Modeling Method And Over-voltage Problem In Lowvoltage Systems Of Multi-voltage Short-transmission Lines On The Same Tower

With the development of the economy,the demand for electricity load is increasing.It is in contradiction with the tight transmission corridor.In order to improve the transmission capacity,the erection of transmission lines on the same tower has become an inevitable trend of power grid development.In regional power grids,the distance of the cross-voltage lines on the same tower is usually short.In addition,there are many types of towers and different kinds of conductors in the cross-voltage lines on the same tower.For the cross-voltage lines on the same tower with a large number of circuit lines,the occurrence of a non-ground fault across the voltage level will increase the voltage of the low-voltage system.This can cause overvoltage problems and can be harmful to the insulation level of the line or equipment.At present,the fault analysis methods for the cross-voltage lines on the same tower are mostly based on simple symmetric systems or neglect the influence of line mutual inductance and conventional modeling methods focus on single tower structure and wire parameters.It is difficult to decouple the actual circuit with asymmetry of parameters by using the existing theoretical method.The simulation results of the line model with errors in the mutual inductance parameters are not accurate enough.In addition,there are few researches and countermeasures on the overvoltage problem of cross-voltage lines on the same tower.Therefore,it is of great significance to construct a high-precision simulation model which suitable for cross-voltage lines on the same tower line.Firstly,this paper analyzes the advantages and disadvantages of the current distributed parameter model and the centralized parameter model in dealing with short-line,multi-tower,and different wire parameters.Furthermore,the influence law of the variation of the wire parameters on the impedance/admittance matrix is studied.The processing method for different wire parameters in cross-voltage lines on the same tower is proposed to reduce the mutual inductance error.It can reduce the mutual inductance error.Considering that the actual line cannot measure all the line parameters,this paper selects the typical tower structure to approximate the impedance/admittance matrix corresponding to each line of the same tower.Finally,based on the correspondence between the matrix elements and the π-type equivalent circuit,a centralized parameter model modeling method of cross-voltage lines on the same tower considering the difference between the tower structure and the wire parameters is proposed.In order to study the over-voltage problem of the cross-voltage lines on the same tower,this paper uses the multi-composite network diagram to qualitatively calculate the cross-voltage level non-ground fault of the cross-voltage four-circuit line on the same tower,and analyzes the factors affecting the over-voltage level of the low-voltage system and the maximum overvoltage multiple.Then use the built cross-voltage line on the same tower model to accurately simulate the non-ground fault across voltage levels.It proves that the multi-composite network diagram fault analysis method is insufficient.At the same time,the rationality of the analysis results of the two is compared and verified,which proves that the overvoltage problem of the crossvoltage line on the same tower will endanger the insulation of the line or equipment.In order to study the overvoltage protection measures of cross-voltage line on the same tower,this paper analyzes the applicability of several common overvoltage protection methods to the overvoltage problem of low-voltage systems with non-ground faults across voltage levels.According to the analysis results,a simple countermeasure strategy for the overvoltage problem of low voltage system is proposed.The work of this paper is mainly based on the actual power grid in Guangzhou.The simulation model of the cross-voltage level of the same tower is provided for the study of overvoltage problems.In addition,it also provides a model basis for the research on relay protection of the cross-voltage lines on the same tower of the grid in Guangzhou area.