A bird droppings flashover analysis method based on the combination of electric field distortion and spatial electric field distribution

To study the flashover of air gap caused by bird droppings shorting air gap and the influence of space electric field change on it, the distortion electric field simulation, space electric field simulation, and related flashover test of transmission line bird droppings falling process were carried out. By analyzing the specific influencing factors of bird droppings flashover fault, it is pointed out that the electric field distortion caused by bird droppings will affect the spatial electric field distribution around the insulator and thus affect the air gap flashover. This study analyzes the specific causes of bird dropping flashover of transmission line insulators, reduces the number of bird damage trips of transmission lines, reduces the cost of operation and maintenance of transmission lines, improves the intrinsic safety of transmission lines, and ensures the safe and reliable operation of power grids.


Introduction
Bird damage is an important factor affecting the safe operation of transmission lines.In recent years, as the country's environmental protection efforts continue to increase, bird survival environment has been greatly improved.According to statistics, at present, the proportion of transmission line failures caused by bird activity is second only to lightning strikes and external damage, ranked No. 3 [11].Bird-related tripping has posed a serious threat to the safe operation of transmission lines; therefore, it is of great significance to carry out the study and analysis of the aberrant electric field simulation and spatial electric field simulation of the dropping process of bird droppings on transmission lines.
At present, a variety of research methods have been practically applied to the analysis of birddropping flashovers.In [1], by changing the structural parameters of the insulated lead wire and applying the industrial frequency voltage analysis obtained to obtain the insulated lead wire insulation thickness, conductor diameter, end shape, and other parameters of the insulated lead wire flashover characteristics of the influence of the law.The flashover research platform was used to study the effect of bird droppings viscosity, conductivity, and the presence or absence of pressure equalizing ring on bird droppings flashover while combining with electric field simulation to study the effect of bird droppings dropping path on bird droppings flashover [2].In [3], through a 220 kV transmission line tripping fault analysis of the tripping cause for the bird droppings flashover, the cause of the flashover was analyzed at the IOP Publishing doi:10.1088/1742-6596/2771/1/012030 2 same time the use of electric field simulation to verify the optimal thickness of the line insulation sheath at different voltage levels and measures to prevent similar bird damage failure were put forward.However, the above studies did not take into account the electric field distortion caused by the dropping of bird droppings and its effect on the electric field in space.Therefore, it is necessary to carry out relevant research to reduce the operation and maintenance cost of the line and provide references to ensure the safety of the line.
This paper establishes a simulation model of bird droppings flashover of composite insulators of transmission lines under different voltage levels and completes the analysis of electric field distortion of bird droppings flashover of composite insulators through electric field simulation results, and at the same time, combines with its impact on the spatial electric field of bird droppings when they fall to the bird droppings flashover to complete the analysis.

Simulation of electric field distortion during bird droppings process
Compared to DC composite insulators, the operation of the continuous DC voltage role is different.AC composite insulators in operation due to the influence of the frequency alternating voltage, and the frequency voltage cycle change than the line transient response time are about four orders of magnitude higher.The voltage wavelength is much larger than the transmission line itself size.Therefore, it can be considered that the insulator operation in a small period of the voltage borne by a constant DC voltage, in the AC composite insulator electric field simulation calculations, consider the most stringent circumstances apply the amplitude of the highest operating phase voltage.At this time, the composite insulators subjected to the voltage can be viewed as a steady voltage, and other moments of voltage alternation on the composite insulators are not representative of the impact of the electric field distribution, which can be handled by the static field Calculation modeling.
When bird droppings flashover occurs in composite insulator string, the surrounding spatial electric field distribution curve will be distorted compared with the intact one, and the simulation of spatial electric field distribution characteristics of insulator string is of great significance for bird droppings flashover [7~10] .In this chapter, the three-dimensional model of 110 kV and 220 kV composite insulator strings of transmission lines is constructed based on COMSOL finite element simulation software, and the spatial electric field distribution characteristic curve of bird droppings flashover of composite insulators is obtained by simulation calculation according to the actual operation situation.

Description of modeling
The main modeling in this study was established for bird droppings and composite insulators, where: (1) Bird droppings model Since bird droppings is an object with high electrical conductivity, it is usually treated as a conductor for computational convenience.In the simulation analysis, the change in bird droppings morphology is generally ignored, the bird droppings are modeled by a long straight conductor rod, and the end of the bird droppings is smoothed by a spherical structure.Meanwhile, in the simulation model, the diameter of the droppings channel is kept unchanged, and the effects of different droppings morphologies and their spatial positions on the electric field in the gap space are analyzed by changing the length and spatial position of the droppings.
(2) Insulators model Insulators are the most important insulating devices in transmission lines.Insulators are set up in the high-voltage conductor and grounded between the tower, in addition to the need to bear the normal operation of the high voltage, but also to ensure that can withstand a certain degree of over-voltage to match the line relay protection.At the same time, the structural strength of the insulator ensures that it can withstand the connection of fittings and conductor weight, as well as in bad weather, such as wind conditions caused by mechanical stress.
The models developed under 110kV and 220kV are shown in Figure 1 and Figure 2:

Composite insulator bird droppings flashover electric field distortion analysis
In this study, it is believed that the occurrence of bird droppings flashover is related to the change of spatial electric field distribution caused by the electric field distortion near the bird droppings channel.In this chapter, the finite element software will be used to simulate bird droppings flashover of composite insulators of 110 kV and 220 kV transmission lines, and combined with the previously established model, the change of the whole spatial electric field under different conditions during the occurrence of bird droppings flashover will be analyzed in depth.

Bird droppings flashover electric field distortion process.
Bird droppings flashover is divided into two categories, one is the bird droppings channel caused by insulator string flashover, and the other is bird droppings dripping on the insulator on rainy days or moisture together with other filth on the insulator surface to form a conductive liquid caused by the insulator string along the surface of the flashover.The first category is the main cause of insulator bird-dropping flashover failure.Bird droppings channel caused by insulator string flashover mechanism is a bird falling instant distortion of the electric field distribution around the insulator so that the bird droppings channel and the insulator high voltage measurement insulation ring and other parts of the air gap breakdown and lead to the flashover.The entire flashover development process is divided into the following three stages [12] : Stage 1: Formation and extension of bird droppings channels.After the birds discharge their droppings, the droppings generally fall from the top of the crossbar in a free-fall manner, and in the process of falling, they are gradually stretched into elongated channels due to the effect of gravity.As the lower end of the bird droppings channel starts to fall first, the lower part of the bird droppings channel that falls first always falls faster than the upper part of the bird droppings channel.Thus, as time goes by, the distance between the first and last ends of the bird droppings channel will be bigger and bigger, which makes the bird droppings channel much thinner, so the length of the bird droppings channel when it reaches the vicinity of the high-voltage end of the insulator is much longer than that at the beginning of the descent from the cross-bar.Stage 2: When the lower end of the bird droppings channel reaches the vicinity of the high-voltage end, the electric field around the insulator begins to undergo serious distortion, this time due to the bird droppings channel having a certain degree of electrical conductivity and making the bird droppings channel lower end and the insulator high-voltage end of the air gap between the electric field strength increases instantly so that most of the voltage distributed on the insulator are added to this section of the air gap.
Stage 3: When the electric field distortion reaches a certain degree, the lower end of the air gap field strength reaches the critical breakdown field strength and breakdown occurs, in the high-voltage side will produce a local arc.The arc will be along the bird droppings channel to the low voltage end of the development, at this time the bird droppings channel on the voltage will increase instantly, thus making the bird droppings channel on the upper part of the air gap on the upper end of the part of the air gap also began to withstand a lot of voltage, thus making the upper end of the air gap also occurs, so that the localized arc will ultimately cause to become a complete flashover.

Electric field distortion of bird droppings flashover under 110 kV.
After the model is built, it is imported into COMSOL for simulation calculations.
The following are the simulation results and analysis for the 110 kV voltage condition: (1) Normalized composite insulators: Normal composite insulator along the surface of the electric field curve as a whole was asymmetric "U" type, the high-voltage end and the low-voltage end of the field is strong, and the medium-voltage end of the field is small.As shown in Figure 3 in the left area, each opening up the parabola for a complete skirt along the surface of the electric field curve, a total of 12 similar areas.Starting from the upper skirt, the field strength gradually decreases and reaches a minimum at the junction of the upper and lower skirts, after which the field strength gradually increases and reaches a maximum at the lower skirt.As in the right area in Fig. 3, the field strength decreases once at the end due to the installation of the pressure-equalizing ring at the high-pressure end [4~5] .(2) Composite insulators when dropping bird droppings As can be seen in Figure 4, the electric field effect around the lower end of the bird droppings channel, when it is detached from the crossbar, is obvious, while only the end of the upper end shows a weak distortion effect.The electric field on the other side remains essentially unchanged.As shown in Figure 5, the electric field strength on the side of the insulator near the bird droppings is slightly higher than that on the side away from the bird droppings [7] .As shown in Figure 6 and Figure 7, bird droppings fall along the surface of the composite insulator electric field curve as a whole was asymmetric "U" type, high-voltage end and low-voltage end of the field is strong, and medium-voltage end of the field strength is small.Relative to the air side of the "U" type, the bird droppings side of the "U" type of the lowest point to the low voltage side offset.Each parabola with an upward opening is a complete electric field profile along the surface of the umbrella skirt, as shown in the left-center region of the two figures, and there are 12 similar regions in total.As shown in Figure 6, under the influence of the bird droppings model, starting from the upper umbrella skirt, the field strength decreases rapidly to a minimum value, and then rises gradually, and the field strength rises the fastest at the junction of the upper and lower umbrella skirts, and then the field strength increases to a maximum value, and the height of the region where the maximum value is located is the lowest end of the bird droppings.As in Figure 6, the right side of the region, the high-pressure end due to the installation of a pressure equalization ring, so the field strength after reaching the peak began to gradually decline [4~5] .Figure 7 electric field curve distribution is similar to Figure 3, but by the lowest point of bird droppings, the high-voltage side of the electric field strength slightly decreased, and the highest point from the high-voltage side shifted to the low-pressure side.

Electric field distortion of bird droppings flashover under 220 kV.
The following are the simulation results and analysis for the 220 kV voltage condition: (1) Normalized composite insulators: Normal composite insulator along the surface of the electric field curve as a whole is asymmetric "U" type, the high-voltage end and the low-voltage end of the field is strong, and the medium-voltage end of the field is small.As shown in Figure 8 in the left area, each opening up the parabola for a complete skirt along the surface of the electric field curve, a total of 19 similar areas.Starting from the upper apron, the field strength gradually decreases and reaches a minimum at the junction of the upper apron and the lower apron, after which the field strength gradually increases again and reaches a maximum at the lower apron [4~5] .As in the right area in Figure 8, the field strength decreases once at the end due to the installation of the pressure-equalizing ring at the high-pressure end.(2) Composite insulators when dropping bird droppings As can be seen in Figure 9, the electric field effect around the lower end of the bird droppings channel, when it is detached from the crossbar, is obvious, while only the end of the upper end shows a weak distortion effect.The electric field on the other side remains essentially unchanged.As shown in Figure     As shown in Figure 11 and Figure 12, bird droppings fall along the surface of the composite insulator electric field curve as a whole was asymmetric "U" type, high-voltage end and low-voltage end of the field is strong, medium-voltage end of the field strength is small.Relative to the air side of the "U" type, the bird droppings side of the "U" type of the lowest point to the low voltage side offset.Each parabola with an upward opening is a complete electric field profile along the surface of the umbrella skirt, as shown in the middle-left region of both figures, and there are 19 similar regions in total.As shown in Figure 11, under the influence of the bird droppings model, starting from the upper umbrella skirt, the field strength decreases rapidly to a minimum value, and then rises gradually, and the field strength rises the fastest at the junction of the upper and lower skirts, and then the field strength increases to a maximum value, and the height of the region where the maximum value is located is the lowest end of the bird droppings.As in Figure 11, the right side of the region, the high-pressure end due to the installation of a pressure equalization ring, so the field strength after reaching the peak began to gradually decline.The distribution of electric field curves in Figure 12 is similar to that in Figure 8, but the peak value is affected by the bird droppings model.

Summary.
This chapter is based on composite insulators modeled according to the actual size applied in the actual line, after which COMSOL is used to simulate the 110 kV and 220 kV composite insulators as the simulation object and the comparative analysis concludes that the electric field along the surface under the influence of bird droppings undergoes a significant distortion and affects the distribution of the electric field on the air side.

Description of the experimental model and program
The experimental model was constructed in the high-voltage hall and the test arrangement is shown in Figure 13.The conductors were designed and fabricated according to 1:1 dimension.
The bird droppings flashover simulation test model is mainly composed of a bird-dropping simulation system, insulator string, pressurization device simulation tower head, and other parts.In this paper, the bird droppings simulation system in the model is improved and optimized to better meet the needs of field experiments [13] .The rubber insulators used in the test are categorized into two types according to their voltage withstand capacity: 110 kV insulator strings and 220 kV insulator strings.
In this paper, a set of multi-airway timing control bird droppings simulation systems is used, which can avoid the difficult and dangerous manual pole-climbing operation, and can easily meet the needs of field experiments under high-voltage conditions; the device does not need to set up control motors, wireless transmitting and receiving units in bird droppings simulation devices, and the bird droppings simulation device on the stretcher does not have any electronic components, and the actions of the relevant mechanical parts rely on the ground pneumatic part of the control, which avoids This avoids the possibility of damage to electronic components caused by electric arcs when bird droppings flashover occurs.
The whole bird droppings simulation system mainly consists of a bird-dropping excretion device, plastic air tube, air circuit control box, and air pump, and its general arrangement is shown in Figure 14 when the test is carried out in the field.

Test results and analysis (1) 110 kV insulator bird droppings flashover characteristics:
The number of flashovers under different conditions recorded during the test is shown in the table.The test results are shown in Table 2.As can be seen from the table, when the horizontal distance D between the dropping channel of bird droppings and the center axis of the composite insulator varies within the range of 23.5 cm, the probability of bird droppings flashover under both voltages is 100%; when the distance D is more than 25 cm, the probability of bird droppings flashover under both voltages shows a decreasing trend with the increase of distance D; when the distance D reaches 27.5 cm, the probability of bird droppings flashover under both voltages decreases to zero.When the distance D reaches 27.5 cm, the flashover probability of bird droppings at both voltages decreases to zero.
(2) 220 kV insulator bird droppings flashover characteristics: As can be seen from the table 3, when the horizontal distance D between the dropping channel of bird droppings and the center axis of the silicone rubber insulator varies within the range of 51 cm, the probability of bird droppings flashing under both voltages is 100%; when the distance D exceeds 52 cm, the probability of bird droppings flashing under both voltages shows a decreasing trend with the increase of distance D; when the distance D reaches 58 cm, the probability of bird droppings flashing under both voltages decreases to zero.When the distance D reaches 58 cm, the probability of flashover of bird droppings under both voltages decreases to zero.
(3) Flashback analysis When bird droppings fall, due to their charged nature, they may leave an electric charge on the insulators of the transmission line, causing an electric field distortion.This electric field distortion leads to inhomogeneity in the electric field distribution in the surrounding space, which may make the electric field strength around the insulators change.Especially considering the high conductivity typical of bird droppings, about 4000-8000 ȝs/cm [6], the formation of conductive paths may increase the inhomogeneity of the spatial electric field.If the electric field strength exceeds the insulating strength of the insulator, it may trigger flashover failures of composite insulators.Thus, the effect of electric field distortion on the spatial electric field may increase the risk of insulator flashover.To prevent such problems, improved bird-proofing devices, the use of more efficient insulating materials, and regular cleaning and maintenance programs may be considered to ensure that insulators are protected from external substances, thereby maintaining the reliability of the power system.An in-depth study of parameters such as electric field distortion mechanisms and conductivity can help to better understand and prevent this problem.A combination of experimental and simulation data shows that a change in the horizontal distance between the bird droppings drop channel and the center axis of the silicone rubber insulator may have a significant effect on the probability of bird droppings flashover of the insulator.This variation may affect the electrical performance of the insulator through several mechanisms.First, a smaller horizontal distance leads to more intense electric field distortions, making the electric field around the insulator more inhomogeneous, which in turn increases the risk of insulator flashover.Second, the change in horizontal distance may affect the distribution of the electric field in space, resulting in a more concentrated electric field, leading to certain areas where the strength of the electric field exceeds the insulator's dielectric strength, increasing the likelihood of flashover.In addition, the reduction in horizontal distance may increase the influence of bird droppings on the electric field, increasing the probability of forming conductive paths, which in turn increases the risk of insulator flashover.Therefore, changes in the horizontal distance between bird-dropping paths and insulators need to be carefully considered when designing and maintaining power systems to reduce the probability of electric field distortion and insulator flashover and to ensure the reliability of transmission lines [14~15] .

Conclusion
To cope with the challenge of bird damage on transmission lines, this study conducted a comprehensive simulation of bird droppings flashover on composite insulators by using multi-physics field simulation software, and also designed and carried out bird droppings flashover tests combined with the simulation content [16] .This provides a reference for future bird droppings flashover simulations and tests as well as the design and optimization of transmission lines.At the same time, the electric field strength distribution of the composite insulator and its surrounding space during the dropping of bird droppings is obtained, which is helpful for comparison and analysis with the electric field strength of insulators that encounter bird damage in actual operation.
The results of the study show that the closer the spatial distance between the bird droppings and the composite insulators when they fall leads to stronger electric field distortions, which makes the electric field inhomogeneity around the insulators increase, which in turn raises the risk of flashover of the insulators.This is the actual situation of composite power line operation.It provides support for the stable operation of power lines and helps to reduce the risk of power loss and equipment failure caused by corona discharge.Therefore, this study is of practical significance in improving the reliability and efficiency of high-voltage direct current transmission systems.

Figure 1 .
Figure 1.Simulation model under 110 kV.Figure 2. Simulation model under 220kV.The rest of the simulated material parameters are shown in the table 1 below:

Figure 2 .
Figure 1.Simulation model under 110 kV.Figure 2. Simulation model under 220kV.The rest of the simulated material parameters are shown in the table 1 below:

Figure 3 .
Figure 3. Electric field diagram along the surface of a composite insulator under normal conditions.

Figure 3 .
Figure 3. Potential distribution of composite insulator string cut surface.

Figure 5 .
Figure 5. Electric field distribution on both sides of the composite insulator string in section.

Figure 6 .
Figure 6.Electric field along the surface of the bird droppings side of composite insulator.

Figure 7 .
Figure 7. Composite insulator air side along the surface electric field diagram.

Figure 8 .
Figure 8. Electric field diagram along the surface of a composite insulator under normal conditions.(2)Composite insulators when dropping bird droppings As can be seen in Figure9, the electric field effect around the lower end of the bird droppings channel, when it is detached from the crossbar, is obvious, while only the end of the upper end shows a weak distortion effect.The electric field on the other side remains essentially unchanged.As shown in Figure electric field strength on the side of the insulator near the bird droppings is slightly higher than that on the side away from the bird droppings.

Figure 9 .
Figure 9. Potential distribution of composite insulator string cut surface.

Figure 10 .
Figure 10.Electric field distribution on both sides of the composite insulator string.

Figure 11 .
Figure 11.Electric field along the surface of the bird droppings side of the composite insulator.

Figure 12 .
Figure 12.Electric field along the air side of composite insulator.

Figure 13 .
Figure 13.Model layout of bird droppings flashover simulation experiment.

Figure 14 .
Figure 14.Model layout of bird droppings flashover simulation experiment.The flashover test was carried out on the bird droppings simulated liquid to analyze the changes in the probability of flashover by varying the horizontal transverse distance D of the droppings falling channel path from the central axis of the silicone rubber insulators, and the flashover test was done several times for each set of distances.

Table 1 .
Simulation model material parameters.