160 / 2018-08-26 20:59:24

Thermal Aging Properties of Thermally Upgraded Paper Impregnated with Mineral Oil

Thermal Aging Properties,Thermally Upgraded Paper,Mineral Oil

Transformers are the key power components in the power systems. As the insulating medium of power transformers, paper insulation, including insulation paper and paper board, degrades under the interaction with temperature, moisture, oxygen and acid, losing its mechanical performance during the operation. Among these factors, thermal aging is the most common aging factor of paper aging. Therefore, studying the aging properties of paper insulation has long been a concern for power utilities and researchers, as it is of great importance to the safe operation, planned maintenance and replacement of transformers.
Compared to the Kraft paper, the thermally upgraded paper has much more ability to endure the thermal heat and maintain its insulating function, and therefore it has been used in transformers in some cases. However, its aging deterioration properties exhibit some difference compared with Kraft paper. Specific thermal aging experiments should be carried out to study the thermal aging properties of thermally upgraded paper.
In this paper, the conventional naphthenic-based Karamay 25# insulating oil from PetroChina Company Limited and the thermally upgraded paper from Munksjo were used as the accelerated thermal aging experiment materials. Firstly, the insulating oil and insulation paper were degassed and dehydrated separately in a vacuum-thermal chamber at 50 Pa and 80 °C for 48 h to simulate the processing procedures of the oil-paper used in an actual transformer and to control the initial state of the experimental samples. Secondly, the insulation paper was impregnated with the insulating oil at 50 Pa and 60 °C for another 48 h. Thirdly, the well-processed insulating oil and paper were mixed and put into a stainless steel container with a mass weight ratio of oil to paper equal to 10:1. A copper sheet with the dimension of 50×1.5×0.1 cm was submerged in the oil in each container to simulate the winding of in-service transformers. Lastly, the containers were sealed and put into an aging chamber to perform the accelerated thermal aging experiments at atmospheric pressure and 130 ºC for different aging durations. At each aging time, one container was removed from the aging chamber to obtain the aged insulating oil and paper samples for relevant test. The DP of insulation paper and contents of 2-furaldehyde(2-FAL) and carbon oxides dissolved in oil were measured as per IEC 60450, IEC 61198 and IEC 60567, respectively. Besides, the breakdown voltage and water content of paper were measured according to relevant IEC standards.
The results showed that the variation tendency of dissolved carbon oxides and water content has no distinguished difference compared with that of Kraft paper. The breakdown voltage of the aged paper fluctuates with small change but doesn’t decline much compared with that of unaged paper, which should be attributed to combined effect of the water content of paper, the pores characteristics inside the paper and some other influencing factors. The 2-FAL content is constantly small during the whole aging period, and no clear relation with DP of paper was found, which confirms the low sensitivity of 2-FAL content as the aging marker of paper insulation in the early aging period.