IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 56, NO. 5, MAY2009 1665 A Dual Seven-Level Inverter Supply for an Open-End Winding Induction Motor Drive Gopal Mondal, Student Member, IEEE, K. Sivakumar, Student Member, IEEE, Rijil Ramchand, Student Member, IEEE, K. Gopakumar, Senior Member, IEEE, and Emil Levi, Senior Member, IEEE Abstract—This paper develops a seven-level inverter structure for open-end winding induction motor drives. The inverter supply is realized by cascading four two-level and two three-level neutral- point-clamped inverters. The inverter control is designed in such a way that the common-mode voltage (CMV) is eliminated. DC-link capacitor voltage balancing is also achieved by using only the switching-state redundancies. The proposed power circuit struc- ture is modular and therefore suitable for fault-tolerant applica- tions. By appropriately isolating some of the inverters, the drive can be operated during fault conditions in a five-level or a three- level inverter mode, with preserved CMV elimination and dc-link capacitor voltage balancing, within a reduced modulation range. Index Terms—Capacitor voltage balancing, common-mode volt- age (CMV) elimination, multilevel inverter, open-end winding induction motor. I. I NTRODUCTION M ULTILEVEL inverters are increasingly gaining impor- tance for industrial and utility applications due to their numerous inherent beneficial features [1]–[8]. In particular, multilevel inverters allow the operation at higher dc voltages using semiconductor switches connected in series and produce voltage waveforms with better harmonic profile than conven- tional two-level inverters. In general, a pulsewidth-modulation (PWM) inverter gen- erates high-frequency CMV at motor terminals, which results in unwanted leakage current. This unwanted common-mode current may cause premature bearing failure, as well as elec- tromagnetic interferences disturbing neighboring electronic de- vices. Hence, it is necessary to eliminate the CMV. Numerous techniques have been developed for reduction of the CMV in multilevel motor drives. A modified PWM strategy for CMV reduction can be realized by selecting particular inverter states that produce low or zero CMV [9]. Alternatively, a combination of the flat-top technique (keeping one leg of the inverter blocked in one state) and the double-commutation mechanism once per switching period can be used to minimize the switching losses Manuscript received April 21, 2008; revised November 6, 2008. First pub- lished November 25, 2008; current version published April 29, 2009. G. Mondal is with the School of Electrical and Electronic Engineering, The University of Nottingham, Nottingham, NG7 2RD, U.K. (e-mail: gopal. mondal@nottingham.ac.uk). K. Sivakumar, R. Ramchand, and K. Gopakumar are with the Cen- tre for Electronics Design and Technology, Indian Institute of Science, Bangalore 560012, India (e-mail: ksiva@cedt.iisc.ernet.in; rrijil@cedt.iisc. ernet.in; kgopa@cedt.iisc.ernet.in). E. Levi is with the Department of Electrical Engineering, Liverpool John Moores University, Liverpool L3 3AF, U.K. (e-mail: e.levi@ljmu.ac.uk). Digital Object Identifier 10.1109/TIE.2008.2010159 and CMV [10]. Various CMV reduction methods for the single- sided supply using multilevel inverters have been discussed in [6], [9]–[15]. If dual inverter supply is used in conjunction with an open- end winding induction motor, the number of switching-state redundancies per voltage vector increases, and it becomes easier to find the solution for CMV reduction/elimination. Open-end winding induction motor drives have been initially developed for traction applications using dual two-level inverter supply [2] and are nowadays extensively considered for various applications, such as electric vehicles (EVs) and hybrid EVs [16], [17], grid-connected embedded generation systems [18], and electric ship propulsion [19]. When dual inverter supply is used for an open-end winding induction motor, it becomes possible to eliminate the CMV by selecting only the switching states with zero common-mode voltage (CMV) [20]–[23]. A dual two-level inverter supply with CMV elimination has been elaborated in [21], while a dual five-level structure with CMV elimination and dc-link capacitor voltage balancing has been discussed in [22], [23]. A total of 48 switches were used for the power circuit of the dual five- level inverter configuration developed in [22], [23]. An alterna- tive version of the dual five-level inverter supply, with a switch count being reduced to 36, has been elaborated in [24]. By selecting opportune switching states, it was again possible to achieve CMV elimination. In this paper, a dual seven-level inverter structure for the open-end winding induction motor, with both CMV elimination and dc-link capacitor voltage balancing, is developed. The inverter structure is of the reduced switch count type, since it contains only 48 switches (the same as the full structure for dual five-level inverter system in [22], [23]). The dual seven- level power circuit is realized by exploiting the idea introduced in [24] for the dual five-level inverter structure. Four two-level inverters are shared by two supply systems of the open-end winding machine and are cascaded with two three-level neutral- point-clamped (NPC) inverters, one of which is connected at each side of the machine. The induction motor is thus fed from both sides, using two seven-level inverters. Such a power circuit is capable of generating a 13-level voltage vector structure. However, to eliminate the CMV by means of voltage vectors that yield zero CMV simultaneously at both sides of the stator winding, the voltage space vector structure has to be reduced to a seven-level voltage vector structure. Capacitor voltage balancing is achieved in addition to CMV elimination by selecting only the redundant switching states. 0278-0046/$25.00 © 2009 IEEE