Study On Optimal Direct Torque Control Algorithm For Eight-switch Three-phase Inverter-fed Induction Motor Drive System

The multilevel converter is favored by many researchers because of its advantages,and the three-level NPC inverter is also the most widely used topology of the multilevel inverter.However,accompany with the advantages produced by the multilevel structures,the multiplied number of power switching devices reduces the reliability of the system.Therefore,in order to improve the reliability,it is of great value and significance to study the fault tolerant control technology.The eight-switch three-phase inverter(ESTPI),as the post-fault reconfigured topology for the three-level NPC inverter with the fault occurring in a leg,has the advantages of simple structure,less redundant devices and simultaneous reconfiguration of open or short circuit faults of one or more power switches in one leg.Therefore,in this paper,it takes the ESTPI-induction motor(IM)as the research object.On the basis of the fundamental principle of the direct torque control(DTC)algorithm,in order to suppress the dc-link capacitor voltages offset and reduce the torque ripple,the optimized DTC algorithm of the ESTPI-IM drive system is researched.Firstly,the topology,mathematical model and working principle of the ESTPI are studied.In order to lay the theoretical foundation of the full text,the mathematical model of induction motor,the rationale of the DTC algorithm and the control mechanism of the flux linkage and the torque are introduced.On this fundation,the influence of the ESTPI's basic voltage vectors on the variation trend of the flux linkage and the torque is analyzed in detail,and the DTC algorithm for ESTPI-IM drive system is explored.Secondly,in view of the problem that the DC-link two capacitor voltages are imbalance in the traditional DTC algorithm of the ESTPI-IM drive system,the fluctuation mechanism of the DC-link capacitor voltage is deeply studied,the adverse effects of the imbalance phenomenon on the normal operation of the system is analyzed,the formula of the neutral-point current is derived,and the relationship between the basic voltage vectors of the ESTPI and the DC-link capacitor voltages is revealed.By adding a hysteresis regulator for the DC-link capacitor voltages,and redesigning the switching table(ST)according to the output of these three hysteresis regulators for the flux linkage,the torque and the DC-link capacitor voltages,a DTC algorithm based on all of the basic voltage vectors is proposed to suppress the DC-link capacitor voltages offset,which can be named ST1-DTC.The validity of the proposed algorithm is verified by the Matlab simulation.Thirdly,for the problem of large torque ripple in the DTC system,the generation mechanism of the torque ripple is analyzed in depth.By deriving the formula of the variation rate of the torque,the specific factors that affecting the torque variation are obtained.Then the effects of the basic voltage vectors on the torque is quantitatively described,and the extent of the influence of the medium vector and the small vector on the torque variation is revealed.On the basis of the ST1-DTC algorithm,the switching-table is further improved.Therefore,an optimized DTC algorithm based on the small vectors and the zero vector is proposed to not only suppress the DC-link capacitor voltages offset but also reduce the torque ripple,which can be named ST2-DTC.The validity of the proposed algorithm is verified by the Matlab simulation.Finally,the hardware-in-the-loop(HIL)testing system based on DSP TMS320F28335 and RT-LAB is introduced.And the performances of the traditional DTC algorithm of the ESTPI-IM drive system and the ST1-DTC and ST2-DTC algorithm proposed in this paper are compared with the HIL test.It is proved that the ST1-DTC and ST2-DTC algorithms can suppress the offset of the DC-link capacitor voltages,and the effect of the ST2-DTC algorithm is a little better.At the same time,the ST2-DTC algorithm can also reduce the torque ripple effectively.