Flexible Switching Of Motor Power Supply Between The Converter And The Power Grid

In recent years,high power motor motors play more and more important role in important industry such as electric power,metallurgy,coal and so on.It is very common for the motor power supply to switch between the inverter and the power grid.However,severe shock is caused by improper switching of power supply for motor,which will have a serious impact on the power supply system and the drive system.Therefore,it is of great value and significance that this paper focus on the switching between converter and grid.For the switching system of motor power supply between converter and grid,this paper focus on the research of the DPC-based flexible switching control technique to eliminate current shock and overvoltage caused by chopping.Specific contents are shown as follows:Firstly,the working principle of H-bridge cascade multilevel converter is analyzed in detail and its mathematical model in d-q coordinate systems is established.The working principle of carrier phase-shift pulse width modulation technique with higher switching frequency and lower harmonic is analyzed.On the basis of the working principle and mathematical model of motor,constant volts/hertz soft-starting control scheme is derived.Secondly,the working principles of direct switching,asynchronous switching,and synchronous switching are introduced.The generating mechanism of the current shock and overvoltage under traditional switching control schemes are analyzed and switching conditions without shock is summarized.Moreover,the direct switching,asynchronous switching,and synchronous switching are verified by simulation.The results show that under the direct switching and asynchronous switching,the current and voltage shock of the converter and the power grid,and the stator current shock of the motor are all severe.Under synchronous switching,the current and voltage of the converter and the power grid change abruptly,then the chopping causes overvoltage.The overvoltage damages the inverter,power grid and motor.Thirdly,in order to eliminate the current and overvoltage shock,a DPC-based flexible switching control technique is presented and also applied to switching motor power supply between converter and grid.The direct power control strategy is adopted to control the proportion of the power supplied by the converter.The converter and grid output currents change gradually tracking the active power reference and reactive power reference,avoiding the overvoltage caused by chopping and the impact on grid,converter and motor.The five stages and the equivalent circuit when motor is fed by power switching from converter to the 50 Hz grid is represented.The mathematical model of direct power control scheme is established on the basis of instantaneous power.Moreover,in order to solve the problem of phase switching,progressive phase are proposed.Finally,a simulation model consist of the H-bridge cascade multilevel converter model,motor model and power grid model based on Matlab/Simulink is established.The model is composed of the H-bridge cascade multilevel converter model,motor model and power grid model.The process of the motor power switches from converter to grid is simulated.The effectiveness of the improved flexible switching control scheme is verified by simulation.Moreover,a experiment is designed to verify the the performance of the proposed flexible switching control scheme.