Research On Influence Law Of Force On The Bushing Affected By Cable Bending In 10kV RMU

The technology of ring network power supply can improve the reliability and stability of power supply greatly,so it has attracted a lot attention and been employed increasingly in the transformation of urban power grids.At the same time,the RMU(Ring Main Unit)is also widely used for its characteristic of small size,convenient installation and maintenance,and low operating costs.However,most of the RMU is European-style.In European,single-core cables are usually used when connecting with a RMU,each phase is separately installed without considering the enormous force generated by cable bending.In the domestic distribution network,the power cable is usually a three-core cable.It must be bent to fit the installation exactly when the cable is installed with RMU bushing,so it will produce a greater force on the ring network bushing.Due to the neglect of the influence of force when the RMU designed and manufactured,it often suffers from problems such as cracking of the wall bushing and gas leakage inside the cabinet in the domestic distribution network,which poses a major hidden danger to the safety and stable operation of the distribution network.This article has launched a relevant study to the curved cable which causes the bushing to be overstressed to solve the problem of rupture of bushing in 10 kV RMU.Firstly,the stripping structure of three-core cable and the principle of calculation of mechanics of single-phase cable are expounded in this paper.After that he principle of finite element calculation of cable bending deformation is mainly introduced.The balanced differential equations,geometric differentials equations and constitutive equations of the nodes are given.That nodes are junctions of the finite number of microelements which divided from single-phase cable.Then,a single-phase model of the cable is established based on the foundation of finite element analysis,and the influence law of the force on bushing is analyzed in different bushing spacing,cable type,and curved area of single-phase cable under three conditions: vertical installation of the top of single-phase cable,skew installation of the top of single-phase cable,and stripping oversized of cable.And the cable bending test is designed to verify the results.Then,they are discussed in detailed that include the calculated principle of heat source of the single-phase cable,the calculated principle of thermal transmission and the calculated principle of temperature-stress field.After that,the influence law of temperature to the force generated by cable bending is investigated from the aspect of thermal-structure coupling,include that the influence of temperature to the force on the top of single-phase cable is discussed under different length of single-phase cable,different type(cross-sectional area)of cable and different phase spacing.The conductor temperature has a positive correlation with the top force of the single-phase cable,and as the conductor temperature increases,the increase of its acting force slows down;as the length of the single-phase cable increases,the trend of slowing down becomes more obvious;it is greater influence of temperature to the force of the top of single-phase cable with the larger of cross section of it;the narrower of the phase spacing,the more obvious of the influence of the conductor temperature to its force.Finally,the strategy is studied about the optimization of layout of the bushing in the cable compartment of the RMU.Through the experiment of the ultimate bearing capacity of the bushing,the allowable force is confirmed to the bushing of the RMU.When installation with the cable with largest cross-section,the optimized result of the layout of bushing is that: the spacing of bushing is 107 mm and the height of bushing is 765 mm under the situation of single-phase cable installed vertically;the spacing of bushing is 150 mm and the height of bushing is 1053 mm under the under the situation of the stripping oversized of cable.