Study Of Maximum Power Point Tracking And Low Voltage Operation Technology For DFIG

After half a century research and application, the grid-connected wind power technology has made considerable progress. Due to the double-fed induction generator (DFIG) has small capacity power converter, active and reactive power can be adjusted independently and rotor speed can adjust in wide range has been widely used, in the development process of the modern wind power industry. Double-fed wind turbine can convert wind energy into electrical energy very well, play an important role in the energy restructuring, wind turbine unit capacity increased and wind power total capacity rapid increased. How to improve wind turbine efficiency become a hot research topic of wind power industry. Because of wind power’s proportion become larger in the grid, and wind power with randomness and volatility will directly affect the stability of the grid, The grid requirements for wind turbines has improved.This paper first summarizes wind energy and wind power development status of the situation, Analysis and elaboration of the technical characteristics of wind power and hot research issue, proposed the research background and purposes of this paper. Then analysis the mathematical model and control theory of variable speed constant frequency (VSCF) DFIG. Established a mathematical model of the doubly-fed wind generators in the two-phase rotating coordinate system. Based on mathematical model studied maximum power point tracking (MPPT). Studied the method of DFIG harmonics suppression when wind turbine work at unbalanced grid voltage. Do some Research for DFIG Low-voltage ride though (LVRT) where power grid with Low-voltage failure.Studied the principles and the existing control methods for MPPT of DFIG, Pointed out that the existing MPPT control object is only generator, Ignore pitch angle regulation can affect MPPT’s result. Add pitch control system into MPPT loop, give out the reason and method for pitch system.Proposed a new MPPT method based on minute pitch angle regulation and Extreme value method for DFIG. Do the field tests to verified the proposed method, the test results show the proposed MPPT method with superiority.Analyzed under unbalanced voltage DFIG response characteristics, studied the effects of unbalanced grid voltage on generator stator, rotor, generator side converter (GSC) and DC bus current. Pointed out that in the unbalanced grid voltage, the converter will produce control deviation caused by work condition deviate from the design. which will cause rotor current contains obvious harmonics, DC bus voltage shock and GSC current waveform distortion. Based on vector control method, proposed an improved vector control strategies based on notch filter. For GSC and LSC (Grid line side converter) dq-axis current filtering the AC component, the simulation results Verified the proposed control strategy effectively suppress harmonic of DFIG.Analyzed the DFIG transient response process under low voltage fault, focuses on the rotor fault current transient process. There are two components of the rotor fault current, one is the steady-state current during the fault time, the other fault current decay with time. Rotor current rapid rise when there is a grid voltage drop which can reach3-5times of the rated current. The fault current may higher than GSC over-current protection value, rotor fault current will decay with time can decrease to blow the over-current protection value, the GSC restart then the GSC can recovery excitation regulation for DFIG. Proposed an AC and DC Complex Crowbar LVRT solutions based on Dynamic braking resistor (DBR). The proposed scheme adds a group of uncontrolled rectifier bridge between the generator rotor and converter DC bus. For slight voltage fault, wind turbine connected in back-to-back converter way, LVRT works at DC Crowbar way, during the fault period the wind turbine converter work normal can regulate the output power of the stator. Achieve provide reactive power support to the grid during the whole fault period. For severe voltage fault, at the beginning the GSC cut out caused by over current false, the uncontrolled rectifier bridge replace the GSC transports rotor fault energy to the DC bus capacitance. LVRT works at AC Crowbar way avoid the GSC Current capability insufficient. When fault current decrease the GSC restart, the wind turbine recover connected in back-to-back converter way, LVRT works at DC Crowbar way, the proposed solutions can improve LVRT ability of DFIG when there is a severe voltage fault. The proposed scheme can achieve DFIG LVRT. Reactive power support can meet the requirements of the state grid to during different depths grid voltage drop fault.At last, summarizes the main research result and points out the further research direction.