Operation And Control Techniques Of High-capacity Full-Power Modular Wind Power Converters

With the rapid development of global wind generation,especially offshore wind generation,power rating of wind turbine is continuously increasing in order to improve efficiency and reduce unit cost.Meanwhile the grid code for large scale wind generation has become more and more strict,which leads to high demand in efficiency,reliability and grid connection performance of the turbine.For the sake of meeting grid codes,full-power flexible wind system is the most promising grid connection style in large-scale wind power integration.Power rating of single wind turbine has already exceeded 10MW,and it is now in continuous growth.Considering the power handling of existing power electronic devices,full-power wind converter should be composed of integrated power modules.This paper focuses on full-power large-capacity modular PMSG(Permanent Magnet Synchronous Generator)wind generation converters.Two technical solutions are considered,namely low voltage and medium voltage grid connection.Based on the solutions,the following operation and control techniques are studied:grid connection current control of low voltage modular parallel wind converters,sub-module voltage fluctuation suppression control strategy of medium voltage BTB-MMC(Back to Back Modular Multilevel Converter),detection and control of LVRT(Low Voltage Ride Through)and the module combination technique in full-power wind systems.Firstly,grid injection current control technique of isolated low voltage modular parallel wind converters is studied.Simulation model of PMSG and mathematical model of the modular parallel converters are built.Meanwhile the coordinate control strategy and the control parameters of the grid side inverter are designed.A current compensation control method is proposed to solve the dc voltage fluctuation problem during dynamic process of winf turbine or the grid.The effectiveness of the proposed control method is validated through simulation and experimental results.Due to isolated parallel connection between the modules,the proposed strategies can be applied in all the modules.Secondly,control techniques of medium voltage cascade modular BTB-MMC wind converter are studied.MMC is modeled in the first place and then the overall control strategy of BTB-MMC based on carrier phase shift SPWM(Sine Pulse Width Modulation)is proposed,in which the SM(Sub Module)voltage balancing control loop,SM voltage averaging control loop and circulating current control loop are designed.SM voltage of the generator side MMC suffers high amplitude fluctuation because of the low-frequency and large phase current of PMSG.A circulating current injection method is proposed to suppres the voltage fluctuation.The fluctuation of SM voltage is suppressed with the proposed strategy,however circulating current increases.In order to reach a trade-off between fluctuation suppression and circulating current increase,several trade-off coeeficients are defined as criteria.Thirdly,LVRT implementation techniques of full-power wind generation systems are studied,including three phase voltage fault detection technique,coordination LVRT control and design of the braking chopper.An improved loop filter PLL is proposed to realize fast and accurate detection under unbalanced and harmonic three phase voltage fault.Meanwhile a phase order identification method is proposed and validated by experiment.As to the LVRT braking chopper design,two traditional LVRT control method are compared and the design method of the chopper resistor are proposed according to the Chinese national grid code.Furthermore a new distributed braking chopper strategy is proposed in order to avoide instant voltage peak on the dc-link under conventional braking chopper.Braking choppers are paralleled with all the SM capacitors to help riding through grid faults.The correctness and effectiveness of the proposed strategies are validated by simulation.Finally,key technologies of the module combination in multi-modular full-power wind systems are studied,including PWM signal generation and its logical synchronization,the EMI(Electromagnetic Interfere)suppression of multi-modular driving power supplies.An improved PWM generator based on FPGA(Field Programmable Gate Array)is propsed to eliminate the logic pulse error caused by digital control delay.The validity of the improved PWM generator is verified through simulation and experiment.On the other hand,the high-frequency EMI source caused by high-frequency switching devices of the power electronics modules is analyzed and calculated quantitatively.Based on the high-frequency modeling,methods to suppress the interferences are proposed and validated through simulation and experiment.