Numerical Simulation Method of Ion Flow Field in DC Transmission Line Based on Wind Speed Probability Distribution and Wind Shear Exponent

The ion flow field under ultra high voltage direct current (UHVDC) transmission lines is one of the important evaluation indicators for electromagnetic environment. Wind speed, as one of the most common and influential factors, has obvious regional characteristics. At present, scholars often use constant wind speed to replace natural wind speed, which can easily lead to the following two problems. One is the waste of resources, as the local wind speed used in the calculation is not necessarily as high as possible. Every 1 m increase in the height of the line installation will significantly increase the total project cost. Secondly, it is inconsistent with the actual situation. Due to the relationship between wind speed and height, there is a gradient relationship on the near ground side. Therefore, the impact of wind speed changes in the vertical direction on the ion flow field should be considered. In view of this, this paper proposes a numerical simulation method of transmission line ion flow field based on wind speed probability distribution and wind shear exponent. The calculation method is divided into two parts: wind speed calculation and ion flow field calculation. This method builds a Weibull wind speed probability distribution function model based on the wind speed data of the region to be calculated, and selects the critical wind speed with the max wind speed data within 95% as the reference value. It considers the installation height of the wind speed sensor, and calculates the wind speed distribution at different heights according to the wind shear exponent, as the wind speed input at different height nodes in the calculation of the ion flow field. The accuracy of the calculation model after the introduction of wind shear exponent is verified through outdoor tests. The voltage level is ±800 kV, the conductor is 6-split, the radius of the sub conductor is 1.68 cm, the cross-sectional area of the sub conductor is 630 mm2, the height of the conductor to the ground is 16.5 m, the pole spacing is 22 m, the altitude is about 2 000 m, and the weather is sunny. The wind speed measurement point is 1m above the ground, and the wind speed is maintained between 0.5~1 m/s. A total of 9 measurement points are arranged below the line. Due to the high altitude region, this article also corrected the parameters such as the corona field strength, ion mobility, and recombination coefficient. Compared with the traditional wind speed model, after the introduction of the wind shear exponent, the errors between the ion current density and the total electric field strength decreased by 16.2% and 3.8%, respectively. The results show that the method is more consistent with the actual distribution. Taking the actual operating ±800 kV UHVDC transmission line in Kunming area as an example, this method was used for calculation. First, to calculate the wind speed. The parameters of Weibull probability density distribution function in Kunming area obtained by fitting are: α=3.5, β=3.172, δ=0. The critical maximum wind speed within the 95% wind speed range of the probability distribution curve is calculated to be 4.36 m/s. Combined with the height above the ground of the wind speed sensor, the wind shear exponent is used to calculate the wind speed at each node in the solution field as the input of the ion flow field calculation. At the same time, this paper also provides calculated values for different heights as a comparison. The results show that, under the condition that the total electric field strength meets the standard, the minimum height of the conductor could be 20 m, which has a significant margin difference from the actual height of 27.5 m used in the actual line section. In summary, the method proposed in this paper can specifically consider the impact of wind speed in the area where the line is located, provide a reference basis for the design of the line to ground height under the safety limit of the total electric field, especially in areas with high sea level and high wind speed. © 2024 China Machine Press. All rights reserved.