Research On Improved Control Method Of Vienna Rectifier Under Disturbed Voltage And Multi-vienna-rectifiers Analysis

Vienna Rectifier,as a three-phase three-level topology,its input current distortion is smaller,and the power factor of the grid is higher.At the same time,the stress of the switching transistors is also slighter.Consequently it attracts more and more attention of the researchers at home and abroad.Influenced by the power quality problems,such as harmonic,the conventional control method of Vienna rectifier doesn't have enough precision,the grid current usually has large distortion.Thus a improved control method is introduced to improve the current loop performance.With the development of DC power system,a single rectifier can't satisfy the requirement of the capacity.Therefore,multi-Vienna Rectifiers need to run in parallel.Scaled Vienna Rectifiers are easy to enlarge the capacity and realize.They also have higher reliability.Thus the paper aims at the three-phase four-wire Vienna Rectifier,and it puts emphasis on two main aspects: a new control method under disturbed power grid voltage and the multi-Vienna Rectifiers running.In this paper,an improved control method is established which is used in the three-phase-four-wire Vienna Rectifier.It aims at solving three main problems brought by the power grid: the back-ground harmonic?collapse voltage and unbalanced voltage.First of all,to solve the problem of power quality,it improves the reference signal of current loop to shield the disturbed signals.Then the grid voltage feed-forward method is introduced to deal with current zero-crossing distortion and disturbed voltage.Meanwhile considering the poorish current tracking performance and precision in the conventional PI control strategy,a novel control strategy combined PI control strategy with the repetitive control strategy is put forward.In the paper,it makes qualitative analysis on the improved control method,and it also does enough analysis and comparing about repetitive controller design.The superiority of the novel control method can be shown through contrast of two methods on harmonic suppression characteristics.The simulation and experimental results have proved the feasibility and effectiveness of the novel strategy.It has high tracking precision and it can also improve the wave quality of the input current effectively.When the distortion rate of grid current is reduced,the fluctuation of the output voltage is also slighter.So it can maintain perfect output characteristics.Two current-sharing method are introduced to make multi-Vienna rectifiers run in parallel: master-slave method and droop control method.First,the mathematical model of multi-Vienna rectifiers is built up.Then it analyzes the current sharing principle of two methods.And they are proved by the MATLAB Simulation model.At the same time,when the parameters of two drooping curves are different,there will be a strong power oscillation between two machines in the DC side.In the paper,it does some mathematical modeling and analysis about multi-Vienna Rectifiers and put forward two methods to inhibit the oscillation:(1)add a inductor in the DC side;(2)add a diode in the DC side.Both of two methods can inhibit the oscillation from the circuit principle.In the other way,it inhibits the circulating current.Finally the MATLAB simulation model is built.Through the contrast of load currents,the methods prove to be true.