A Fundamental Study Of Surface Discharge Characteristics On Cellulose Insulation Immersed In Ester Oil For Large Power Transformer Application

Zainuddin, Hidayat and Aman, Aminudin and Ab. Ghani, Sharin and Sutan Chairul, Imran and Ahmad Khiar, Mohd. Shahril and Talib, Mohd. Aizam (2018) A Fundamental Study Of Surface Discharge Characteristics On Cellulose Insulation Immersed In Ester Oil For Large Power Transformer Application. Project Report. Universiti Teknikal Malaysia Melaka, Melaka, Malaysia. (Submitted)

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Abstract

Surface discharge at the oil-pressboard interface is known as the development of a conducting path which is characterized by white and carbonized marks.This phenomenon tends to cause damage on the cellulose pressboard insulation which is subsequently promotes catastrophic failure in transformer’s insulation.One of the major defects that may cause surface discharge to occur along the pressboard surface is an excessive moisture content in pressboard insulation.In order to increase the understanding on this failure,this research project presents the investigation on the degradation behaviour of surface discharge along dry and wet pressboard samples (3 % and 6 %) immersed with different viscosity of natural ester insulation (NEI) oils,i.e.palm fatty acid ester (PFAE) and MIDEL eN oil.There are three types of experiments that were discussed in this research project,i.e. surface breakdown,partial discharge inception voltage (PDIV) and surface discharge experiments.These experiments were conducted in oil bath by using a needle-bar electrode configuration under AC voltage stress.Three differences gap distances between the needle tip and earth electrode,i.e.20 mm,30 mm and 40 mm were used in surface breakdown and PDIV experiments.On the other hand,surface discharge experiment was conducted under a long duration of constant AC voltage (30 kV) with a fixed 30 mm gap distance.The development of surface discharge have been analysed by correlating the visual records of surface discharge and phase-resolved partial discharge (PRPD) pattern.The results show that the moisture contents in the pressboard and viscosity of insulation oils play important roles in all experiments. In general,as moisture increases,the PDIV and surface breakdown voltage decreases,whilst the PD number from the surface discharge experiment increases.However,unexpected results are observed when pressboard of 6 % moisture content are used in MIDEL eN oil,whereby the surface breakdown voltage was unexpectedly increased and no PD data was recorded in the surface discharge experiment. This might be due to the trapped vaporised moisture and dissolved gases in the pressboard structure as a result of higher viscosity of MIDEL eN oil compared to PFAE oil. Another effect of high viscosity of MIDEL eN oil is also observed when the MIDEL eN oil-impregnated pressboard has a higher surface breakdown voltage by approximately 27.82 % on average and lower PDIV approximately by 8.98 % on average compared to PFAE oil-impregnated pressboard, regardless the moisture contents in pressboard.In addition,the maximum PD magnitude for the MIDEL eN oil-impregnated pressboard is in the order of 1×10−12C which is lower than PFAE oil-impregnated pressboard that is in order of 1×10−9C.On the other hand,the number of PD is observed higher for MIDEL oil-impregnated pressboard compared to PFAE oil-impregnated pressboard.

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