Evaluation And Compensation Of Connecting Railway Substation With Untransposed Transmission Lines

Electrical railway load is a typical three-phase unbalance load.When electrical locomotives or electrical EMUs are running,it’s hard to avoid injecting negative sequence current into utility grids.Railway substations and utility grids are connected by an exclusive power lines.Those power lines are transposed three phase power lines.Transposing lines can be realized by installing transposing structure that supports these lines and changes their relative position.However,transposing structures are complicated structures,with high cost and low reliability.This paper proposed a novel method that using untransposed transmission lines to connect railway substations and utility grids.This method exploits the intrinsic three phase unbalance to compensate the load unbalance of railway substations,which can reducing the negative sequence current injected by railway substation.With untransposed transmission lines installed,reliability is improved and cost is reduced,and more importantly,negative sequence current is reduced.The specific researched contents of this paper are as follows:1.According to the current transformation matrices and voltage transformation matrices,established a unified mathematical model for different types of traction power transformers.And,based on these transformation matrices,purposed a power transformation method that convert power at secondary side to the power of primary side.In addition,this paper purposed a three phase power flow calculation method to evaluate the effect of using untransposed transmission lines in the following chapters.2.Both untransposed and transposed power lines has been analyzed by decoupling multiconductor parameters and lumped-circuit can be acquired.Then equivalent circuit of both two types of transmission lines is acquired by eliminating internal nodes by Kron reduction.3.This paper evaluated the effect of grid capacity,length of transmission lines,type of traction transformer and power of locomotives affecting the unbalance of utility grids.In addition,the validity of reducing voltage unbalance and current unbalanced of utility grid by using untransposed power lines has been proved by simulation.4.For some cases that current unbalance exceeds standard,this problem can be solved by installing serial inductors or serial capacitors to further reducing negative sequence current.In addition,parallel inductors and parallel capacitors can also be installed to compensate the load unbalance of railway substation.A theoretical calculation method for deciding the reactance of compensating inductors and capacitors has been purposed.Based on simulation,both serial compensators and parallel compensators can effectively reduce current and voltage unbalance.