Research On Control Strategy Of Three-phase PWM Rectifier In Unbalanced Power Network

PWM rectifier,which can realize green conversion of electric energy,is widely used in industrial production because of its characteristics of bidirectional energy transmission,sinusoidal current on the grid side,unit power factor and so on.In this dissertation,the mathematical models of PWM rectifier under balanced and unbalanced power grids are established,and the design of related parameters is elaborated.When the three-phase PWM voltage source rectifiers are in unbalanced grid voltage faults in general,the input voltage can be equivalent to a sum of three phase voltage in positive,negative and zero,which requires complex coordinate transformation and loses control of both the grid side current and DC side voltage.Based on the extended instantaneous power,this dissertation propose two control strategies.(1)Firstly,the released energy and inductance absorption are inconsistencies in unit period of unbalanced power grid through the power analysis of rectifier under extended instantaneous power.Secondly,a method of output power in stationary coordinate system is proposed.The calculation of extended reactive power needs to calculate the delay,the traditional method is to delay 1/4 period,which is easy to introduce time delay into the system,which is improved in this dissertation.Thirdly,it is difficult to calculate the parameters of the PR controller of the current inner loop in practical production,so the quasi-PR controller and the PR controller are compared in this dissertation,and conclusion is drawn that the quasi-PR controller can realize the sinusoidal signal's static control at the resonant frequency.Therefore,quasi PR controller is used to replace the traditional PR controller.(2)Firstly,modeling and analysis of PWM Rectifier,get the predictive power control model in stationary coordinate system.Secondly,the vector expression of the control voltage is obtained,by making the active power and reactive power equal to the given reference value in the unit period.And deduces the power compensation that can meet the control requirements under the unbalanced network.The simulation model of the two methods is built on Matlab/Simulink,and the correctness and effectiveness of two methods are verified by compared and analyzed.