Multi-port Cascaded Multilevel Converter And Its Application For Motor Drive

Energy crisis and environmental pollution are becoming more and more serious,which has promoted the energy saving demand of industrial application and the rapid development of multi-port interconnection technology.Corresponding,multi-port converter appears.In recent years,the multi-port converter for the low voltage and small power application has been reserarched widely,but it remains to be explored in view of the high voltage high power applications.In real application,multiple motors are frequently required to cooperate with others.However,lots of discrete traditional converters are needed to drive the motors because which can be controlled freely only with the way of "one by one".So,based on the background of motor drive application,the multi-port cascaded multi-level converter and its control characteristics are discussed in this paper.The proposed converter,through module reuse and system integration,can realize the interconnection of multiple ports and the energy exchange among them,which can not only drive multi-motor in free operation modes,but also provide a new way for the regenerative energy utilization.In which,the regenerative energy of the motor in generating mode is supplied directly to the electric motor.Thus,the energy can be used effectively and the negative impact on the power grid will be reduced.The main research contents and the achievements of this paper are as follows.(1)This paper analyzes the application status of multi-port converters,the requirements of multi-motor,its drive technology and the methods of regenerative energy utilization.And then,the research topic is brought:build a high-voltage large-power multi-port converter with energy interaction capability,which can integrate multiple ports.Through the topology analysis of the existing multi-port topologis and their applicability,a kind of three-port cascaded multi-level converter topologies are proposed on the basis of hexagonal H-bridge cascaded multilevel converter.,which is suitable for the two-motor drive in the high-voltage and large power application scenarios.(2)The topology model is established in view of the first proposed topology:three port cascade multilevel converter without energy feedback capacity.And the parameters relation and power distribution law are analyzed in detail,and then the mechanism of energy flow is explored.Based on this,a control strategy for two-motor asynchronous drive is designed.Then the capacitance parameters are calculated according to the limit state based on the analysis of the generating mechanism of motor.Finally,the feasibility of topology and control strategy is verified by high and low voltage simulation.Though the topology,the regenerative energy can be directly utilized to improve energy efficiency when one motor operates in motoring mode and the other works in braking mode.So the capacity demand of capacitor is reduced,but the synchronous braking condition of the two motors is limited.(3)Based on the application limitation of the first topology,a hybrid three-port cascaded multilevel converter with the energy feedback capacity is proposed.In order to ensure the voltage stability of the capacitors in the passive branches,the circulating current control is introduced.The power balance condition and key control quantity are deduced by analyzing the power of each branch.Then,the model and quantity analysis of circulation circuit are obtained by means of the mean model analysis,and the control parameters are obtained.Based on the above analysis,a control strategy suitable for this topology is designed.Finally,the effectiveness of topology and circulation regulation is verified by simulation.Through the system,the regenerative energy can be used directly between the motors,and the surplus energy is fed back to the grid by the active branches,which not only improves the efficiency of energy utilization,but also reduces the number of full-controlled switches and reduces the pressure of the grid.At the same time,the number of secondary side windings of the phase-shift transformer and the switches is reduced because there is no front end in each cell of the passive branch.The system is suitable for all operation conditions.(4)To verify the feasibility of the proposed converter and its control strategy,a downscaled three-port multilevel converter cascaded with two units in each branch has been built in the laboratory.It used to drive two 380V/1.1kW AC induction motors.The asynchronous operation of two motors and the distribution of regenerative energy among the branches were tested on the experimental platform.The feasibility of the converter and the control method proposed and the validity of the theoretical analysis are verified.The simulation and experiment results show that the proposed system is able to realize the integration and energy interaction of discrete converters.In which,two motors can operate separately,and the mutual utilization of regenerative energy between motors is achieved.In the topology one,the surplus energy is stored in the capacitors temporarily,which reduces the capacitance demand.In the topology two,that is fed back to the grid through a small number of PWM cells.The two topologies can be chosen according to the operation conditions.This kind of converter,without significant increase of the cost,can meet the demand of multi-motor drive,in which the regenerative energy can be transferred and utilized between motors.It provides a new form for the interconnection of high-voltage and large-power multiple ports and also provides a new way of energy utilization for the energy-saving transformation of motor load.