| With the steady progress of the "carbon peaking and carbon neutrality goals"development strategy,new energy is gradually replacing traditional fossil fuels as an essential part of future energy.It is estimated that the total installed capacity of wind power and solar energy in China will reach more than 1.2 billion kilowatts by the end of 2030.The connection of large-scale new energy increased the number of power electronics in the power system and has gradually developed into a "double high" trend:a high proportion of new energy and a high proportion of power electronic equipment.The control way and structure of electronics are different from traditional synchronous generators.The system’s interaction at different time scales is also more complex,and the frequency range of the power system oscillation is also wider.At present,the oscillation generation mechanism of large-scale wind farm gridconnected systems is not completely clear,and the existing impedance analysis methods also have certain limitations.In addition,a full-order model for dynamic stability analysis is difficult to establish because of the high model order of large-scale wind farms and the complex interaction between wind turbines and grid systems.Therefore,reducing the order equivalent of the wind farm reasonably and effectively is the premise of an in-depth study of the dynamic interaction between the wind turbines and the AC system.Therefore,this paper focuses on the oscillation problem of large-scale wind farms connected to the grid.Based on complex vector impedance modeling,this paper studies the oscillation generation mechanism of the wind farm grid-connected system,the multi-frequency coupling characteristics,the dynamic equivalence of the wind farm,and the stability optimization control.The main work and contributions are as follows:(1)A complete frequency-domain refined impedance modeling method for Permanent Magnet Synchronous Generator(PMSG)is proposed.The proposed method utilizes the basis vector in the complex number field and the physical meaning of periodic component to directly perform harmonic linearization on the steady-state periodic running trajectory of the wind turbine.This method uses the exponential rotation factor and time-domain transfer function to represent the dynamic characteristics of periodic components and the nonlinear control structures of the interconnected system,avoiding repeated transformation of the coordinate system and reducing the complexity of analytical modeling.Combined with the Euclidean norm.the PMSG machine-network coupling degree index is defined,quantitatively analyzing the influences of the machine-side inner loop and outer loop control bandwidth,wind turbine operation mode,DC capacitance,etc.on the PMSG impedance characteristics and grid-connected stability.(2)A proposed PMSG modeling method based on harmonic state space is proposed using the complex vector impedance modeling method.With this method,the influence of multi-frequency coupling on the dynamic stability of the PMSG gridconnected system is analyzed.This method can transform a linear time-domain periodic system into a complex-frequency-domain linear time-invariant system.This method can also derive the state space matrix containing harmonic components of any frequency and establish the equivalent impedance model of the PMSG AC side containing the harmonic of each frequency.The method is suitable for the analysis of multi-frequency coupling characteristics of the system.(3)A wind farm equivalent method is proposed considering the internal topological structure and dynamic characteristics of wind farms.The method firstly divides the wind farm into several wind power clusters with the same model and similar working conditions according to the aggregation characteristics of wind turbines.Then,through matrix similarity transformation,a wind power cluster including M wind turbines and internal ties is equivalently transformed into a series of decoupled wind turbine subsystems.On this basis,the dominant eigenfunction model of the equivalent subsystem is derived.Furthermore,based on the matrix perturbation theory and Padéapproximation,a dynamic equivalent method for the wind farm is proposed using the dominant eigenfunction of the equivalent wind turbine subsystem combined with the improved asymptotic waveform estimation method characterize the original wind farm.Compared with the existing single-machine equivalent model,this method can reflect the dominant dynamic characteristics of the external AC system of the wind farm and dynamically characterize the influence of the interaction between wind turbines on the dynamic stability of wind farms.(4)Asymmetric PLL control method is proposed,aiming at the problem of the coupled oscillation frequency of wind power grid-connected system caused by the traditional phase-locked loop(PLL)asymmetric control structure.This method equivalently characterizes the dynamic response characteristics of the wind farm’s point of standard coupling voltage on the d-axis and q-axis by introducing the concept of complex phase angle and can suppress the system frequency coupling effect caused by the control asymmetry of the traditional PLL.On the basis of this,the equivalent impedance of the AC side of the wind power grid-connected system is reshaped to improve the impedance characteristics of the wind power grid-connected system,suppress Broadband Oscillation,and improve the stability of the grid-connection system between the wind farm and weak grid. |
