Investigation On Grid-Friendly Control Strategy Of DFIG Wind Power System

With more and more renewable power generation systems,including wind power generation,being connected to the grid,the impact of renewable power generation systems to the power grid has attracted wide attention.Doubly-fed induction gererator(DFIG)has become the mainstream of wind power generation due to its low cost and low requirement for converter capacity.However,DFIG is very sensitive to the grid faults because its stator is directly connected to the power grid.As one of the most common grid faults,the grid unbalance will cause DFIG to produce much current harmonics,power and electromagnetic torque ripples,which not only affects the output power quality of DFIG,but also damages the structure of the generator itself.At present,most of the improved control strategies of DFIG under unbalanced grid are based on vector control(VC)strategy and direct power control(DPC)strategy using linear controllers.While the researches on the nonlinear control strategy under unbalanced grid are very limited.In addition,due to the randomness and intermittentity of wind power generation,most of the DFIG systems cannot provide stable support for the power grid similar as traditional synchronous generators without energy storage module.If large-scale wind power generation systems are integrated into the grid,the frequency and voltage response characteristics of the power system will become worse,and the fluctuation and uncertainty of the grid will increase,which is very harmful to the grid safetyIn order to deal with the problems above,grid-friendly control strategies of DFIG system are studied in this paper.The "grid-friendly" mainly contain two aspects:On the one hand,the output power quality of DFIG system can be guaranteed under unbalanced power grid;on the other hand,the DFIG system can provide effective frequency and voltage support to power grid when disturbance occers.From these two perspectives,this paper studies the low-complexity model predictive control(LCMPC)strategy and sliding mode control(SMC)strategy for the rotor-side converter,as well as virtual synchronous generator(VSG)strategy for the grid-side converter under both balanced and unbalanced grid conditions.The contributions of this paper can be summarized as follows:1.A LCMPC strategy is proposed for DFIG under both balanced and unbalanced grid conditions.The computation burdens are effectively reduced by using sector judgerment to replace the calculation of cost functions.The control precision is improved by using two-vector and three-vector based modulation methods.In addition,a unified power compensation strategy is proposed and combined with the LCMPC strategy to deal with the influence of the unbalanced power grid.Based on the studies above,a muti-objective LCMPC is proposed,which can achieve various control targets under unbalanced grid condition,and the dynamic response is also satisfactory when the control target changes during operation.2.A resonant based SMC strategy and a extended power theory based SMC strategy are proposed for DFIG under both balanced and unbalanced grid conditions.The impact of the unbalanced grid voltage to the SMC of DFIG is analyzed,and a resonant based SMC is proposed,which can achieve different control targets by selecting different variables as the feedback values of the resonant controller.Thus,the decomposition process is no longer needed.In addition,a extended power theory is proposed and combined with SMC to further simplify the control system.By selecting different combinations of extended and traditional powers as the control variables,different control targets can be achieved under unbalanced grid condition without decomposition process and phase-locked-loop(PLL).3.A self-synchronized VSG strategy is proposed for grid-side converter under both balanced and unbalanced grid conditions.The VSG strategy is introduced and the impact of unbalanced grid voltage to the VSG is analyzed.On this basis,an improved self-synchronized VSG strategy is proposed under unbalanced grid condition by adding a resonant based compensator to the traditional VSG,which can achieve precise self-synchronization and restrain the negative sequence current components in the power control mode.In addition,a extend power theory based muti-objective VSG strategy is proposed to deal with power ripples at the PCC point.Different control targets including restraining PCC power ripples can be achieved by selecting different combinations of extended and traditional powers as the control variables.