Analysis Of Harmonic Measurement Error And Prediction Of Transfer Factor For Capacitor Voltage Transformer

CVT is a kind of signal sampling device,which is widely used in the high voltage system of 110 k V and above,which is used for measuring voltage and relay protection.However,there is an error in the harmonic measurement results of the CVT.Therefore,it is very meaningful to study the factors that affect the CVT harmonic measurement error and use the CVT harmonic transmission coefficient to correct the measured value of the CVT harmonic voltage.The structure and principle of CVT is analyzed.The circuit model and transfer function are established.Simulation of the influence of CVT internal parameters on the measurement error of CVT harmonics is analyzed.The external factors such as the secondary load,ambient temperature,and external electric field are analyzed.Although they may affect the harmonics of the CVT measurement,they have little impact and can be ignored in practical use.The results of analysis simulation show that: The stray capacitance on the primary side has the greatest influence on the harmonic measurement of the CVT;changing the harmonic content in the fundamental wave does not affect the change of the CVT transmission coefficient.The article further simulated and analyzed the transmission characteristics of harmonics(mainly 5,7,11,13,and 23 times)on a transmission line with 110 k V voltage level.Through analysis of the simulation results,it can be seen that For overhead lines and cable lines with CVT and PT connected at both ends,under certain conditions,the output value of each voltage at the line PT can be taken as the reference value,and the 5,7,11,13,and 23 of the CVT at the other end of the line.The output of the sub-harmonic voltage is compared with the corresponding harmonic voltage value of the PT output,and this ratio is taken as the 5th,7th,11 th,13th,and 23 th harmonic transmission coefficient of the CVT.This paper summarizes several existing methods for improving the harmonic measurement accuracy of voltage transformers and studies a method for combining the harmonic transmission coefficients of the 5th,7th,11 th,13th,and 23 rd harmonics based on CVT and neural networks.Predict the on-site CVT transfer coefficient.The new CVT error correction system studied in this paper can correct the transmission coefficient of each harmonic of the CVT on site by storing the amplitude-frequency characteristic curve,and correct each harmonic transmission coefficient of the CVT and correct the measured value of CVT harmonic voltage.