Research On The Control Technology Of Three-Phase Four-Wire Diode-Clamped Three-Level Inverter

With the rapid development of science and technology in society nowadays,network systems,medical systems and some other systems propose higher requirements of power supply.UPS has become an important power security equipment in various industries.Due to the introduction of energy saving,emission reduction and green requirements,high frequency UPS has been more and more widely studied and applied because of its small size,light weight and low power consumption.Inverter,as an important part of high frequency UPS,will directly affect the performance of the whole high frequency UPS.High frequency UPS has many kinds of loads and complex working conditions.Three-phase unbalanced load and non-linear load will bring harmonic effects,resulting in distortion of the output of the inverter.Therefore,it is of great significance to study the control technology suitable for the post-stage three-level inverters of high frequency UPS.The three-phase four-wire diode-clamped three-level inverter topology is used in the post-stage inverters of high frequency UPS.This paper analyses the working principle of three-level inverters and establishes a mathematical model with switching function.According to the model in rotating coordinate system,the reason why the three-phase output can be controlled without static error is analyzed.Combining with the simplified equivalent circuit of the inverters,the reason why the four-wire inverters have stronger unbalanced load capacity than the three-wire inverters is analyzed.In the unbalanced load and the non-linear load,the changes of harmonic content before and after coordinate transformation are analyzed,which provides a theoretical basis for the design of the control mode.In this paper,the traditional SVPWM is introduced.When it is used in four-wire three-level inverters,the two-dimensional modulation in the αβ plane needs to be transformed into three-dimensional vector modulation based on the αβ0 axis.On this basis,a new three-dimensional modulation method is introduced,which can reduce the coordinate transformation and trigonometric function calculation required by traditional methods,and is easy to be realized digitally.The sampling quantity of three-phase output is directly based on abc axis for sector judgment,vector synthesis,action time calculation and vector allocation.Finally,the validity of the new three-dimensional modulation method is verified by the simulation model and s-function programming.In this paper,the PI control of voltage and current double loops is used to realize the three-phase output control without static error when the load is balanced.In the case of unbalanced load and non-linear load,the control mode is improved,and the voltage outer loop adopts repetitive control.A low-pass filter is used to suppress the resonant peak of the controlled object when the inverter is idle,and a leading link is used to compensate for the low-frequency phase lag caused by the low-pass filter.Because of the leading link depending on the accuracy of the controlled object model,under the premise of digital control,a bubble-sort method is proposed to select the optimal order of the leading link.Finally,the effectiveness of the above control methods is verified by simulation.Finally,a 6 kVA prototype is built based on digital control chip TMS320F2812 and IGBT power module.The new three-dimensional vector modulation mode is verified by experiments.The digital implementation is convenient,and the power transistor opening logic is consistent with the design.At the same time,the improved repetitive control method is used to verify the strong load carrying capacity of the designed inverter under unbalanced load and non-linear load conditions.