attributed to the decrease in permittivity and the increase in conductivity of ELIP at this ageing stage. The change in electric field distortion with ageing followed the same trend of total amount of charges in both types of samples. Under electric field of 20 kV/mm, electric field distortion in ELIP rose slightly first from 5.3% to 5.4% during early-stage of ageing (DP ≈ 950:830). Then, it decreased to 4% at middle-stage of ageing (DP ≈ 600). For MOIP, electric field distortion rose remarkably from 0.36% at fresh condition (DP = 950) to 2.4% at early-stage of ageing (DP ≈ 750) and to 6.7% at middle-stage of ageing (DP ≈ 600). Although electric field distortion in fresh ELIP was higher than that in fresh MOIP, electric field distortion in MOIP became higher than that in ELIP at middle-stage of ageing (DP ≈ 600).
Thus, ELIP can provide better life-long performance, provided that the initial high electric field distortion was considered at the design stage of insulation system. In addition, impact of ageing on electric field distortion must be also considered in the design of insulation by choosing an appropriate safety factor for preventing insulation failure and for optimal lifetime management of converter transformers.
Conductivity of physically aged Kraft papers re-impregnated with fresh insulating liquids at dry conditions was measured and correlated with space charge dynamics. For ELIP, conductivity decreased first during early-stage of paper-ageing, then it increased and reached its maximum at middle-stage of ageing. On the other hand, conductivity of MOIP decreased slightly with paper-ageing until middle-stage of ageing, then it increased remarkably with further ageing. Thus, strong correlation can be observed between these trends and the variation in total amount of charges in samples with DP. The decrease in conductivity can attribute to the increase in accumulated charges in a sample. In addition, the increase in conductivity can attribute to the decrease in accumulated charges in a sample as more charges can reach the opposite electrode and be neutralized.
Finally, a model was proposed for dielectric response of liquid-impregnated Kraft paper (LIP) under DC fields. It can represent each insulating material separately. Thus, type of insulating liquid and ageing state of Kraft paper can be modelled. Modelling showed that Kraft paper’s conductivity has a significant impact on overall conductivity of liquid-impregnated Kraft paper and it cannot be masked by high conductivity of insulating liquids. In addition, the model was proposed for evaluating DP of Kraft paper based on measured dielectric properties of LIP. | en_GB |
