Research On Modified Modulation Strategy And Control Method Of High Power PWM Current Source Converter-fed Motor Drive System

The use of medium voltage(MV)high-power(HP)adjustable-speed drives in the industrial field has effectively alleviated the impact of energy shortage and environmental pollution,and improved the energy utilization efficiency and economic benefits.Among them,the HP PWM current source converter(CSC)has become a widely used solution in the field of MV HP adjustable-speed drive because of its advantages of simple topology,reliable overcurrent and short-circuit protection capabilities,friendly output waveform to the motor and no transmission distance limitation.However,due to the limitation of the efficiency of DC energy storage link and the supply of power switching devices,the market share of CSC-fed drive solutions is still lower than that of voltage source motor drive.Moreover,the PWM CSC is currently mainly used in occasions with low requirements for dynamic response,such as fans and pumps,while it has not yet achieved commercial applications in such fields as wind power generation and HVDC transmission,which is both an opportunity and a challenge for domestic researchers and manufacturers.In order to solve the problems of large low-order harmonic content,inaccurate modeling and limited control performance that still existed in the practical application of HP PWM CSC-fed drive system,this paper studies the modified modulation and control strategy of HP PWM CSC-fed motor drive system to improve the operation performance of the system.The main research contents are as follows:The basic modulation strategy and control method of PWM CSC is firstly introduced,which provides a fundamental for the following research on modified modulation and control.At the same time,for the space vector modulation(SVM)strategy,a modified modulation strategy for the dynamic distribution of the overlap time is studied to eliminate the problems of the increase of the low-order harmonics and the decrease of the fundamental wave caused by the introduction of the overlap time,and improve the harmonic performance of the system.The problem that the dwell-time deviation of HP PWM CSC-fed drive system leads to the increase of low-order harmonic current at low switching frequency is studied.According to different goals of reducing dwell-time deviation,two modified dwell-time SVM strategies are proposed,which can eliminate the dwell-time deviation of SVM strategy at low switching frequency,reduce the low-order harmonics converter current and improve the harmonic performance of the system.The problems of modeling inaccuracy and control coupling of HP PWM CSC-fed drive system at low switching frequency are studied.Firstly,the complex vector model of the drive system in the discrete domain is established,which can accurately reflect the influence of the coupling effect on the system stability and dynamic performance.On the basis of this precise complex vector model,the stator current modified decoupling control method under low carrier wave ratio is studied to eliminate the coupling problem of control variables in the drive system.The proposed modified decoupling control can not only ensure the stability of the drive system,but also improve the dynamic response performance of the system.The discrete-time state-space modeling and modified speed control of HP PWM CSC-fed drive system at low switching frequency are studied.The mathematical model of CSC-fed drive system based on state space equation in discrete domain is established to reflect the multi-input and multi-output(MIMO)characteristics and the discrete characteristics of CSC-fed motor drive system model at low switching frequency.Based on this model,the open-loop transfer function of the speed control loop is derived,and the relevant parameters of the speed controller are analyzed and optimized according to the change of the root locus of the open-loop transfer function.The optimized parameters designed can improve the dynamic response performance of the speed while ensuring the stability of the drive system.For higher power applications,a drive system topology of cascaded CSCs combined with multiphase motor and its optimal modulation and control are studied.Taking advantage of the flexible arrangement of the modulation sequence of the modular CSC in the drive system,a coordinated SVM strategy of both sides is proposed,which can reduce the current ripple of the DC bus.A fault-tolerant control method for the drive system in the case of motor single-phase winding open-circuit fault is proposed by taking advantage of the fault tolerance of the cascade converter and multiphase motor windings redundancy in the drive system,which can achieve stable operation under system fault.The proposed modulation strategy and fault tolerant control method can improve the performance and reliability of cascaded CSCs-fed multiphase motor drive system.