Research On Sliding Mode Control Of DFIG Rotor-Side Converter

The Doubly-Fed Induction Generator(DFIG)has been widely used in wind power generation systems because of its low cost,high efficiency and adjustable power factor.The DFIG system in practice has internal parameter uptake and external grid voltage dip disturbance,and the linear control method usually has the disadvantages of long regulation time and large power oscillation.In addition,active power oscillation injects harmonic currents into the grid,causing drastic fluctuations in motor torque and accelerating the aging of the mechanical system,while reactive power oscillation injects excess reactive currents into the grid,leading to increased system losses.Sliding mode control has the advantages of simple algorithm,easy engineering implementation and good robustness,and has been applied to DFIG system control.In this paper,the DFIG rotor-side converter is used as the research object,and the relevant control algorithm is studied based on the sliding mode control theory,which is mainly as follows:Firstly,the structure and operation principle of DFIG system is explained,the mathematical model of DFIG under three-phase stationary and two-phase rotating coordinate system is analyzed,the SVPWM principle is relatedly introduced,and the implementation method of SVPWM is given to provide the theoretical basis for the subsequent research.Secondly,a new power convergence law is proposed for the large jitter problem in the traditional sliding mode control algorithm.The proposed convergence law adds an adaptive term to the traditional power convergence law,which not only realizes the adaptive adjustment of the convergence speed,but also achieves the smooth cutting of the sliding mode surface,which effectively reduces the sliding mode jitter.On this basis,the DFIG active and reactive power sliding mode control laws are designed in combination with the DFIG direct power control strategy,and stability analysis is performed.The simulation results show that the control algorithm proposed in this paper can effectively reduce the power fluctuation compared with the DFIG double-closed-loop PI control and the traditional convergence-law sliding-mode control.Finally,in order to improve the fault ride-through capability of DFIG under grid voltage symmetrical dips,a DFIG sliding mode direct power control algorithm considering the dynamic changes of stator magnetic chain is proposed.Firstly,the effect of grid voltage symmetrical dip fault on DFIG stator magnetic chain is analyzed,and the mathematical model of DFIG considering the stator magnetic chain change process is established.The algorithm treats the cross-coupling terms as disturbances and uses an exponentially converging observer for feedforward compensation to realize the decoupling control of system active and reactive power.The simulation results show that the proposed control algorithm has a certain suppression capability for rotor-side overcurrent and power oscillation under the grid voltage symmetry dip fault,which effectively improves the fault ride-through capability of DFIG.