Research On Mechanism And Suppression Of Grid-Connected Subsynchronous Oscillation Of Direct Drive Wind Turbine

With the rapid economic development,the energy required for development is also increasing.It is the premises for the rapid development that the adequately supply of energy.The traditional energy has located in a bottleneck period with that large number of new energy need to be supplied to fill the energy gap.A large amount of energy generated by new energy power generation technology is connected to the grid,which changes the existing energy structure,resulting in obvious changes in the operating characteristics and grid form of the grid.Among them,the sub-synchronous oscilation caused by the large-scale wind farm connection to the grid The problem is particularly prominent.In this paper,we will study the SSO problem in the grid connected of direct-driven wind turbine generator and its fundamental reasons.According to the research results,a method,adding damping controller to the grid-side controller of the system,is proposed to suppress the SSO in the grid-connected system.The main contents of this paper are as follows:(1)A mathematical model of the direct-drive wind power generation system is introduced.Based on the SSO problem,the grid-connected system of direct-drive wind turbine planet,which include Phase-locked loop model,DC link model and the control strategy of the grid-side converter,is established.Then,The building of the model on the Matlab/Simulink simulation platform verifies the correction of the introduced model and provides the support for the subsequent study.(2)Research on the stability of the grid-connected system and the mechanism of SSO.Firstly,a sequence impedance model of the grid-connected for the wind turbines is established by the method of harmonic line arization.Secondly,several stability criterion include Nyquist stability criterion,frequency characteristic criterion and frequency domain impedance stability criterion are applied to judge the stability of the system.Finally,we get a result that the main factor leading to grid-connected SSO is the interaction between the wind turbine and the grid due to the capacitive and inductive effects,by analyzing the frequency characteristics.Furthermore,by the frequency domain impedance analysis,we find that it is the negative equivalent damping of the grid-connected system that causes the SSO.(3)Develop a basis for the grid-side parameter setting by combining sensitivity in frequency and sequence impedance model.Calculate the impedance sensitivity of each parameter in the grid-side theoretically.Then,according to calculated results,we can obtain the effect to SSO for each parameter,thus,we can develop the basis for the grid-side parameter setting.Using the same model built in(1),injecting a disturbance voltage to simulate the SSO in the system,we can verify the effectiveness of the proposed basis.(4)A damping controller is designed to restrain and eliminate the SSO in the system.When SSO occurs in the grid-connected system,The SSO can be restrained by adding a property damping controller to the the grid-side converter control system,it is the same to provide positive damping compensation to the oscillating system.By using the same model as(3)with the same condition,we can verify the effectiveness of the proposed damping controller.The simulation results shows that adding the damping controller to the grid-side control system make sense for restraining SSO in the grid-connected of direct-driven wind plant.