Study Of Inertia And Damping Controls And Coherency Identification For Wind-Integrated Power System Based On Phase-Locked Synchronization Mechanism

Faced with the dual pressures of energy shortage and environmental pollution,China has formulated a blueprint for energy development strategies for 2030 and 2050.By the time,the proportion of renewable energy power generation will increase to 30% and 60% respectively.The power system in China is gradually transformed into a highpenetration renewable energy integrated state with low inertia,power electronics widely distributed and weak synchronization.The integration of high-permeability new energy represented by wind power will bring new challenges to the small signal stability,frequency adjustment and identification of coherent generator groups for traditional power systems.In this paper,the traditional state space modeling method based on the detailed model is not adopted.By establishing the phase motion equation of the inner electric potential in the doubly-fed wind generator(DFIG),the DFIG is equivalent to a virtual synchronous motion rotor with inertia constant,damping constant and synchronous torque.The inertial response,small interference stability and coherent grouping characteristics of wind power systems are studied from the perspective of the synchronous generator.The specific content is divided into the following three aspects:(1)The differential algebraic equations of the general model of doubly-fed wind turbines are introduced in detail.The electromechanical time scale and the core state variables strongly correlated with the electromechanical mode are determined by characteristic analysis.The wind power system linearization model under the mode reduction estimation is constructed.The dynamic process of the drive train,speed controller,current inner loop,phase-locked loop,and virtual inertia control is considered.Based on the internal synchronization mechanism of the phase-locked loop,the electromechanical dynamic model is equivalent to the phase motion equation with the internal potential phase-electromagnetic power as the input-output frequency domain relationship.(2)According to the similarity of the phase-locked loop and the rotor motion equation in the synchronization mechanism,the phase motion equation is transformed into the equivalent swing equation with inertia,damping,and synchronous torque.Based on the equivalent swing equation,the frequency domain expression of the DFIG inertia and the equivalent damping ratio of the interconnected system are derived.The effects of different phase-locked loop parameters on the inertia of the wind turbine and the small disturbance stability of the system are studied respectively.The research shows that the decrease of parameters in the phase-locked loop will jointly improve the inertial response capability of the wind turbine,but the proportional gain has an inverse relationship to the damping characteristics against with the integral gain.According to the research,a phase-locked loop parameter optimization design scheme is proposed to realize the synthetic optimization control of wind turbine inertia and system damping.(3)The coherent generator groups identification method for the wind power system is proposed with the phase motion equation.The research on the traditional generator group theory shows that the swing dynamics of the rotor describes the slow coherent grouping process with the power angle as the coherent criterion.Similarly,the phase-locked state quantity of the phase motion equation,the phase-locked angle dynamic,reveals the degree of coherent separation between the potential of DFIG and the AC system.According to this,the slow coherence theory is used to combine the second-order phase motion equation with the synchronous machine rotor equation to establish a coherency model with the wind power system.The generalized characteristic method and K-means algorithm are introduced to solve the eigenvector corresponded with the electromechanical mode and the phase information is clustered to identify the coherent generators.At the same time,the influence of the phase-locked loop parameter on the system coherence is studied.In summary,based on the inherent stable operation mechanism of wind turbine and synchronous machine,the DFIG is equivalent to the swing equation with rotor synchronous motion characteristics.Therefore,the general modeling method for wind turbine and synchronous generator is proposed.This modeling method is proposed to solve the main problems in the large-scale wind power systems: inertial response,small signal stability and identification of coherent generator groups.