Research On Key Technologies Of Three-phase Vienne Rectifiers

Nowadays,the generation,transmission and distribution of electric power are almost accomplished by AC system.However,many applications need to provide DC power under strictly regulated voltage,such as LED lights,electric vehicles,mobile phones,computers and servers,and other information and communication technology equipment.High-power load is usually supplied by three-phase AC system.Compared with single-phase AC system,threephase AC system can reduce the impact of wiring and avoid low-frequency power pulsation of single-phase AC system.In many applications,rectifiers are required to achieve near sinusoidal input current,phase with grid voltage,and avoid low frequency harmonic distortion caused by other devices connected to the grid.As an excellent rectifier topology,three-phase Vienna rectifier has the characteristics of high power factor,low harmonic content of input current,less switching devices,low switching stress and high reliability,so it provides important theoretical significance and engineering value for the control research of rectifier.On the basis of studying the mechanism of traditional model predictive control,an improved model predictive control algorithm based on optimal switching sequence is proposed.The algorithm can directly track AC variables without converting the control variables into twophase rotating DQ coordinates,thus simplifying the algorithm flow and avoiding the control deviation caused by inaccurate phase information in DQ coordinates.In addition,considering the introduction of the optimal switching sequence and the input variable of the controller with continuous operation time,the algorithm realizes three-phase Vienna integration.The current operates at a fixed switching frequency,which effectively reduces the difficulty of filter design.Considering the switching characteristics and constraints of three-phase Vienna rectifier,an optimal solution is proposed to solve the non-negative solution generated in the process of solving the optimal solution of model predictive control.The objective function is minimized by introducing control variables with circular level set,which improves the original control variables of the system.It can't realize the optimal control.The feasibility and superiority of the algorithm are analyzed under the verification of the experimental results.In order to reduce input current ripple and realize neutral-point voltage balance control,an improved space vector modulation strategy is proposed in this paper.Aiming at the problem of high switching loss of CPWM strategy,this paper reduces the total switching times in the switching cycle by adjusting the combination of switching states,thus realizing intermittent pulse width modulation and reducing switching loss,effectively improving the efficiency and power density of three-phase Vienna rectifier,and in high power applications,smaller switching loss can also improve the power converter.The service life of the device and the development cost of three-phase Vienna rectifier are reduced.In the intermittent pulse width modulation strategy,although deadbeat tracking of reference voltage vector can be achieved,in order to further reduce the input current ripple,this paper solves the switching mode which minimizes the input current ripple through the system model,thus realizing that the input current ripple can be reduced by changing the switching mode at different amplitudes and phases.It has also been verified by experiments.In addition,the traditional intermittent pulse width modulation strategy can not achieve deadbeat control of neutral-point voltage in a single switching cycle.Therefore,a hybrid modulation strategy is proposed in this paper,which enlarges the range of neutral-point voltage regulation in three-phase Vienna rectifier by using non-nearest threevector switching combinations.The experimental results show that the improved modulation strategy improves the three-phase Vienna rectifier.The ability to achieve neutral point voltage balance.When the power grid is normal,the three-phase Vienna rectifier using the above method has excellent steady-state and dynamic performance.When the grid voltage drops or is unbalanced,the amplitude and harmonic of the side current will increase rapidly,and the operation performance of the three-phase Vienna rectifier will also be affected.Therefore,this paper puts forward the direct power control strategy when the power grid is unbalanced.Aiming at the power analysis when the three-phase Vienna rectifier operates in the unbalanced state of the power grid,it puts forward the direct power control strategy to control the active power oscillation,the reactive power oscillation,the negative sequence current at the grid side and the voltage oscillation at the DC side.In addition,to solve the problem of the traditional direct power control strategy can not accurately track the fixed value,an improved direct power control strategy is proposed.At the same time,the problem of delay in digital control system is studied,and an improved compensation strategy is proposed to eliminate the adverse effects of digital control delay.Simulation and experimental results verify the feasibility and effectiveness of the improved algorithm.