Lightning impulse voltage quality transformer oil test platform

作者： GOZ Electric 时间：2024-08-28 09:15:35 阅读：11

In order to study the development mechanism of streamer discharge of gas-made transformer oil under lightning impulse voltage, this paper built an impulse voltage discharge experimental platform, and observed the changes in streamer propagation-related characteristic parameters caused by the ionization process and thermal effect of the streamer tip in gas-made oil from the streamer starting voltage stage to the acceleration voltage stage, so as to clarify the specific role of the streamer tip ionization process and thermal effect in each stage of streamer development. The research results show that in the streamer starting stage, the physical essence of the streamer is a low-density gas channel formed by local gas-liquid phase change. In the streamer propagation and breakdown stage, the physical essence of the streamer is the photoionization process caused by the high field strength at the head of the streamer, and the continuous generation of the gas-liquid phase interface during the direct ionization of the liquid. The change of the streamer morphology from a dark channel with weak photoionization to a bright channel with strong photoionization is the mechanism causing the sudden change of the streamer propagation mode near the acceleration voltage.

Keywords: streamer discharge; gas-made transformer oil; breakdown voltage; photoionization

Traditional transformer oil is obtained by fractionation of petroleum. Due to its good insulation and stability, it is widely used in modern power systems. Although traditional transformer oil has stable performance, it has disadvantages such as being difficult to degrade, containing corrosive sulfides and the risk of environmental pollution. In recent years, a technology for preparing insulating oil based on natural gas cracking has gradually emerged. By extracting hydrocarbons from natural gas and converting natural gas into a liquid medium using Fischer-Tropsch synthesis technology, natural gas is converted into a liquid medium. Compared with traditional mineral transformer oil, the main component of gas-to-oil is isomeric paraffins, and the aromatic hydrocarbon content is significantly lower than that of mineral transformer oil. It has the characteristics of high purity, degradability and no corrosive sulfides. It is a new type of insulating oil that is expected to replace traditional transformer oil in the future.

The breakdown phenomenon caused by high voltages such as lightning impulse voltage or operating impulse voltage during the operation of the transformer is essentially a streamer discharge breakdown caused by the conductive streamer short-circuiting the oil gap caused by the ionization of the insulating oil under the action of local high field strength. Studying the generation and development characteristics of streamer discharge of insulating oil is an important way to study the insulation performance and insulation mechanism of insulating oil. At present, a large number of experimental and simulation studies on streamer discharge in mineral transformer oil and plant-type transformer oil have been carried out at home and abroad, and there are relatively few related studies on streamer discharge in gas-to-oil. Wang Jianyi and others from the China Electric Power Research Institute analyzed the typical insulation properties of gas-to-oil and found that gas-to-oil has higher lightning impulse breakdown voltage and stability. Wang Zhongdong and others from the University of Manchester in the United Kingdom found that compared with mineral oil, gas-to-oil has higher positive polarity lightning impulse breakdown voltage, but lower positive polarity acceleration voltage; under the action of negative polarity lightning impulse voltage, gas-to-oil has higher breakdown voltage and acceleration voltage than mineral oil. However, these studies mainly focus on the testing of the insulation and discharge properties of new gas-to-oil. The mechanism of gas-to-oil having more stable discharge characteristics and the physical mechanism of discharge in gas-to-oil need further in-depth research.

In this paper, a 25 mm needle-plate electrode system insulating oil streamer test platform was built. With traditional mineral oil as a reference, an impulse voltage generator was used to apply a 1.2/50 μs standard lightning impulse voltage under positive and negative polarity to the gas-to-oil transformer oil sample. A high-speed camera was used to observe and record the propagation characteristics of the two oil samples such as the streamer propagation velocity and streamer propagation length under different voltage levels, and the corresponding change law was obtained. Combined with the chemical composition analysis of gas-to-liquid, the physical nature of the initiation, propagation and final stages of streamer discharge in gas-to-liquid is discussed. Furthermore, based on the specific role of the streamer head ionization process and thermal effect in different stages of streamer development, the mutation mechanism of the propagation mode of the streamer near the acceleration voltage is discussed.

Test platform

The structure of the insulating oil streamer measurement test platform used for streamer discharge is shown in the figure. A cubic stainless steel sealed test chamber with a volume of 1.5 L is used. The chamber cover and the chamber base have access holes for filling and discharging the insulating oil inside the chamber. Symmetrical 40 mm transparent quartz glass windows are opened on both sides of the chamber for streamer discharge observation. The high-voltage electrode is insulated and isolated from the stainless steel chamber by a ceramic bushing to prevent surface flashover when high voltage is applied, ensuring the safety of the test.