Sequence Impedance Modeling Analysis And Stability Control Of Virtual Synchronous Generator

Due to the time-varying characteristics of line impedance of medium-and low-voltage distribution network,interactions exist between the virtual synchronous generator(VSG)algorithm based distributed power generation units and grid impedance,leading to system instability.Therefore,research on the impedance modeling,analysis and stability control of VSG with frequency coupling characteristics is addressed in this dissertation.a completed VSG sequence impedance model considering the frequency coupling effect is established which can analyze the oscillation mechanism of the interaction system between the VSG and grid A multi-objectived designed virtual impedance is proposed to improve the stability of the VSG grid-connected system.These above researches can provide theoretical and practical guidance for the realization of large-scale VSG grid-connection.The main research contents of the dissertation are as follows:(1)Considering the power loop and voltage loop comprehensively,a VSG sequence impedance model with frequency coupling effect is established by applying the multi-harmonic linearization method.After that,the characteristics of the frequency coupling loop of the VSG are analyzed with this proposed VSG model which indicates that the asymmetric power loop structure is the essential reason of frequency coupling of the VSG.Moreover,the frequency coupling effect reduces the amplitude of the positive sequence output impedance in low-frequency band and causes the oscillation and instability of the VSG.The impedance frequency-sweeping measurement results verify the accuracy of the proposed VSG sequence impedance modeling method.(2)In order to analyze the influence of system parameter perturbation on the output impedance of VSG precisely,a frequency domain norm-indexed sensitivity analysis is presented.The parameter sensitivity of VSG output impedance characteristic is quantitatively analyzed,and the main parameters affecting the output impedance characteristics of VSG in low frequency band are obtained.Based on the Nyquist stability criterion,the multi-parameter stability region of the VSG grid-connected converteris analyzed,and the influence of parameter perturbation on the stability of the VSG grid-connected system is obtained.A 10 k W VSG grid connected converter is built on Starsim hardware-in-the loop experimental platform,and the experimental results verified the correctness of the stability analysis.(3)Aiming at the instability problem of VSG under strong network caused by frequency coupling effect,a multi-objective constrainted adaptive virtual impedance control method is proposed.According to the difference in the amplitude-frequency characteristics of the positive sequence output impedance of the VSG with and without the frequency coupling effect,the frequency range of the frequency coupling effect in VSG is defined.And virtual impedance is adaptively adjusted with the variation of the grid impedance to avoid the above frequency range.To improve the stability without japadizing the power transfer capability,virtual impedance optimization design method based on power decoupling,extreme power output capacity and grid-connection stability is established Experiments on StarSim experimental platform,verified the effectiveness of the proposed oscillation suppression strategy and the optimality of the virtual impedance parameter design method.