| Four-quadrant converters are often used in energy-feedback applications such as oil pumping units,elevators,and elevators because of the advantages of bidirectional energy flow,low grid-side current harmonics,high power factor,and controllable DC bus voltage.However,the traditional four-quadrant inverter still has problems such as poor mechanical characteristics of low-frequency under constant voltage-frequency ratio control,irreversible periodic disturbances on the rectifier side,large bus voltage fluctuations during sudden load changes under independent control of rectifier and inverter,and the system stability will be affected by the circulating current generated by the paralleled inverters.Aiming at the above problems in the four-quadrant inverter system,this thesis conducts research from the aspects of enhancing the mechanical characteristics of the motor at low frequencies,reducing the input current harmonics on the rectifier side,reducing bus voltage fluctuations,and reducing the inverter parallel circulating current.Aiming at the inverter,the asynchronous motor is taken as the control object,the steady state model of the asynchronous motor is analyzed and the performance advantages and disadvantages of the variable-frequency variable-frequency speed-regulating system with constant voltage-frequency ratio control are discussed,and the low frequency voltage compensation based on the stator voltage vector is studied.The scheme has obtained a good voltage compensation effect and achieved the safe start and stable operation of the asynchronous motor.For the rectifier,because traditional dual closed-loop PI controllers cannot track periodic disturbance signals,a compound control strategy combining repetitive control and PI is studied based on actual application conditions.According to this research,a scheme using PI controller model instead of traditional repetitive control compensator is proposed,which can further reduce the low-frequency harmonics of the input current on the rectifier side.For a four-quadrant converter,in order to reduce the DC bus voltage fluctuations,the control schemes of load current and capacitor current feedforward are studied,which can make the rectifier track the load power change in time,and use the calculated current value to replace sensor measurements to reduce material and installation costs for current sensors.Aiming at the circulating current problem caused by the parallel connection of converters,the causes and effects of the parallel circulating current are analyzed in detail,and a mathematical model of the magnitude of the parallel circulating current is established.It is deduced that the magnitude of the fundamental circulating current is related to the amplitude difference and phase difference of the in-phase voltage of the inverter.Based on this,a scheme of connecting the coupled inductor with the parallelled output of the inverter is proposed,which greatly reduces the circulating current while hardly affects the output characteristics of the inverter,and this coupled inductor considering the magnetic saturation and inductance requirements is designed.In this thesis,the low-frequency voltage compensation strategy for asynchronous motors,the new type compensator composite control of rectifier,two types of current feedforward coordinated control,and circulating current suppression based on the converters with coupled inductor are simulated.Based on this,a back-to-back four-quadrant converter with a power of 15 k W is used as an experimental platform to complete the performance comparison and circulating current suppression test of the inverter under different working modes.Simulation and experimental results prove the effectiveness and feasibility of the research work in this thesis. |
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