| As power electronic devices are widely used in actual production and life,its problem of power grid harmonic pollution is growing. Although the cost andthe structure of traditional uncontrolled rectifiers used by diodes and phasecontrolled rectifiers used by thyristors is low and simple, it can input a largenumber of harmonic and reactive power to grid, leading the power grid pollution.Because of many advantages of PWM rectifier such as approximate sine currentwave in the grid side, low harmonic wave, low power grid pollution, energyfeedback and unity power factor, it has become more and more widely appliedand gradually become one of research hotspots in the field of power electronics.Based on the PWM rectifier as the research object, this thesis focuses on thestudy of control strategies and online parameters identification in the grid sidefor the PWM rectifier, including parameters identification, control strategywithout AC voltage sensors and proportional resonant control.As in the operation of PWM rectifier, parameters in the system are notconstant, and offline parameters identification cannot be used in the operationprocess, this thesis proposes an online parameters identification method for thePWM rectifier based on least square method. The standard form of least square isderived by reduction of system model. Based on the least square method withforgetting factor, the system parameters can be updated in real-time. Through thesimulation results, it can be seen that this method can identify parametersaccurately and the stability and convergence is satisfactory.The control of PWM rectifier usually needs a variety of voltage and currentsensors. Large quantity of sensors increases the cost of the system and at thesame time brings testing errors in communication and makes system complex.Therefore, this thesis proposes a control strategy without AC voltage sensors.The virtual grid flux is introduced to set up the mathematic model under d axis invirtual flux oriented control. The method of observation error compensation isalso introduced to eliminate the observation error. The feasibility of the controlscheme without AC voltage sensors is verified in the simulation.Because of the transformation and inverse transformation of coordinates inthe traditional PI control of PWM rectifier, the operation process is complicated.Therefore the proportional resonant control based on internal model controlprinciple in put forward. The grid current is regulated in the two-phase stationarycoordinate. So the phase-locked loop, the transformation of rotating coordinatesand the feed-forward decouple process is omitted. The control parameters for operation are designed based on the mathematic model of PWM rectifier and theeffect in the performance of the system by control parameters in the proportionalresonant controller. The availability of this controller is verified by simulation.Finally, the hardware experiment platform is introduced, and the modules ofparameters identification and control strategies and the software program aredesigned. The system is debugged in the DSP hardware experiment platform andthe control strategies for the PWM rectifier are implemented. |
