Overvoltage Characteristics Analysis And Suppression Methods Of Tuned Half-wavelength AC Transmission Lines

As an important foundation of national economy and people’s livelihood,electric power energy is continuously accelerating in order to meet the needs of the rapid development of contemporary society and economy.The continuous growth of energy and power demand means that power construction will face more challenges in the future.And the reverse distribution pattern of energy resources and load demand has become a key issue.Thus,the development of more economical and efficient large-capacity,long-distance transmission methods is imminent.Among them,the UHV half-wavelength AC transmission line has gradually attracted worldwide attention due to its good transmission characteristics and economic advantages.Considering that the distance between the actual energy and the load may not meet the length requirements of a standard half-wavelength line,a tuning network is required to construct a tuned half-wavelength transmission line,which can be flexibly applied to any non-half-wavelength distance transmission range.But its transmission characteristics are not only different from conventional transmission lines,but also different from natural half-wavelength transmission lines.For this reason,the thesis focuses on the operating characteristics and overvoltage problems of UHV tuned half-wavelength transmission lines.The main research contents and results are as follows:This paper first introduces the theoretical mathematical model of a non-destructive half-wavelength AC transmission line,analyzes the steady-state power transmission characteristics of the half-wavelength transmission line,and derives the voltage extreme value calculation formula under steady-state operation.Then the tuning technology is introduced,and the calculation formulas of different tuning network parameters are derived based on the transfer matrix of the standard half-wavelength line.The influence of the tuning network on the steady-state characteristics of the half-wavelength line is discussed and analyzed: The π/T type tuned network has a more serious impact on the steady-state voltage along the line than the capacitive type tuning network.Overvoltage suppression measures should be implemented for both substations along the line and along the line.Starting from the internal overvoltage generation mechanism of the UHV tuned half-wavelength transmission system,the formulas for solving overvoltages along the line under different operating conditions are derived.The influencing factors of different overvoltages,distribution characteristics along the line,and the location ofextreme values are studied.The corresponding conclusions are obtained: The line no-load and over-load overvoltage levels of the π/T type tuned line are more serious than the natural half-wavelength line,which puts higher requirements on the insulation of the substations at the head and end of the line.The overall level of overvoltage is the most serious when a single-phase ground fault occurs,and when a fault occurs near the end of the line,an overvoltage extreme point appears 1500 km from the fault point.Its most serious fault point and over-voltage extreme point change with the equivalent impedance of power supply as well as the tuning conditions.Then combined with the over-voltage distribution characteristics under different operating conditions,from the comprehensive consideration of the installation position of the arrester,the number of installation groups and the current capacity limitation,the specific suppression strategies for various over-voltages of the tuned half-wavelength transmission line are proposed.Finally,PSCAD simulation software is used to establish a UHV tuned half-wavelength transmission line simulation system model.The overvoltage distribution and waveform characteristics of the tuned half-wavelength line and the natural half-wavelength line are compared and analyzed as well as the tuned half-wavelength line before and after the suppression measures.The simulation results not only verify the theoretical analysis of the internal overvoltage,but also confirm the effectiveness of the overvoltage suppression measures,and provide a theoretical reference for the engineering application of UHV tuned half-wavelength AC transmission lines.