Condition Assessment Of Transformer Oil-Paper Insulation Based On Analysis Of Dielectric Characteristic Parameters

Oil-impregnated transformers are one of the most important equipiments of electric power transmission and transformation in power systems. There is an important engineering significance to guarantee the safe operation of the serious aged transformers by reliably assessing the ageing condition of the main insulation of these transformers. The polarization and depolarizationcurrents(PDC) and frequency domain spectroscopy(FDS) technique based on dielectric response theory were introduced to assess the ageing condition of transformer insulation due to the advantage of nondestructive measurement. Until now, it is a hot and difficult issue about how to extracte the time-frequency domain dielectric parameters from PDC results and FDS results, and sepeate the influence of moisture and ageing of oil-impregnated pressboards on PDC results or FDS results. Therefore, there is an academic value to launch a series of in-depth studys about above-mentioned issues.In this paper, the transformer oil-paper insulation system is used as the research object, and a series of accerated thermal ageing experiments and moisture absorption experiments were firstly launched under controlled laboratory conditions to prepare the oil-impregnated pressboards with different ageing states and moisture contents. Then the PDC and FDS were measured. After that, the time-frequency domain dielectric characteristic parameters which can represent the ageing of oil-paper insulation were proposed. Finally, the elimination method of the temperature effect on time-frequency domain dielectric characteristic parameters was also proposed after studying the influence of test temperature on PDC and FDS results in depth. The main research results of this literature are as following:① In terms of the domestic standards(DL/T 596-2005 and DL/T 984-2005), the quantitative classification of the ageing condition and moisture level in oil-impregnated pressboards were performed, and 9 reference conditions were proposed. After that, the grey correlation assessment method of ageing assessment of transformer main insulation, which is used the time-frequency domain characteristic parameters extracted from PDC and FDS result as the input datas, was proposed.② The TDDC(larger time constant in extended Debye model), which represent sensitively the ageing condition of transformer pressboard, was firstly proposed and the TDDC is independent of the geometric dimensioning of pressboard according to the theoretical analysis. In addition, it was found that the TDDC increased with the increasing thermal ageing time. There is an exponential relationship between the TDDC and DP values. Therefore, the TDDC can be utilized to quantize the ageing states of oil-impregnated pressboards.③ After investigating how to distinguish the influence of moisture and ageing on FDS results, the FDDC(dielectric loss integration factor) was firstly proposed to quantize the ageing state and moisture contents of oil-impregnated pressboards according to the integration method on the FDS results, and the quantitative relations among FDDC, moisture contents, DP values were firstly establised. The moisture contents and ageing conditions of pressboards can be calculated by using the quantitative relations.④ The assessment program which can perform the combination of qualitative evaluation and quantitative evaluation of the transformer main insulation was proposed. The grey correlation assessment method, larger time constant in extended Debye model and dielectric loss integration factor can be utilized to assess the ageing condition of field transformers by using the proposed assessment program. In order to verify its effectiveness, the oil-impregnated pressboards prepared in lab and an actual transformer(220k V, 3 windings) were used to perform the PDC and FDS measurements, and the final evaluation results initially verify the feasibility of the proposed assessment program, TDDC and FDDC.