Research On Modulation Strategy Of T Type Three-level Inverter Based On Variable Virtual Vector

With the rapid development of social economy and power electronics technology,inverter is more and more widely used in AC motor speed regulation system and other fields.With the increase of voltage and power level,multilevel technology has become the focus of inverter research.Compared with two-level inverter,multilevel inverter has the advantages of high conversion efficiency and small harmonic of voltage and current under the same conditions.At present,three-level inverter technology has been widely used.In this thesis,a T-type three-level inverter topology is selected,and the key problems such as the common-mode voltage(CMV),the simplified SVPWM algorithm and the mid-point voltage are studied.In order to overcome the disadvantage of complex implementation of classic SVPWM algorithm in FPGA,a SVPWM algorithm based on 60-degree coordinate system is proposed in this thesis.Through coordinate transformation and unitization,the process of sector judgment and the calculation of basic voltage vector acting time of SVPWM algorithm is greatly simplified.The validity of the proposed method is verified in Matlab simulation.Aiming at the common-mode voltage problem in the three-level inverter,the harm of the common-mode voltage is analyzed,and the influence of each basic voltage vector on the common-mode voltage is obtained by mathematical modeling.Then a common-mode voltage suppression algorithm is proposed.In order to solve the mid-point voltage balance problem,the influence of each basic voltage vector on the mid-point voltage is analyzed.In this thesis,a method to realize the mid-point voltage balance control by allocating the positive and negative small vector action time is introduced in detail.However,there is a large midpoint voltage fluctuation in this method,and the regulatied ability is limited under the condition of high modulating ratio.In order to solve these problems,a method of variable virtual medium vector is investigated,which can reduce the fluctuation of mid-point voltage by constructing virtual vector reasonably,and adjust the mid-point voltage by changing the size of virtual medium vector.The validity of the proposed method is verified in Matlab simulation.On the basis of the current research,an algorithm which can control the mid-point voltage in the range of full modulation ratio and reduce the common-mode voltage is proposed.By constructing the virtual vector reasonably,the common-mode voltage output of the inverter is suppressed and the fluctuation of the midpoint voltage is reduced.By adjusting the virtual small vector and the virtual medium vector variable at the same time,the mid-point voltage balance control in the full modulating ratio range is realized.The effectiveness of the proposed method is verified by simulations and experiments.