Effects of Temperature and Liquid Flow on the Pre-breakdown and Breakdown Phenomena in an Ester Liquid under AC Stress

Abstract

The insulation system used in the power transformer basically consists of a transformer liquid and the cellulose-based solid insulation materials including paper and pressboard. This combination has been widely applied in the transformer industry for decades and the pre-breakdown and breakdown phenomena in the transformer liquid and on the liquid/solid interface have been extensively investigated. However, the majority of the studies were carried out in a static liquid condition and at the room temperature. In practice, the transformer is operating at an elevated temperature rather than the room temperature, and the transformer liquid is flowing to fulfil its cooling function. Meanwhile, the increasing application of ester liquids in power transformers requires a better understanding of their performance at the operating conditions. Therefore, this PhD study focuses the experimental investigation on the effects of temperature and liquid flow on partial discharge, streamer and breakdown phenomena in a synthetic ester liquid, as well as the effect of liquid flow on surface tracking on the liquid/pressboard interface under AC stress. A liquid circulation system with a heating function was built up for this study and the electrical circuit for PD measurements was made in accordance with the IEC 60270. The temperature range was set from 20 to 80 ºC and the liquid flow range was controlled from 0 to 0.2 m/s. A needle-to-plane electrode configuration was employed to produce the divergent electric field and two plane-needle-plane electrode configurations with various lengths of needle protrusions were applied to simulate the semi-uniform electric field. The results revealed that, in the divergent electric field, the increasing temperature facilitates the positive (those occur in the positive half cycle) PDs but hinders negative (those occur in the negative half cycle) ones. When the temperature increases, the magnitude and repetition rate of positive PDs increase slightly but these of negative PDs decrease dramatically. In addition, the streamer stopping length and area of negative streamers reduce significantly at a higher temperature. The increasing temperature also results in a lower AC breakdown voltage. The temperature effects were further explained in details based on the EHD and space charge theories. The pre-breakdown and breakdown phenomena were also studied at different liquid flow velocities. It was found that the pre-breakdown and breakdown characteristics of the synthetic ester under a divergent electric field are not sensitive to the liquid flow velocity in the investigated range. The increasing liquid flow velocity only causes a slight reduction of negative PD magnitude, and this impact is too small to be verified by the negative streamer investigation. However, under a semi-uniform electric field, the increasing liquid flow will result in a higher AC breakdown voltage and this effect is more profound when the electric field became more uniform. The surface tracking tests were performed under different moisture conditions and at various liquid flow velocities. It was concluded that the development of white mark takes longer time with the increasing liquid flow velocity or the decreasing moisture content in pressboard. In addition, it was found that the increase of overall development time for white mark at lower moisture content is due to the significant extension of the initiation time. On the other hand, the increasing liquid flow velocity will reduce the growth rate of white mark while it has a negligible effect on the initiation time.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Zhongdong Wang (Supervisor) & Qiang Liu (Supervisor)