Research On The Dynamic Characteristics And Control Strategy Of Sub/super-synchronous Oscillation Of Direct-drive Wind Farm

With the increasingly serious shortage of energy resources and environmental pollution and other issues,focusing on the development of clean energy and promoting the green development of energy have become an inevitable trend in the development of the power industry.Wind power is one of the key directions for the development of new energy.As the scale of wind farms continues to increase,sub/super-synchronous oscillation occurs ever more frequently,which seriously affects or even threatens the stability of the system.This paper takes the direct-drive wind farm as the study object,analyzes the dynamic characteristics of devices and control links of direct-drive wind farm during the sub/super-synchronous oscillation from the energy point of view,and gives a double-layer control strategy for direct-drive wind farm oscillation.The main contents and achievements of this paper are as follows.In view of the research on sub/super-synchronous oscillation of direct-drive wind turbine,the interaction of the oscillation components in the control links of the wind turbine are not fully considered.First,the transient energy model of direct-drive wind turbine containing grid-side converter control links is built in this paper.And the expressions of the variation rate of transient energy of each control link in grid-side converter are derived.Further,based on Lyapunov’s second stability theorem the transient energy change rates of each control link of direct-drive wind turbine are calculated and its energy change characteristics are analyzed.It is found that the positive and negative damping energy in the transient energy model are important factors that characterize the stability of the wind turbine.Finally,the model of system is built for simulation tests,and simulation results verify the correctness of the analysis results.Aiming at the problem that stability analysis of direct-drive wind farm does not consider the interaction between devices,the transient energy model for direct-drive wind farm considering the interaction between devices is built based on the stand-alone model in this paper.The interaction energy between the wind farm and the grid is divided into three parts—the disturbance energy of each device,the coupling energy and the interaction energy between devices,which respectively represents the initial disturbance effect of the device,the self-induced effect and interaction between devices during the oscillation process.And the detailed expressions of each part of the wind farm is derived.On this basis,energy change characteristics of each energy in the wind farm are explored based on Lyapunov’s second stability theorem.It is found that the interaction energy between devices is the key factor that determines the divergence or convergence of the wind farm oscillation.And then the influence of wind turbine connection distance,wind turbine operation status and SVG operation mode on the stability level of the wind farm is revealed.Finally,the simulation results verify the correctness of the analysis results.In view of the problem that the existing oscillation suppression measures are difficult to achieve adaptive control of multiple oscillation modes,and the influence of additional control on the fundamental frequency characteristics and dynamic response characteristics is not considered,the double-layer control strategy for direct-drive wind farm oscillation is proposed in this paper.It strives to minimize the energy interaction between the wind farm and the grid,that is,the upper-layer realizes the maximum interaction energy dissipation between devices in the wind farm,and the lower-layer realizes the minimization of the negative damping energy accumulation of direct-drive wind turbine.The upper-layer control strategy is constructed based on the interactive energy characteristics of devices in the wind farm.It optimizes the operation mode of the wind turbines and reactive power equipment with the goal of minimizing the change rate of the interactive energy between devices.The lower-layer control strategy is constructed according to the energy characteristics of direct-drive wind turbine.And the energy compensation branches are designed through backward deduction,and the supplementary energy branch set is optimized to maximize the compensation energy of each branch and minimize the increment of fundamental-frequency voltage.At the same time,with the power of the units,the frequency-domain characteristic and fundamental-frequency voltage characteristic of control links being satisfied as the constraints,a scheme to optimize the coefficients of double-layer control strategy for direct-drive wind farm is established.Finally,the correctness and effectiveness of the double-layer control strategy for direct-drive wind farm oscillation are verified by simulation.