Study In The Influence Of Overhead Ground Wire On The Lightning Coupling Voltage Of Lines And The Shielding Failure Of The Lines

Overhead transmission lines are highly vulnerable to various hazards under natural conditions.The danger of lightning can not only directly hit phase lines to cause an accident of power supply system,but also can cause the phase lines to generate an suddenly overvoltage through the electromagnetic field coupling,causing part of the damage of lines or even the entire power supply system to be paralyzed.In order to ensure the normal operation of the power system,shielded wires(overhead ground wires)are usually set up above the transmission lines,which reduces the damages of direct lightning strikes to the phase lines and weakens the lightning coupling voltage on the phase line.In previous studies,there were many studies on the coupling voltage of the phase lines,but relatively fewer studies on the shielding effect of the shielded wires.Therefore,this paper mainly analysis the shielded wires' role on protecting the transmission phase lines,and the mainly results obtained are showed as follows:(1)In the cylindrical coordinate system,the Finite-Difference Time-domain method in two-dimensional and the field-line coupling model are used to analyze the coupling voltage and shielding effect of the shielded wires under different influencing factors.The three-phase lines can be represent by the vertical and horizontal structures.As for the three-phase line in vertical structure,the number of overvoltage reduces to 67.2% when the shielding effect is best;but for the horizontal structure,the number is 55.97%.When the soil conductivity is changed within a certain range,the change of the horizontal lines' shielding effect is larger than that of the vertical lines.But when the distance and the shield line's height are changed,the change of vertical lines' shielding effect is relatively larger.(2)The coupling voltage at the end of the phase lines can be decomposed into the incident voltage and the scattered voltage.The influence of the shielded wires on the scattering voltage and the incident voltage of the phase lines is analyzed.The shielding lines make no difference on the incident voltage at the end of the phase lines.But the scattering voltage reduces significantly onder the influence of shielded wires.When the conductivity,distance and shielded wire's height are changed,the variation rule of the incident and scattering voltage's peak value at the end of the phase lines in two structures are the same.When the shielded wire's height and lines' horizontal spacing Changes within a few meters,the incident and scattering voltage' at the end of lines do not change significantly.(3)The analysis and verification of the "two-step" algorithm and field-line coupling in this study ignored the influence of shielded wires on the lightning electric field,and the integrated electric field and lines model in three-dimensional could calculate the value of the electric field when the shielded wires exists.Through the formulas of Coory-Rubstein(C-R)and the Finite-Difference Time-domain method,the ideal perfectly term and the surface impedance term component of the horizontal electric field generated by the lightning return storke are analyzed.When the conductivity changes,the negative peak value of the generated voltage by the surface impedance term changes significantly,but as for the ideal perfectly term,the result is contrary.(4)Comparing the electrical geometric method with the leader progression model,an idea to improve the electrical geometric method is proposed.Under the circumstances set in this paper,the shielding failure outage rate of the transmission lines by the improved algorithm is smaller than that by the original algorithm.