The morphologies and velocity of ionic wind induced by direct-current (DC) corona discharge were experimentally measured. DC voltage was applied to short air gaps of high-voltage electrode–needle electrode and grounding electrode–net electrode to generate coronas, so as to allow the generation of ionic wind induced by corona discharge. The purpose of the study is to optimise the multi-needle-to-net electrode structure to improve the velocity of the ionic wind and keep it in a relatively stable state. For this purpose, the influences of various parameters on the velocity of the ionic wind were explored. The research results showed that the side length of meshes and the distribution mode of needles significantly affected the velocity of the ionic wind. The maximum velocity of the ionic wind, when the side length of meshes was 0.85 mm was 3 and 1.28 times those when the side lengths of the meshes were 2 and 0.425 mm, respectively. This indicated that there was an optimal side length that can maximise the velocity of the ionic wind. Increasing the number of needles and optimising the distribution mode of needles can effectively improve the velocity, that is, the improvement of the number and distribution uniformity of needles was conducive to increase the velocity of the ionic wind. Increasing the voltage amplitude can increase the velocity of the ionic wind, however, it was possible that noise was significant and in this case, the corresponding voltage cannot be considered as the working voltage of the ionic wind generator.

Lightning has a certain degree of potential threat to the safe operation of overhead transmission lines. In order to make targeted lightning protection arrangements and reduce the impact of lightning on overhead transmission lines, it is necessary to conduct lightning risk assessments on overhead transmission lines. This paper proposes a lightning failure risk assessment method for overhead transmission lines based on a modified Dempster–Shafer theory. First, analyze the historical lightning failure data of the line, determine the lightning failure impact factors, and use confidence to express the relationship between the lightning failure and the impact factor; then, use entropy weight theory and gray relational theory to calculate the value of mass function, and modify it on this basis; finally, use Dempster–Shafer theory to determine the trust degree and fit this with the calculated lightning trip rate to produce the risk assessment. This paper analyzes the lightning failure data of overhead transmission lines in some areas of Hebei Province. The results show that, compared with the evaluation method of the Dempster–Shafer theory, the accuracy of the evaluation is improved to a certain extent after correcting the mass function value. It can be seen that this method can integrate and comprehensively consider different data and can provide a reference for preventing damage to transmission lines by lightning strike weather.

The construction of ultra‐high‐voltage direct‐current power transmission projects in high‐altitude areas calls for the research into the discharge characteristics of the air gaps in the valve hall of a power converter station in such areas. Tests on the switching impulse discharge characteristics of the sphere‐plane air gap were conducted at Yangbajing test base (Tibet, China) at an altitude of 4300 m on conditions that there were burrs on the 1.3 m diameter shield balls. The discharge characteristics, under the influences of burrs with different lengths and locations, were obtained and compared with test data from low‐altitude areas. The results indicate that the arrangements of burrs with different lengths and locations can significantly reduce the switching impulse discharge voltage of sphere‐plane air gaps, while the voltage sag gradually decreased down with increasing burr length. Moreover, the larger the sphere‐plane air gap is, the smaller the influence of such burrs have. High altitude also substantially decreased the switching impulse discharge voltage of the air gap. When the surface of shield balls was smooth, the U50 saturation of the air gap was more significant than at low altitude; however, if there were burrs on the shield balls, the saturation effect was less significant.

In this work, a combined photoelectric detecting system is established using devices such as a photomultiplier, an electric field sensor based on optoelectronic integration, and a high-speed camera. The system is used to measure the instantaneous optical power, spatial electric field strength, and optical image during discharge of a 3-m-long sphere–plane gap under standard switching surge voltage of positive polarity. The test results show that the initial streamer of the 3-m-long sphere–plane gap can be divided into strong and weak ionization according to the rate of rise of optical power and electric field strength. Within the dark discharge period in the sphere–plane air gap, the intensity of the spatial electric field slightly jumps when the streamer-to-leader transition is completed, with an increase of about 162.59 kV/m. It is proposed to take the time when the intensity of the spatial electric field jumps in the dark discharge period as the criterion for the streamer-to-leader transition. The measured duration t_a of the streamer-to-leader transition is calculated according to instantaneous optical power and spatial electric field strength. Based on the thermodynamic equilibrium equation, a Gallimberti simulation model for this streamer-to-leader transition is established. The duration t_s for completing the streamer-to-leader transition is obtained according to the quantity Q of injected electric charges. By comparing the measured duration t_a with the calculated duration t_s of the streamer-to-leader transition, the rationality of the criterion is verified. The research results provide a basis for establishing an improved model for the inception of a leader in a long air gap.

The study established a test platform for a long sphere‐plane air gap in the test hall of the ultra‐high voltage (UHV) direct current(DC) Testing Site. Under the sphere‐plane gaps with 3, 5 and 7 m distance, the study carried out the experiment on discharge characteristics under switching impulses on a shield ball on which surface tips had different lengths at different locations. By comparing with the test data obtained under conditions with a smooth surface of the shield ball and the same gap distance, the discharge laws under the influence of surface tips on the shield ball are shown as follows: the presence of the tips greatly decreased the discharge voltage of the sphere‐plane gap with the maximum decrease amplitude of 39.0%. Moreover, the decrease degree was increasingly significant with the growing length of tips. The influence of the same length of tips on the discharge voltage of the gap gradually declined with the growth of the gap distance. The presence of tips significantly reduced the discharge voltage, especially in the perpendicular line of the shield ball. The results indicate that it's necessary to strictly control the machining and installation to avoid the occurrence of tips in UHV DC engineering.

Air density is an important factors affecting the morphology characteristics of long air gap discharge channels. Analyzing characteristic parameters, such as the propagation velocity and angle of the streamer-leader system at high altitude areas is indispensable in studying the discharge mechanism of long gaps. In this paper, optical images and optical power of the discharge channel were captured. The propagation velocity of the streamer-leader system and angle distribution characteristics of the streamer region were obtained. In accordance with the findings, the downward velocity of the streamer-leader system at low pressure ranges from 1 × 10⁴ to 5 × 10⁴ m s⁻¹. Simultaneously, the velocity decreases slightly under the voltage with a low rise rate, and it is often accompanied by the phenomenon of leader re-luminescence. In addition, the optical images were processed by the pseudo-color algorithm, and the angle of the streamer region of the downward leader head was obtained between 50.3° and 71.6°. The angle of the streamer region is inversely correlated with the voltage rise rate but positively correlated with the gap distance. The results obtained in this paper have academic significance for improving the theoretical system of long gap discharge in high-altitude extreme environments.