Research On The Control Strategy Of Rectifier Unit In AFE System

The power supply system of marine crane is usually converted from AC network to DC-link by uncontrolled rectifier,and then the AC required by asynchronous motor is generated by inverter.However,uncontrolled rectifier leads to large harmonic currents on the grid side,which seriously affects the power quality of marine AC network.In addition,the redundant energy generates by falling weights cannot be fed back to the grid side,resulting in a waste of energy.Because the three-phase voltage source PWM rectifier has low harmonic current,can operate with unit power factor and has the bidirectional energy flow,the active front end(AFE)rectifier unit with PWM rectifier technology as the core is gradually applied to the power supply system of marine crane.Firstly,aiming at the core of AFE rectifier unit,namely three-phase voltage source PWM rectifier,the working principle is studied,and the mathematical model is established.The main circuit parameters are designed on the basis.Then,this paper studies the direct current control strategy and direct power control strategy of AFE rectifier unit.With regard to direct current control,current and voltage double closed-loop control system based on CFPWM,SPWM and SVPWM are designed.The simulation model is built.Considering that frequent switching of working conditions and redundant energy generated by falling weights will lead to excessive fluctuation of DC-link voltage,the effective and fast suppression of DC-link voltage disturbance caused by sudden load change is studied.The load current feed-forward control method is introduced and improved.The results show that the improved direct current control strategy can reduce voltage overshoot and shorten regulation time on the premise of ensuring small harmonics and unit power factor.With regard to direct power control,the traditional direct power control system is designed.Aiming at the runaway area of reactive power,a three-level power hysteresis comparator is adopted.At the same time,the increasing or decreasing range of active power is controlled,the interval judger of the difference between actual active power and given value is added,and a new switching table is constructed.The simulation model is built.The results show that the new control method effectively eliminates the runaway area of reactive power,makes the active power control more precise,and improves the power factor.Next,according to the typical working conditions of the marine crane,the improved direct current control strategy and the improved direct power control strategy are simulated respectively,and compared with the uncontrolled rectifier.It provides a reference for effectiveand rapid suppressing the output disturbance of AFE rectifier unit of marine crane,and improving power factor and system response speed.Finally,the hardware experiment platform of three-phase voltage source PWM rectifier is built,and the hardware and software are debugged.The correctness of the control system and the feasibility of the control strategy are verified by the resistance load sudden change experiment.