| With the development of economy and society in our country,the demand of electric power is increasing rapidly.The safe and stable operation of power system and the reliability of power supply are faced with more severe challenges.Transformer is the core equipment to link the power grid between different areas converting and transmitting electric power.The electric field characteristics and insulation performance of transformer internal insulation under impulse voltage is the key link in the assessment.The impulse electric field distribution is calculated based on capacitance dividing by the commercial simulation software,and the design value is aquired by the reasonable safety margin up to the allowable field.The calculation results can not only take into account the distortion of the electric field by the space charge,but also lack the support and verification of the measurement method.At present,there have been many fault cases of transformers caused by lightning insulation damage in the operation or flashover in the ex-factory routine impulse test.Therefore,the existing simulation methods can not meet the design requirements of transformer insulation structure.It is necessary to study the electric field of typical oil-paper insulation structures under impulse voltage,providing the basis for modifying simulation method.In this paper,based on the principle of Kerr effect,the rapid electric field measuring apparatus of oil paper insulation under impulse voltage is established by using the improved single point measuring method.The error sources in the measurement are analyzed from 5 aspects:the beam incidence angle,the electrical noise,the temperature,the non-ideality of the optical device and the calculation method of the electric field.By optimizing the design of the optical path,restraining the noise level of the photoelectric conversion device,and adjusting the precision of the beam incident window,the measurement effect is improved.The performance of the apparatus is tested with parallel plate electrodes in transformer oil.The results show that the measured electric field are in good agreement with the applied electric field,with the sensitivity of 3.49kV/mm,the maximum error of less than 3.56%,and the uncertainty of 4.41%.The electric field in transformer oil under impulse voltage was measured,and the space electric field and charge distribution characteristics were obtained,and also the influence of electrode materials,voltage amplitude and wavefront time on the space electric field and charge characteristics.Using brass,steel and aliuminium as electrodes,the impulse electric field measurement result in oil showed that The charge injection of aluminum electrode was the highest,followed by stainless steel and brass at least,and the electric field distortion of aluminum electrode was 19.25%higher than that of brass electrode,which resulted from the different work functions of mentals.Longitudinal comparison of different voltage amplitudes at the same wavefront time showed that the time required for negative charge to move from cathode to anode was 278ns and the average velocity was 1.1*10-3 m2/(V·s).The transverse comparison of different wavefront time with the same voltage amplitude showed that the time for negative charges to reach a relatively stable state was at least 2?s.With the increase of the wavefront time,the peak value of the measured electric field decreased,which was 17.6%lower when the wavefront time is 500ns than 40?s.Due to the long wave head,injected charge became more and the weakening effect was stronger.The result of the electric field distribution measurement in transformer oil showed that there was a dynamic process of the charges with the variation of the applied electric field.The positive and negative charges produced near the anode and cathode enhanced the electric field in oil.Tthe voltage rising gradually,and because the negative charges moved faster than the positive ones,so the positive charge is stationary and the negative charges were moving to the anode.When achieving the anode,the formation of negative charge layer weakened the electric field.After the peak voltage,with the voltage decline,the amount of charge injection reduced and neutralization lasted,and it returned to the initial state that positive and negative charges stayed near the anode and cathode respectively,so that the oil field was slightly higher than that of the applied electric field.Based on the actual measurement of the electric field in oil of insulating pressboard model and insulating paper model,the effect of the insulation pressboard structure and the number of insulating paper layers on the characteristics of the electric field and charges was aquired.It was proved by experiments that the pressboard could capture charges.In asymmetric pressboard model,there was no significant difference between the positive and negative electric field of the brass electrode,but the positive and negative electric field of the aluminum electrode were different.In symmetrical pressboard model,the measured electric field and was close to the applied electric field,regardless of the electrode material.Insulation paper had a barrier effect on the charges,the increase of the paper layers would lead to the retention of the charge in the layers,the electric field in oil could be enhanced by 5.4%at most.The aluminum electrode needed more layers of insulating paper,because the charge injection of aluminum was greater than that of brass.It was found that the electric field peak value decreased by 11.2%,according to the actual electric field measurement of oil-paper insulation structure with low moisture and high moisture.The results show that the moisture in oil played a dominant role in the weakening of electric field,while the moisture in paper had little effect on electric field.This is because the paper moisture is strongly bound by hydrophilic groups,difficult to ionize.Water molecules in transformer oil under high electric field ionized to the formate H+ and OH-,and the numbers of carriers in oil was increased.The electric field formed by the charges around the electrodes exerted weakening effect on the applied electric field.The charge injection and motion model in transformer oil under lightning impulse voltage was proposed,and the source of positive and negative charges in oil was explained.The negative charge motion time was estimated with the mean free path,in good agreement with the exprimental results.The model was improved considering the influence of insulating paper and moisture. |
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