| In the context of the accelerated transformation of the global energy structure,"carbon neutrality" is an inevitable requirement for my country to achieve energy independence and ensure energy security.The realization of the goal of "carbon neutrality" will be accompanied by an increase in the installed capacity of renewable energy power generation.High proportion of power electronic equipment and low inertia levels will become the basic characteristics of future power systems.Virtual Synchronous Generator(VSG)technology endows power electronic equipment with inertia support capability,but increases the complexity of the control system.With the requirement of friendly grid connection,the stability of VSG grid-connected system has become a current research hotspot.This paper focuses on the power oscillation analysis and suppression methods of VSG grid-connected systems.The main research contents and innovations are as follows:(1)The applicability of the typical VSG reduced-order model is analyzed.It is proved that the traditional third-order model is only suitable for the analysis of large inertia and high damping system through the unilateral Fourier analysis.The accuracy of the electromagnetic fifth-order model is more sensitive to the control parameters through the output impedance analysis.Aiming at the weak time-scale separation property of VSG,a model reduction method based on time constant is proposed,which can preserve the cross-time-scale interaction.A reliable and concise improved electromagnetic model is derived based on the reduction method,which effectively improves the accuracy and scope of application of the model without changing the order.(2)By analyzing the dynamic coupling characteristics of the VSG power loop,the dynamic decoupling conditions of the active/reactive power loop and the theoretical basis for independently analyzing the subsynchronous/synchronous frequency characteristics of the active/reactive power loop are obtained.The mechanism of subsynchronous/synchronous resonance and its influence on system stability are revealed,that is,the interaction between line impedance and output impedance leads to the decrease of system stability margin,or even resonance.By analyzing the influence of control parameters on resonance,a resonance suppression strategy based on output power proportional differential feedback is proposed,and the specific design method of each feedback coefficient is given.The output power proportional differential feedback strategy can effectively suppress the resonance and improve the system stability while maintaining the performance of the power loop.(3)The phenomenon of VSG output power oscillation is analyzed through characteristic root locus,and the feasibility of suppressing resonance is demonstrated through the combined value method of steady-state damping and dynamic damping.The virtual torque model of VSG is further established,and the mechanism of low-frequency oscillation of VSG output power is intuitively revealed through damping torque analysis:the subsynchronous resonance and the negative damping effect of the phase-locked loop lead to the damping torque and damping coefficient of the VSG,and the synchronous torque and Synchronous power coefficients are inconsistent.A VSG virtual torque correction method based on phase compensation is proposed.By quantitatively correcting the virtual torque,low-frequency oscillation can be effectively suppressed,and a compensator design method is given.(4)In view of the discrepancy between the actual indicators of the VSG and the design indicators,the necessity of testing the design parameters of the VSG is demonstrated,and an indirect testing method of the design parameters of the VSG based on the system identification is proposed,which converts the test problem of the design parameters into a system identification problem..Through mechanism analysis,a grid-connected model structure of VSG/ droop control is built,and the system identification problem is simplified to a parameter identification problem.The numerical model of the inverter data sampling system is deduced,and the parameter identification problem is transformed into a parameter optimization problem through an intelligent optimization algorithm.The verification results show that the indirect test method based on system identification can realize the rapid identification and calculation of VSG parameters.(5)Based on the microgrid system experimental platform,the basic control strategy experiments of single machine such as PQ,droop and VSG,and the basic control strategy experiments of microgrid such as secondary voltage regulation,secondary frequency regulation and grid-connected pre-synchronization were carried out.The theories,methods and related conclusions described in the article have been experimentally verified,including the flexibility of the VSG control strategy,the accuracy of the improved electromagnetic model,the effectiveness of the output power oscillation suppression strategy and the feasibility of the VSG design parameter test method,etc.It further confirms the engineering value of related research results. |
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