Design And Applied Research On Non-powercut Spanning Method In Construction Of High-voltage Transmission Line

With the development of economy and technology,power consumption in industry and agriculture has been increasing in recent years and power grid construction has also been developing rapidly.As the capacity and voltage level are growing,the crossing of transmission lines are becoming more and more common in practice.Non-powercut spanning method is generally used in constructions of new transmission lines.In this paper,several common methods of stringing over live lines are introduced and analyzed by comparing their performance features and application conditions.Then suggestions and guidance on the choice of spanning method are presented.The key process in spanning project is the block of the live lines using insulation nets and the safety and reliability of the insulation net can guarantee the smooth launch out of the project.Support to the insulation net can be achieved by using scaffoldings,gin poles and the cross arm of the tower of the line being constructed.Scaffoldings can be utilized in most of the conditions but the constructing process is complex and the cost is relatively higher.Set up of scaffoldings is avoided when using the tower to hang the insulation net but this method is limited by the height of the tower and the span of the new line.When choosing spanning method the construction conditions should be studied comprehensively and safety demonstration should be carried out after the design of the construction plan.In the process of non-powercut spanning,magnetic field will be generated by the live line.In the stringing step,the conductor of the new line will swing in the magnetic field,thus induced electromotive force will be generated,which may cause a threat to the safety of people involved.So in this paper,the model of the magnetic field both two-dimensional and three-dimensional and the transmission line is established.In the two-dimensional model,the transmission line is simplified as a straight line with infinite length which is parallel to the ground.And in the three-dimensional model,the sag of the transmission line is considered and the magnetic field in the space around the line is calculated much more accurately.As the current in the transmission line being crossing over is alternating,the magnetic field generated by it is also time-varying.So the calculation will be complex and the results will be in large data scale.The three phases of the transmission lines will operate symmetrically,so the sum of momentary values of the currents is zeros.The magnetic field is generated by the spatial asymmetry from the observation point to the transmission lines.It is not necessary to get the accurate magnetic field on every point of the space around the lines in engineering practice and model in this paper uses time vector to represent the currents and the difference of the three phases is considered as phase difference.In the stringing process,the transmission line and the shell of tensioner can form a loop via the ground.The new line swings when wind blows and there can be an induced electromotive force to the ground.By using the three-dimensional model established,the maximum value of the electromotive force is calculated and it is recommended that when the crossing angle is small,the safe distance should designed larger and the lines should be kept smooth and steady during string process.In the of the paper,an engineering instance of the spanning of 500 kV line over 275 kV live line in Malaysia is presented to introduced how the processes of the non-powercut spanning is utilized in practice.The demonstration of reliability and detailed regulations on safety are analyzed in line with the practical construction plan.Based on the experience in the participation of the project,the insufficient and issues are analyzed and directions for improvement and optimization are indicated and explained.