| With the proposal of China’s "double carbon" strategy,the power system will access more and more new energy and related power electronic equipment,the broadband signal they input will bring new broadband voltage disturbance problems to the power system.As a large disturbance of non power frequency,the characteristics of high amplitude and high frequency of power system overvoltage will have a great impact on the reliability,stability and security of power system.In this paper,the capacitive voltage transformer(CVT)wildly used in power system is studied,and the CVT non-linear broadband model is established.Through theoretical research and experimental verification,the distorted waveform output at the secondary side of CVT can be inverted by using the established inversion algorithm to obtain the original lightning overvoltage waveform input at the primary side,which expands the frequency bandwidth of CVT measurement system.The paper focuses on the following aspects:Firstly,based on the traditional CVT physical model,this paper combines it with the black box model by using the improved extended model principle,and considers the nonlinear characteristics of the non-linear elements in the electromagnetic unit,and establishes the CVT non-linear broadband model.The steady-state parameters of a CVT with the model of "WVL2 110-10H" are measured.The capacitance of its capacitance unit is accurately measured by LCR digital bridge device.Other relevant parameters of the electromagnetic unit are measured and calculated based on the open and short transformer experiment,and the polynomial fitting formulas of non-linear resistance and non-linear inductance are fitted by using the idea of piecewise linearization.Then,based on the overall scattering parameters(S parameters)of CVT measured by vector network analyzer,the S parameters of black box module are obtained through transformation and calculation;Each module in π type black box module is expressed as partial fraction sum form by vector fitting method.In order to ensure the stability of the model,the macro passive optimization of the model is carried out by using the Hamilton matrix discrimination principle.Finally,based on Foster type II equivalent circuit,each module in the black box module is represented as parallel branches of multiple RLCG circuits.So far,the CVT non-linear broadband model with all known parameters is established.Conduct multiple groups of lightning impulse tests on the same CVT and record the input and output waveforms.Based on Bergeron equivalent model,time-domain recursive convolution method,Adams explicit formula and Kirchhoff current and voltage law,an inversion algorithm based on CVT non-linear broadband model is built in MATLAB.The measured secondary side output waveform is brought into the algorithm to obtain the inverted CVT input waveform.Compared with the measured waveform,it is found that they have high similarity,the waveform wavefront time is the same,and the relative error of waveform peak time is 0.76%,the relative error of half peak time is 16.07%,which are all within the allowable error range.The research purpose is achieved and the bandwidth of CVT measurement system is expanded,which has a certain reference significance for the broadband research and protection of power system in the future. |
