On Line Measurement Of Impedance Based On PWM Optimal Modulation

With the wide application of new energy,the power electronics-dominated power system has nonlinear impedance.The grid connected inverter,which widely exists at the end of the system,will interact with the grid impedance in a wide frequency band,resulting in resonance instability.The grid impedance measurement based on inverter can obtain broadband grid impedance in real time,which is an effective means to suppress broadband resonance of inverter and realize intelligent control of inverter.In order to expand the frequency band of power grid impedance measurement and realize the impedance estimation in the middle and high frequency band,two on-line impedance measurement methods based on PWM optimized modulation of inverter are proposed in this dissatation.The main contents of this paper are as follows:(1)From the aspects of disturbance signal type,disturbance injection mode and signal processing method,the advantages and disadvantages of existing on-line measurement schemes of power grid impedance are investigated and analyzed comprehensively.The influence of different disturbance injection positions on the impedance measurement bandwidth is analyzed.It is shown that the valued of harmonic signal injected at the point of current reference are greatly decreased above the controller bandwidth,which affects the measurement accuracy above the bandwidth.(2)In order to obtain the impedance of the power grid in the frequency band above the control bandwidth,a quasi random PWM disturbance method based on Sobol sequence is proposed to measure the impedance of the power grid in the middle and high frequency band by using the spectrum dispersion characteristics of random PWM.The relationship between the difference degree of random sequence and frequency spectrum is established,which shows the advantages of Sobol quasi random sequence in random PWM and impedance measurement.Based on the constraints of system stability and frequency spectrum distribution,a method for selecting parameters such as upper and lower limits of disturbance frequency is proposed.The proposed impedance measurement method based on Sobol quasi random PWM disturbance can achieve broadband impedance measurement in medium and high frequency band on the premise of ensuring power quality and system stability.A 6k W grid connected inverter system is built on the Starsim hardware-in-the-loop simulation platform to verify the feasibility of the proposed method.(3)Aiming at the problem of uncontrollable current harmonic energy distribution in random PWM disturbances,according to the analytical relationship between the inverter side PWM output voltage and switching frequency,the paper proposes an impedance measurement method based on the optimal design of switching frequency,directly designing the switching frequency The sequence realizes the controllable adjustment of the output current spectrum,and further improves the impedance measurement effect in the middle and high frequency bands.Based on the idea of frequency division constraint,the current frequency division interval is obtained according to the high-frequency attenuation characteristics of the filter,and the PWM output voltage spectrum constraint conditions that meet the uniformity,consistency and power quality of the output current spectrum distribution in each frequency band are established.Based on the double Fourier analysis and frequency conversion interval,the analytical expressions of the switching frequency sequence and the PWM output voltage spectrum are simplified,and the genetic algorithm is used to obtain the switching frequency sequence that meets the PWM output voltage spectrum,thereby optimizing the harmonic current in the disturbance frequency band The distribution characteristics improve the impedance measurement effect in the middle and high frequency bands.The 6k W single-phase grid-connected inverter system was built on the Starsim hardware-in-the-loop simulation platform,and the advantages of high precision and small disturbance were verified by the impedance measurement method based on the optimization design of switching frequency.