Research On Low Voltage Ride Through Technology Of DFIGs Under Unbalanced Grid Fault

As more and more wind turbines were connected to the grid wind power impact on thepower grid is increasing Doubly-fed induction generators (DFIGs) have become themainstream in the large capacity wind turbines Due to the stator windings were connected tothe grid directly any fault of the grid would impacted on DFIGs Single-phase earth faultoccurs most frequently When unbalanced faults happening it causes unbalanced heat in thethree phase windings even causes over currents Active power reactive powerelectromagnetic torque appears2ndpulse Negative sequence currents would appear in thegrid-side convertor The DC voltage would appear2ndpulse All above these would affect thenormal operation of the systemIn this thesis firstly it analyzed and researched the DFIGs model and control methodsunder normal grid voltage It designed a speed sensorless based on rotor currents and wasoptimized with fuzzy control technology It presented a DFIG control method with fuzzyspeed sonsorlees based on rotor currents By simulation experiment it proved the fuzzy speedsensorless had higher estimation precision and better dynamic performanceSecondly it analyzed the mathematical models of the rotor-side convertor and grid-sideconvertor under unbalanced grid voltage It designed dual dq-PI controller and presented dualdq nonlinear control strategy based on passivity theory according to the passive characteristicof the grid-side convertor and rotor-side convertor The controller could realize the controlobjectives of the DFIG under unbalanced grid voltage by controlling the positive andnegative dq currents of the convertors By simulation experiment it realize the control of thegrid-side convertor and rotor-side convertor separately Then it simulated the control strategyof grid-side convertor cooperating with rotor-side convertor with nonlinear controller The results showed that the dual dq nonlinear controller base on passivity theory could restrain theelectromagnetic torque active power and DC voltage2ndpulse of DFIG system and thenonlinear controller was affected less by the changes of the motor parameters Compared tothe dual dq-PI controller the nonlinear controller based on passivity theory had betterrobustness and dynamic performance It improved the unbalanced low voltage ride throughability of the DFIG systemAt last the paper designed the hardware experiment platform of the grid-side convertorand test it...