The impact of contamination on audible noise in AC transmission lines

The objective of this study is to investigate the impact of accumulated pollutants on the surface of a 1000 kV AC transmission line in a specific location in China on its audible noise generation. The line with diatomaceous earth, kaolin, and other pollutants attached to it is modeled by using finite element software, and the study results indicate that the conductivity of the material plays a significant role in influencing the audible noise. In addition, diatomaceous earth has the least effect on the amplitude and lateral attenuation of audible noise, and sodium chloride and carbon powder have the greatest effect. The research in this paper can be used for the reason that dirt particles affect the magnitude of audible noise and these findings offer valuable insights for the management of audible noise.


Introduction
The scarcity of land resources has led to a reduction in the distance between transmission lines and the public, resulting in an increase in environmental complaints due to the disturbance caused by audible noise, posing numerous obstacles to engineering construction and management [1][2][3].Taking Shandong Province as an example, there are a considerable number of ultra-high voltage transmission lines in Shandong that pass through areas with varying degrees of contamination, leading to complex distributions of audible noise.Studies by scholars [4][5] have shown that the surface condition of conductors has a significant impact on corona sparking and the resulting interference (acoustic noise, radio interference, etc.); the presence of contaminants on the conductor surface reduces the onset voltage of corona sparking and increases audible noise.Research on audible noise in transmission lines [6][7] primarily investigates the impact of environmental conditions, such as rainy weather or high altitudes, on the audible noise emitted by clean conductors, with limited studies on the impact of surface contamination on transmission lines.Based on the distribution map of power pollution in Shandong Province, this study selects several transmission line spans in the polluted areas above the medium pollution level in Shandong, monitors the audible noise around the transmission lines, analyzes the impact of polluted areas pertaining to the audible noise in transmission lines, and further investigates the impact of contaminated particles on the distribution of noise around high voltage lines by using finite element simulation software and transmission line prediction formulas.

Field monitoring plan for acoustic noise
To investigate the distribution of audible noise in ultra-high voltage transmission lines with different levels of contamination, preliminary investigations and on-site surveys are conducted.Based on the distribution map of power pollution in Shandong Province, the same transmission line is selected for monitoring in different polluted areas.The chosen line is the China Ximeng-Shandong 1000 kV ultra-high voltage AC transmission line for audible noise monitoring.The selected spans are located in flat farmland near Jinan City, Shandong Province, without the presence of nearby tall buildings or other high-voltage transmission lines.The measurements are carried out during the summer season, and people standing beneath the line can clearly hear the hissing sound.
According to the reference standard of this measurement scheme, the sensor is set at a height of 1.5 m and the background noise projection point is positioned 50 m beyond the ground projection point of the side phase conductor.According to Figure 1, we set the monitoring point every five meters, and the endpoint is the position at 40 m.Each point is monitored for 20 seconds.The B & K2250 sound level meter is utilized for monitoring, and the measurement results are processed by using A-weighting.

Audible noise prediction model for transmission lines
The two-dimensional finite element method is a common numerical calculation method used to solve the surface electric field of transmission lines.It is known for its high accuracy, strong applicability, and scalability, which makes it widely used in the research and design of transmission lines.The model used in this study refers to the transmission line observed in the field.The conductor chosen has a specification of LGJ-500/45, which is an eight-split conductor.The cross-section of the steel core aluminum stranded wire is simplified according to Figure 2, the ground wire utilized is an aluminum-clad steel stranded wire.The outer diameter of the conductor is approximately 12 mm, with a horizontal spacing of 18 m between layers.The conductivity of aluminum used is 3.8×107 S/m, and the conductivity of steel is 2×106 S/m.The entire simulation domain is considered an air domain.The entire transmission line is constructed as a double-circuit arrangement on the same tower.The conductors are arranged vertically in an umbrella-shaped configuration, with a spacing of 20 m between conductor layers.The lowest conductor is positioned 48 m above the ground, and the horizontal spacing between layers increases from bottom to top, with values of 15.5 meters, 14.5 meters, and 13.5 meters, respectively.Figure 3 illustrates the schematic diagram of the conductor arrangement on the tower.
For the simulation of contamination particles on the surface of the conductor, a conical shape is mainly used to simulate the particles.Simulating the particles with a conical shape has the advantages of realistic particle shape, easy control of geometric parameters, high computational efficiency, and wide application in research.The schematic diagram of a single split conductor simulation is shown in Figure 4, where the cone has a radius of 0.2 mm and a height of 0.4 mm.Regarding the selection of contamination particles, simulations are carried out by using diatomaceous earth, kaolin, sodium chloride, and carbon black.The material parameters for these particles are shown in the following Table 1.As the transmission line conductors are rated at 1000 kV, the electric field intensity being solved for is the static electric field at t=0.Therefore, the applied excitations on the three-phase transmission lines are as follows, from bottom to top: UA=816 kV, UB=-408 kV, UC=-408 kV.The phase angles for A, B, and C are 0°, 120°, and -120°, respectively.
The calculation formula of noise is the BPA noise prediction formula, as the following Formulas (1-2): (1) In the formula, E represents the surface gradient of the conductor, in kV/cm; deq represents the equivalent radius of the conductor, in mm; n represents the number of conductor segments; d represents the diameter of each sub-conductor, in mm.

Simulation results of surface electric field on the transmission line
Finite Element Electric Field Analysis is conducted on both clean conductors and four types of contaminated particles.An AC steady-state solver is used, and a classical mesh with adaptive mesh refinement is employed.Table 2 presents the maximum surface electric field values for the three-phase conductors of clean wires and wires with attached contaminated particles.From the data analysis in the table, it is evident that the wire's surface with attached impurities experiences a substantial increase in maximum electric field strength, approximately 3.5 times higher than that of a clean wire.
Due to the equal relative permeability between carbon powder and diatomaceous earth impurities, the surface electric field strength of the three-phase line with carbon powder is approximately 7.4 kV/cm higher than the average difference of diatomaceous earth.This indicates that the main factor influencing the surface electric field strength of the line is the conductivity of the impurity particles.

Line noise analysis
The noise distribution of the line can be calculated.The calculation is done with the reference point at the zero coordinate of the wire sag, and the distribution is determined based on the schematic diagram of the field-measured data.Following the principle of symmetry, this study only considers the data on one side of the conductor when calculating the lateral distribution.
In order to analyze the errors in the results of this model, the audible noise simulation results of the conductor with diatomaceous earth particles, which have relatively low electric field strength, are selected for comparison with the line data monitored in various polluted areas, as shown in Figure 5. Through the analysis shown in the figure, it can be observed that the measured audible noise data increases in amplitude as the pollution level of the area increases.The data from heavily polluted and very heavily polluted areas show disturbances due to background noise, but overall exhibit a decreasing trend.Comparing the simulation results with the data from different polluted areas in the figure, it can be noticed that the measured results in the field are slightly lower than the simulation results.This is because the BPA predictive formula calculates the audible noise under heavy rainfall conditions, while the field measurements are conducted on a clear day.However, by observing the trends, it can be noted that the attenuation amplitude for moderate pollution areas is 11%, for heavily polluted areas, it is 0.9%, for very heavily polluted areas, it is 3.5%, and the simulation results show an attenuation amplitude of 1.6%.Comparing these data, it can be concluded that the attenuation amplitude of diatomaceous earth is within 2% of the measured results from heavily polluted and very heavily polluted areas, thus demonstrating the accuracy of this model.
Based on the above analysis, the results of the four types of impurity particles and the clean wire are compared, as shown in Figure 6.Based on Figure 6, it is evident that the sound pressure level amplitude of audible noise in the transmission line increases substantially with the presence of impurity particles.Among them, carbon powder and sodium chloride particles exhibit the largest increase in amplitude, while diatomaceous earth shows a smaller increase compared to the other three impurity particles.Additionally, as the distance increases, the change in audible noise of the transmission line shows a relatively small attenuation rate within a lateral distribution range of 50 m from the wire sag.The attenuation rates are as follows: clean wire 18.8%, carbon powder 1.6%, diatomaceous earth 2.2%, sodium chloride 1.6%, and kaolin clay 1.4%.From the above data, it can be observed that impurities can also affect the lateral attenuation of audible noise in the transmission line.The ranking of impurity particles in terms of their impact on audible noise is as follows: diatomaceous earth < kaolin clay < sodium chloride < carbon powder, with carbon powder and sodium chloride having a similar impact on the audible noise.

Conclusions
In this paper, finite element modeling is conducted on the cross-section of a 1000 kV ultra-high voltage transmission line, and the results are validated through field measurements.Based on this, the following conclusions are drawn: 1) By comparing diatomaceous earth with kaolin clay, it is found that the main factor affecting the audible noise of the transmission line is the material conductivity.Therefore, when addressing the audible noise control of transmission lines, research on suppression strategies can be focused on the direction of material conductivity.
2) Different impurity particle coverages not only affect the amplitude of audible noise but also impact the lateral attenuation of audible noise.The degree of influence on audible noise can be ranked as follows: diatomaceous earth has the smallest impact, followed by kaolin clay, and sodium chloride and carbon powder have the greatest impact.This indicates that when considering audible noise control strategies for transmission lines, it is necessary to comprehensively consider the impact of different impurity particles based on the actual situation.

Figure 2 .
Figure 2. Cross-section of transmission line conductors.

Figure 4 .
Figure 4. Schematic diagram of contaminants on a single split conductor.

Figure 5 .
Figure 5.Comparison of errors in line simulation data.

Table 1 .
Material Parameters of Contamination Particles.

Table 2 .
The maximum surface electric field intensity of the line.