Simulation Analysis of Electric Field on the Body Surface of Personnel Engaged in Interphase Live Working on 750 kV Double Circuit Transmission Lines on the Same Tower

This article focuses on the assistance of electric lifting devices for phase-to-phase live working. Taking a typical 750 kV double circuit vertical arrangement line on the same tower as the research object, a three-dimensional simulation model of a real tower conductor simulated human is established. The field strength on the human body surface is calculated and analyzed during the process of operators entering the equipotential (conductor) while riding the electric lifting device in order to determine the risk points of the operation and determine the safety protection measures for the human body. Through simulation, it is found that when simulating human entry (exit) and phase-down operations, the human body is located at different distances of 0.4 m-1.5 m directly below the conductor. When not wearing shielding clothing, the maximum field strength of the human head is much greater than the allowable value. If using shielding clothing with a shielding efficiency of 60 dB for safety protection, the surface field strength inside the worker’s clothing can be reduced to 2.25 kV/m, meeting the requirements of live working. Operators should pay attention to the electric field protection at each phase wire when using an electric lifting device to enter (exit) the interphase wire. Equipotential operators must wear a complete set of shielding clothing for 750 kV live working.


Introduction
The 750 kV double circuit line on the same tower is a backbone grid project and the main channel for the implementation of China's "West-East Power Transmission" strategy.It is crucial to carry out the operation, maintenance, and live working of the 750 kV double circuit line on the same tower, which is of great significance for ensuring the safe and reliable operation of the power grid.For a long time, research on live working on double-circuit vertical arranged lines on the same tower has been focused on the safety distance of phase-to-ground operations, with less research on phase-to-phase operations [1][2] .The tower height and conductor spacing of 750 kV double circuit lines on the same tower are small, and traditional operation methods cannot directly carry out maintenance operations from the ground or the air [3][4][5] .It is urgent to carry out the calculation and analysis of the surface electric field of personnel engaged in phase-to-phase live working on 750 kV double circuit lines on the same tower.
This article establishes a three-dimensional simulation model of a typical 750 kV double circuit transmission line with vertical arrangement and tower type, consisting of a true tower conductor simulator [6][7][8] .By calculating and analyzing the electric field distribution of operators entering and exiting the interphase conductors, the proposed method provides a theoretical basis for the development of safety protection measures for interphase operators.It promotes the application and promotion of live working technology for 750 kV double circuit transmission lines [9][10] .

Analysis of Phase to Phase Live Working Conditions
The characteristics of double circuit lines on the same tower are, firstly, the height of the tower and the size field of the cross arm, the arrangement of the upper, middle, and lower phases of the conductors (double circuit vertical arrangement of drum shaped towers or umbrella-shaped towers), and thirdly the existence of electromagnetic effects between the two circuit lines.The distribution of static electricity field in the space around the tower is analyzed.Research is conducted on the specific layout of the line structure and conductors.Potential risk scenarios are considered based on actual operational needs, and safety analysis of phase-to-phase operations is conducted.
Operators take electric lifting devices or helicopters to carry maintenance personnel into the area between the two wires for operation.When the electric lifting device or helicopter suspension maintenance personnel are between the two phases, the presence of manned equipment and the human body will affect the spatial electric field distribution between the two phases.There is a risk of the discharge path between the phase wire, manned equipment, human body, and another phase wire.Different from working condition (1), the manned equipment and the simulated person are suspended between the two phases of the wire, and the discharge probability and discharge path of the simulated person at different positions are observed.The hazardous points of operations are obtained when operators pass through different locations.
The situation where live working personnel use electric lifting devices to enter and exit the conductor for phase-to-phase operations can be divided into three types: ① Entering and exiting the lower phase conductor operation; ② Operation of incoming and outgoing phase conductors; ③ Operation of incoming and outgoing phase conductors.

Line Tower and Simulated Human Simulation Model
The tower adopts a typical 750 kV double circuit line on the same tower, with a straight tower ZGU 218 and a three-phase conductor model of 6 × JL/G1A-400 × 35.For the convenience of simulation, the six-bundle conductor is simplified and its equivalent radius is calculated as follows: In Equation ( 1), r eq is the equivalent radius of the wire, in millimeters; R is the radius of a single bundled wire, in millimeters; d 12 , d 13 ,... d 1n are the distances between the first sub wire and the second, third... (n-1) sub-wire, in m.
During the equipotential process of assisting lifting operators with electric lifting devices, two insulated ropes are required.One rope is equipped with an electric lifting device, and the other rope is used as backup protection.The schematic diagram of the hanging point position of the insulated rope is shown in Figure 1.During the simulation, an insulation rope hanging point was set at a distance of 5 m from the tower, and the far insulation rope hanging point was set at a distance of 6 m from the tower.However, due to the relatively slender insulation rope and its contact with operators and wires, and the insulation rope itself being made of insulation material, it had little impact on the surface electric field of the human body.Therefore, the insulation rope part was removed during the simulation.The operator adopts standard human body parameter dimensions, as shown in Table 1.

Selection and Segmentation of Simulation Parameters
The voltage amplitude of the 750 kV busbar is 800 kV * √ 2/√ 3=653 kV, and the voltages of the three-phase conductors A, B, and C are set to have a phase difference of 120°.The phase sequence arrangement of ABC/BAC is selected for the vertical arrangement of the double circuit lines on the same tower.
During simulation, different dielectric constants are set for wires, towers, and simulated humans, with the relative dielectric constant of the human body set to 80 and the relative dielectric constant set to 1e4 after wearing shielding clothing.
A half cylinder with a radius of 200 m and a height of 100 m is selected as the outer boundary, as shown in Figure 2. Due to significant differences in the dimensions of the outer boundary, towers, wires, and simulated personnel, in order to ensure more accurate calculation results, a cylinder is used to wrap the simulated personnel in the areas that may occur under different working conditions.This allows for different maximum and minimum unit sizes to be defined for different areas during grid generation in order to obtain more accurate results.

Discussion of Calculation Results
Taking the example of operators entering and exiting the lower phase conductor, if the operators are not wearing shielding clothing, and the operators are wearing shielding clothing, the surface electric field distribution of the operators is calculated and analyzed.Using the finite element method, a three-dimensional simulation model is established to calculate and analyze the distribution of the electric field on the human body surface when operators enter and exit the lower phase conductor.The electric field values of each part of the lower phase, where the human body is located directly below the lower phase, are obtained in Table 2.The electric field distribution cloud map of the lower phase conductor surface and the human body surface is shown in Figure 3.   2, it can be seen that when a simulated human enters the lower phase operation by riding an electric lifting device, the human body is located at different distances below the wire.When not wearing shielding clothing, the head field strength of the human body is the highest, with a maximum field strength of 1900 kV/m, which is far greater than the specified limit value.When there is shielding, the electric field on the head and toe of the human body surface (outside the shielding suit) increases.If a shielding suit with a shielding efficiency of 60 dB is used for safety protection, the surface electric field strength inside the operator's suit can be reduced to 2.25 kV/m, which is less than the allowable value of 15 kV/m specified in the standard, meeting the requirements of live working.

Analysis of Job Risk Points and Safety Protection
At present, the main methods for entering the high potential of conductors on single circuit lines are: ① Equipotential electricians climb from the ground to enter the equipotential through insulated soft ladders hung on conductors or ground wires.This method is only applicable to the lower phase conductors.Due to the vertical arrangement of three-phase conductors in double circuit lines, this method has adverse factors in operation and safety for medium and upper phase lines.② Assemble vertically or obliquely with an insulated hard ladder on the wire level, and equipotential electricians ride along the ladder to enter the equipotential.This method is due to the long and different sizes of the three-phase cross arms of the double circuit line.The tower body and the wire are overlapped with a horizontal hard ladder, which has the defects of large weight, large deflection, long overlapping size, and inconvenient use of tools during three-phase operation.The upper end of the insulated ladder is hung, with a hanging basket, seat, or soft ladder near the hanging point of the linear insulator string.The equipotential electrician should relax the control rope and enter the equipotential position on the seat, hanging basket, or ladder.This method is inconvenient to operate due to the long cross arm, and the insulation performance of the control rope will significantly decrease when the humidity is high.Therefore, it is not suitable to work in environments with high humidity.
The first method is only applicable to the lower phase conductors.Due to the vertical arrangement of three-phase conductors in double circuit lines, this method has adverse factors in operation and safety for middle and upper phase lines.The second method is due to the long and different sizes of the three-phase cross arms of the double circuit line, where a horizontal hard ladder is used to overlap the tower body and the conductor, which has the defects of large weight, large deflection, long overlap size, and inconvenient universal tools during three-phase operation.The third method is also inconvenient to operate due to the long cross arm.The insulation performance of the control rope will significantly decrease when the humidity is high, so it is not suitable to work in environments with high humidity.
The structure, size, and wire arrangement of double circuit lines on the same tower are different from those of single circuit lines.Therefore, it is necessary to combine the characteristics of phase-to-phase operation of double circuit lines on the same tower and determine the safe distance to study new operation methods for entering the upper, middle, and lower three-phase wires.For the phase-to-phase operation of 750 kV double circuit lines on the same tower, it is recommended to use an electric lifting control device to lift operators into equipotential positions for phase-to-phase operation.When operators ride the electric lifting device to enter (exit) the lower phase conductor, they should pay attention to the electric field protection when working under the lower phase conductor.When entering and exiting the middle phase conductor, they should pay attention to the electric field protection of the lower phase conductor and the middle phase conductor.When entering and exiting the upper phase conductor, they should pay attention to the upper and lower phase conductors.For electric field protection of neutral and lower phase conductors, equipotential operators must wear a complete set of shielding clothing for 750 kV live working, including jumpers, hats, masks, gloves, socks, and shoes.

Conclusion
This article first analyzes the working conditions and risks of live working, including two aspects: air gap impulse discharge and operating overvoltage.By establishing a three-dimensional real simulation model of a "tower conductor simulated human", the study analyzes the different surface field strengths of the human body under different working conditions when electric lifting devices lift workers into equipotential processes.The simulation results show that the simulated human body enters the lower phase operation by riding an electric lifting device, and the human body is located at different distances of 0.4 m-1.5 m directly below the wire.When the shielding suit is not worn, the field strength of the human head is the maximum, which is much greater than the specified limit value.If a shielding suit with a shielding efficiency of 60 dB is used for safety protection, the surface field strength of the operator's body inside the suit can be reduced to 2.25 kV/m, meeting the requirements of live working; Operators using electric lifting devices should pass through the electric field protection at each phase wire when working in (out) of the interphase wire.Equipotential operators must wear a complete set of shielding clothing for 750 kV live working.

Figure 1 .
Figure 1.Schematic diagram of insulation rope hanging point position.The operator adopts standard human body parameter dimensions, as shown in Table1.Table1.Size table of operators.

Figure 2 .
Figure 2. Schematic diagram of the outer boundary.

Figure 3 .
Figure 3. Cloud diagram of electric field distribution on the surface of the lower phase conductor and the human body surface.From Table2, it can be seen that when a simulated human enters the lower phase operation by riding an electric lifting device, the human body is located at different distances below the wire.When not wearing shielding clothing, the head field strength of the human body is the highest, with a maximum field strength of 1900 kV/m, which is far greater than the specified limit value.When there is shielding, the electric field on the head and toe of the human body surface (outside the shielding suit) increases.If a shielding suit with a shielding efficiency of 60 dB is used for safety protection, the surface electric field strength inside the operator's suit can be reduced to 2.25 kV/m, which is less than the allowable value of 15 kV/m specified in the standard, meeting the requirements of live working.

.
Size table of operators.

Table 2 .
Electric field values of each part of the human body directly below the lower phase in and out of the lower phase.