# Modeling of Positive Switching Impulse Discharge of UHV Transmission Line Air Gaps
## 1 Introduction  
In order to solve the problem of reverse distribution of energy resources and load centers, China has carried out a great deal of research and construction work on Ultra High Voltage (UHV) transmission lines since the beginning of the 21 st century. UHV transmission lines stand for alternating current (AC) transmission lines with voltage levels of 1000 kV and above, and direct current (DC) transmissions with voltage levels of \(\pm\)800 kV and above, which have the characteristics of large transmission capacity, long transmission distance, and low line loss. At present, China has put into operation several 1000 kV AC and \(\pm\)800 kV DC UHV transmission lines [1].  
In transmission lines, the air gap is the main external insulation mode of towers. In the design process of transmission lines, the choice of air gaps is very important. Reasonable design is not only related to the safe and stable operation of the system, but also to the cost of the project. Compared with power frequency and lightning overvoltage, the air gap required to withstand switching impulse overvoltage is the largest. Moreover, according to past test results, the positive switching impulse breakdown voltage of the same gap is significantly lower than the negative switching impulse breakdown voltage [2]. Therefore, the positive switching impulse discharge characteristics are an important basis for the external insulation design of transmission line towers.  
The air gaps in transmission lines belong to long air gaps. The discharge characteristics of long air gaps are complex and have been the focus of research in high voltage transmission projects for many years. In past decades, scholars have carried out a lot of research on long air gap discharge characteristics. In the 1960s, some scholars studied the discharge characteristics under switching impulse, obtained the breakdown characteristics of different gap structures through a large number of tests, and established several empirical formulas for calculating the discharge voltage [3; 4; 5]. Since the1970s, some scholars have carried out tests on the discharge characteristics of power transmission and transformation projects to provide a basis for Extra High Voltage (EHV) and UHV transmission projects [6; 7; 8]. In the 1980s, some scholars established a typical gap breakdown voltage calculation model based on the leader discharge mechanism [9; 10]. Since the 21 st century, with the continuous progress of technology, scholars have carried out long air gap breakdown voltage prediction calculation and observation tests [11; 12; 13; 14; 15; 16; 17; 18; 19], further promoting the research process of long air gap discharge. At present, scholars mainly adopt real tower discharge tests to obtain the positive switching impulse discharge characteristics of air gaps in UHV transmission lines. Since the research on the discharge model is not perfect, test designs are not reasonable and therefore have high costs. In order to solve this problem, this paper studies the discharge characteristics of air gaps in UHV transmission lines, and obtains the discharge models.