Research On Gird Side Converter Control Of Doubly Fed Induction Generator System During Voltage Sag

Recently,the LVRT control of DFIG system has becoming a hot reach scholar.As is well known to all,the mainly focus is on the rotor side converter control,however,if the grid side converter is controlled improperly,DFIG system will samely lose the LVRT capability.Thus,in this paper,we will take the grid side converter as the research target and control strategy under grid voltage sags is analyzed.The mathematical model is the basis of the analysis and controlling of the grid side converter,therefore,the model under the stationary and synchronous reference frame is established at first.The control strategy used in this paper is the double closed loop control strategy of voltage and current which is based on d axis voltage in the traditional synchronous reference frame.Then,the parameter design method of voltage loop and current loop PI controller is introduced.Simulation and experimental results validate the correctness of the parameter design method.When grid voltage sags,according to the current control ability of grid side converter,the control procession of grid side converter during voltage sag can be divided into the out of control area and the control area.The current out of control area is caused by the delay of digital control,during this area,the grid side converter lose the control ability of the current,which will further lead to an inrush transient current(ITC).This ITC may trigger the overcurrent protection of the inverter and in the worst case may even destroy the semiconductor switches,causing the disconnection of the PGI from the grid.Thus,in this paper,a thorough study of the ITC influence factors are carried out.Based on these findings,the appropriate ITC hardware and software suppression approaches are then discussed.By changing the DSP control logic,changing the PWM loading mode to immediate load mode,using a smaller computational DSP SVPWM realization algorithm,the delay of feedforward grid voltage can be decreased,then the ITC is suppressed.Hardware current limit method is based on the principle of hysteresis comparator,the PWM signal is blocked when the current value is bigger than the threshold value.According to the simulation and experiment,the validity of the correctness of the theoretical analysis and the effectiveness of the current suppression approaches are proved.The control strategy of the grid side converter in the control area is divided by the symmetrical voltage sags and asymmetrical voltage sags.In the case of symmetrical voltage sags,if voltage sags and DC voltage PI controller output is saturated,the control authority of the DC bus voltage is switched to the rotor side.When asymmetric voltage sags,the symmetrical current control method is used,and if the output of DC voltage PI controller is saturated,the control method is same with symmetric voltage sags situation.The feasibility of the above scheme is verified by simulation.The key technology of the above control strategy,whcih is the positive sequence grid voltage phase locked and fast grid voltage drop detection technology,were studied in this paper.In order to ensure whether to suppress DC voltage fluctuation is a critical problem under the condition of unsymmetrical,the fluctuation of DC bus is analyzed.