Research On Harmonic Voltage Measurement And Correction Technology Based On CVT

Capacitor Voltage Transformer（CVT）is a voltage signal measurement device widely used in power systems at present.Although CVT has the ability to accurately measure the fundamental voltage,it cannot be used to measure the harmonic voltage.In recent years,the number of high-voltage direct supply users has gradually increased,and CVTs have become more and more widely used.It is of great significance to study how to effectively use CVT to master the accurate harmonic content of the power grid.Due to the uncertainty of the stray capacitance parameters,the CVT harmonic transmission has a large error,so how to obtain the accurate value of the stray capacitance parameters is a key step to solve the problem that the CVT is used to measure the harmonic level.In order to solve the problem that the CVT that is currently running in large numbers cannot accurately measure the harmonics.In this thesis,Firstly,two equivalent circuit models of CVT are established,and the transfer functions of the two equivalent circuit models of CVT are deduced,and the influence of the fluctuation of stray capacitance parameters and other parameters on the harmonic transfer characteristics is studied.Secondly,the influencing factors of the stray capacitance value change are analyzed,and the stray capacitance parameters are numerically estimated according to the characteristics that the stray capacitance is difficult to measure directly.Then the harmonic voltage measurement and correction method is introduced,and the advantages and disadvantages of the method are analyzed,and the correction curve correction method is taken as the guiding method of this thesis.Finally,in order to accurately obtain the parameter value of stray capacitance and eliminate its influence on the harmonic transfer characteristics.Based on the complete equivalent circuit model of CVT and the related methods in the circuit principle,a method（VI method）to measure the voltage and current of the secondary side of the CVT is proposed.This method regards the CVT complete equivalent circuit model as a two-port network,and establishes the relationship between the CVT secondary side voltage U_s,the high voltage side current I₁and the unknown parameter stray capacitance based on the relevant knowledge of the Z parameter equation in the circuit principle.with the help of the Newton iteration algorithm and the improved Newton iteration algorithm,the stray capacitance parameter values contained in the nonlinear equation system can be obtained more accurately,so as to eliminate the influence of the fluctuation of the stray capacitance value on the harmonic voltage measurement,so as to achieve the harmonic voltage measurement.The method does not need to disassemble the CVT,and can also ensure the integrity of the CVT measurement equipment,so the method has certain feasibility,and the simulation test also shows that the method has better accuracy.