Nonlinear Control Of Grid-side Converters For Doubly-fed Induction Generator Under Unbalanced Grid

Human beings have to face the environmental problems caused by energy such as the increase of greenhouse gases and global warming,as well as the resource problems caused by the increasing scarcity of fossil fuels,with the overexploitation of fossil fuels.Humanity’s demand for new energy has become more urgent,and the development and utilization of renewable energy resources represented by wind power has been put on the agenda.In recent years,the installed capacity of wind power has grown rapidly,and the wind power technology represented by double-fed wind power system has been widely used in the power grid.However,the doubly-fed induction generator is exceptionally sensitive to grid faults and the grid-side converter is the entrance of energy control,the control of the grid-side converter under unbalanced grid voltage has become the focus of current academic attention.Therefore,it is of great theoretical importance to study the control strategy of doubly-fed induction generator netside converter under unbalanced grid.In this paper,a nonlinear control strategy for a grid-side converter is propoesd under unbalanced grid voltage conditions,combining sliding-mode control technology and feedback linearization theory to improve the dynamic response performance of the system compared with traditional cascade control.This simplifies the system control structure,reduces the impact of positive and negative sequence separation on the control accuracy of the current controller,and improves the adaptive capability of the doubly-fed wind power system under unbalanced grid.The main research contents are as follows.(1)The nonlinear control strategy of DFIG grid-side converter under ideal grid conditions is studied.The affine nonlinear mathematical model is established in αβ coordinate system,and the feedback linearization is decoupled with this model to realize the static difference-free tracking of the reference current;the error between the reference value and the actual value of the DC bus is selected as the sliding mode surface,for which the sliding mode controller is designed to stabilize the DC bus voltage at the voltage reference value.(2)In order to make the DFIG operate stably under the unbalanced grid voltage condition,the control strategy of the network-side converter under the unbalanced grid is studied.The positive and negative sequence components of the unbalanced grid voltage and the positive and negative sequence separation technology are analyzed.By studying the influence of the positive and negative sequence grid voltage on the active and reactive power output of the network-side converter,the calculation method of the reference current is derived,and combined with the nonlinear control strategy proposed in this paper,the negative sequence current suppression,active power and DC bus voltage fluctuation suppression,and reactive power fluctuation suppression of the network-side converter can be achieved respectively.The control objectives are(3)An experimental platform based on TMS320F28335 controller for the grid-side converter of doubly-fed wind power system was designed and developed.Under the laboratory conditions,the hardware and software platform was designed to simulate the unbalanced grid voltage environment and two sets of unbalanced experiments were conducted on the grid-side converter to prove the effectiveness and practicality of the control strategy proposed in this paper.