Virtual Synchronous Control Strategy For Doubly-fed Wind Turbines Under Unbalanced Grid Voltage

As the global energy crisis continues to spread and people’s awareness of environmental protection gradually increases,renewable energy provides a new way for countries around the world to solve the energy problem.As the backbone of the global deployment of renewable energy,wind power has also received increasing attention.However,with the rapid increase of fan grid-connection capacity,the equivalent inertia of power grid is gradually reduced.Especially in the case of power grid voltage imbalance fault,it is difficult to maintain the stable operation of the power grid.Therefore,this paper mainly studies the control of doubly-fed asynchronous wind generator under the unbalanced power grid.This paper firstly models the doubly-fed induction wind generator and its grid-side converter,and then the steady-state equation and small signal model of the virtual synchronous control applied to the side converter are derived,and the characteristics of the virtual synchronous control are compared with that of the vector control.Then a mathematical model of the grid-side and machine-side converters of the doubly-fed machine under unbalanced power grid is established,and the adverse effects of the unbalanced power grid on the operation of the doubly-fed machine are theoretically analyzed and simulated.Considering that virtual synchronization control can produce friendly voltage and frequency support for the power grid,therefore,under the unbalanced power grid,this article proposes an active damping control strategy based on virtual synchronous control to reduce the unbalance of grid-connected current and voltage respectively,and the control effect is verified by simulation.Based on the limitations of the active damping control strategy,this paper proposes a coordinated suppression strategy for negative sequence voltage and current.First,the unbalanced information of stator voltage and current is extracted by the resonance regulator,and then suppressed by the proportional integral regulator,and the voltage and current compensation coefficients can be changed to achieve the coordinated suppression of negative sequence voltage and current.On the basis of this analysis,this paper designs the primary and secondary control loops respectively,and carries out the stability analysis and simulation verification of the control strategy at the same time.In order to make full use of the inherent advantages of doubly-fed motors,this paper also adds active and reactive power synchronous suppression strategies to the grid-side converter to achieve the suppression of the total output power double-frequency pulsation.Finally,based on the 11 kW doubly-fed motor experimental platform,this article further verifies the effectiveness of the proposed coordinated suppression strategy through experiments.It not only shows that the control strategy can significantly inhibit the negative sequence voltage and current,but also shows that it can maintain its excellent control effect when the grid asymmetry degree and the active power instruction value has a step.