Research On Capacitor Voltage Transformer Transmission Characteristics And Correction Method For Internal Overvoltage Measurement

With the increasing voltage level of the power grid,the power equipment is more affected by different waveform and amplitude transient voltage,mainly including operating overvoltage,temporary overvoltage,direct lightning overvoltage,induced lightning overvoltage and other electromagnetic transient phenomena.Capacitor Voltage Transformer(CVT)has the advantages of better insulation performance,cheaper price,and simpler maintenance and monitoring than the Potential electromagnetic voltage Transformer(PT).Therefore,it is more used in the power system as a monitoring and measuring voltage equipment.However,due to the transient characteristics of CVT,the measurement errorunder overvoltage is larger.In some transient cases,the error is further changed due to the saturation of the intermediate transformer.Based on this,this topic analyzes the transient characteristics of CVT,further analyzes the errors caused by the transient characteristics of CVT in the case of overvoltage,and puts forward the corresponding error correction scheme to reduce the errors existing in the measurement of transient voltage of CVT,providing guarantee for the system relay protection and reliable transient voltage measurement.The main research contents of this paper are as follows:First,the transient equivalent circuit of CVT is constructed according to the actual structure and component characteristics of CVT.Secondly,the transmission process of CVT transient voltage is analyzed,and the component of secondary transient voltage is deconstructed.Then,based on the equivalent circuit,the mechanism of CVT transient response and ferroresonance is explained respectively,and the frequency domain characteristics of secondary transient voltage are derived,and the CVT transient response and ferroresonance are simulated to verify the theoretical derivation results.Secondly,the measurement error of internal overvoltage based on CVT is studied.Firstly,the internal overvoltage in the power grid is classified to find out the types of overvoltage that may affect CVT,and the frequency range of error analysis is determined.Then,the expression of CVT transmission error under overvoltage is derived,and the relationship between the transient characteristics of CVT and the measurement error is established.Three types of internal overvoltage conditions are simulated:no-load line switching,capacitor bank switching and single-phase ground fault.The error of overvoltage measured by CVT during fault and circuit breaker operation is analyzed.Finally,several factors that may affect the internal overvoltage error of CVT measurement,such as transient initial state,CVT load parameter and damping coefficient,are simulated and analyzed.Thirdly,a digital filter is designed to correct the transient measurement error of CVT.Firstly,the functions of IIR and FIR digital filters are compared to determine the selected digital filter type.Then,based on the frequency characteristics of CVT,the IIR digital filter suitable for transient error correction of CVT is designed by indirect method,and the expression of the filter transfer function is obtained.Finally,the error between CVT transient output voltage and output voltage processed by digital filter is compared by simulation to verify the effectiveness of the proposed correction method.