0278-0046 (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIE.2018.2868014, IEEE Transactions on Industrial Electronics IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS 1 Low Switching Frequency Model Predictive Control of Three-Level Inverter-Fed IM Drives with Speed Sensorless and Field-Weakening Operation Yongchang Zhang, Member, IEEE, Yuning Bai, Haitao Yang, Student Member, IEEE, and Boyue Zhang Abstract—This paper proposes a single-vector-based model predictive control (MPC) for induction motor (IM) drive supplied by a three-level neutral-point-clamped inverter operating at low switching frequency. Different from the torque and flux control in conventional MPC, the proposed MPC tries to minimize the error between the applied voltage vector and a reference voltage vector obtained based on the principle of deadbeat control, hence reducing the number of weighting factor in the cost function. To reduce the computational burden and restrict the high jumps in both phase and line voltages, two switching tables are proposed and compared for the pre-selection of candidate voltage vectors. The neutral point potential fluctuation and switching frequency are also included in the cost function to achieve the balance between the upper and lower dc voltages and relatively low switching frequency. Furthermore, a speed adaptive stator flux observer with novel gain matrix is proposed to achieve speed sensorless operation, which has higher speed and flux estimation accuracy than conventional fixed gains. Finally, the proposed MPC is extended to field-weakening operation by adjusting the torque and stator flux reference online, which significantly widens the speed range and improves the practical value of MPC. The effectiveness of the proposed method is confirmed by the presented experimental results obtained at average switching frequency of less than 600 Hz. Index Terms—Induction motor drives, predictive control, speed-sensorless, field-weakening I. I NTRODUCTION T HE three-level neutral-point-clamped (3L-NPC) topology is more suitable for high power medium voltage drives than two-level inverter owing to its lower voltage stress across semiconductor devices and less harmonic distortion in ac side [1], [2]. For high power converters, it is essential to maintain low switching frequency to minimize the switching losses while increasing the utilization of power capability for medium Manuscript received January 23, 2018; revised July 4, 2018; accepted August 10, 2018. This work was supported in part by the National Natural Science Foundation of China under Grant 51577003, and in part by Beijing Natural Science Foundation under grant 3162012. (Corresponding author: Haitao Yang). Y. Zhang, Y. Bai and B. Zhang are with Inverter Technologies Engi- neering Research Center of Beijing, North China University of Technology, Beijing, 100144, China. (email: yozhang@ieee.org; whyorwhnt@qq.com; lgddlzby@163.com). H. Yang is with Inverter Technologies Engineering Research Center of Beijing, North China University of Technology, Beijing 100144, China, and also with the Faculty of Engineering and Information Technology, University of Technology Sydney, Ultimo, NSW 2007, Australia (email: yhtseaky@gmail.com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2018.××××××× voltage inverters [3]. For 3L-NPC inverter, arbitrary switch between voltage vectors in practical industrial applications is not allowed for the aim of preventing the inverter from being damaged or producing higher harmonics [1]. Hence, a smooth vector switching strategy is required. Furthermore, the neutral point potential fluctuation should also be considered, otherwise the lifetime of capacitors in the upper and lower dc link may be influenced and the power quality of output voltages and current is deteriorated. Compared to two-level converter, the 3L-NPC converter has to consider the limitation caused by the topology itself while satisfying the requirements of torque and flux control in motor drives, hence the control complexity is significantly increased. Vector control (VC) and direct torque control (DTC) are two most popular control methods of motor drives [4]. In VC, the issues of neutral point balance and high voltage jumps for 3L-NPC inverter-fed motor drives can be solved in the modulation stage, hence decoupling the high-level toque/flux control from the low-level modulation. However, when the switching frequency is well below 1 kHz, the harmonic content of conventional space vector modulation (SVM) increases quickly. Using optimal modulation like selective harmonic elimination PWM (SHEPWM) can improve the harmonic performance [5], [6], but it is difficult to be used in closed- loop control system for achieving high dynamics [7], [8]. DTC considers the inverter and the machine as a whole system and it directly selects the desired voltage vector from a predefined switching table according to the torque and flux error signs. Due to the use of hysteresis comparators and heuristic switch- ing table, it presents high torque ripples and variable switching frequency. In [1], two kinds of modified DTC schemes based on improved switching tables are proposed, which not only achieves high performance torque and flux control but also considers the limitations caused by the 3L-NPC topology. However, the control complexity is increased. Furthermore, the average switching frequency is still relatively high, which is unfavorable for high power applications. MPC has recently emerged as a powerful control method for high performance closed-loop control of power converters and motor drives, especially in the applications requiring low switching frequency [9]. To apply MPC to 3L-NPC, the neutral point voltage balance and average switching frequency reduction should be taken into account. In [10], a finite- state predictive torque control (PTC) is proposed for 3L-NPC inverter-fed IM drives, which achieves good steady state and dynamic performance with the consideration of neutral point