Research On The Neutral Voltage Balance For VIENNA Rectifier Based On SVPWM

With the rapid progress of science and technology and the great improvement of people's quality of life,the requirements for electrical equipment have become higher and higher,the demand for power levels has gradually increased,and the standards for harmonic contents have become increasingly strict.The three-level VIENNA rectifier is a three-level PFC circuit.The voltage stress of the power device is lower than the two-level topology,the power factor correction effect is good,and the harmonic content is low.In addition,the VIENNA topology does not need to set the dead time,so it has been widely used.First of all,this paper analyzes the energy flow paths of different current flows under the single-phase VIENNA topology.Due to the symmetry of the three-phase power grid voltage,by dividing the area of the grid-side voltage,taking one of the sections as an example,the working principle of the three-phase three-level VIENNA rectifier is analyzed.In addition,the VIENNA rectifier is established based on the state space equation,and the mathematical model in the three-phase stationary coordinate system and the two-phase synchronous rotating coordinate system is derived,and the corresponding coefficient matrix is derived.Secondly,the space vector distribution of VIENNA rectifier is listed.By comparing the traditional three-level SVPWM modulation method with the two-level space vector theory modulation algorithm,the modulation method based on the two-level space vector theory is selected,and the sector judgment formula,action time calculation formula and so on for this method are given in detail.This method is simpler than the traditional three-level SVPWM modulation method without trigonometric function calculation.Furthermore,the control system of the three-phase three-level VIENNA rectifier is designed.Aiming at the control objective of the system,to achieve the output voltage regulation,a dual PI controller with voltage outer loop and current inner loop is designed,and the reference design formula for PI parameters is given.Among them,due to the mutual coupling of dq axis currents,the current inner loop adopts the feedforward decoupling strategy.In order to realize the voltage equalization of the upper and lower capacitors on the output side,a midpoint potential balance controller based on fuzzy control is designed.By collecting the difference between the capacitor voltage and the variation of the error before and after the period,making decisions based on the established fuzzy rules,and sending them to the SVPWM module after defuzzificaton,dynamically adjusting the action time of the redundant small vector to achieve the balance of the midpoint potential.Finally,the simulation verifies the effect of fuzzy control on suppressing the midpoint potential shift.Finally,based on MATLAB / Simulink simulation platform,the simulation model of the three-phase three-level VIENNA rectifier is established,and detailed model of SVPWM,phase locked loop and other modules are given.The analysis of the simulation results verifies the correctness of the theory in this paper.An experimental prototype with a power of 8kW and the out voltage of 700 V was built.The commissioning and waveform analysis of the prototype verified the feasibility of parameter design.