722 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 19, NO. 3, MAY 2004 Limits of the Neutral-Point Balance in Back-to-Back-Connected Three-Level Converters Josep Pou, Member, IEEE, Rafael Pindado, Member, IEEE, Dushan Boroyevich, Member, IEEE, and Pedro Rodríguez, Member, IEEE Abstract—This paper explores the limits of neutral-point cur- rent control in back-to-back connected three-level converters. The theoretical analysis used is based on a mathematical neutral-point current model, which can be extended to apply to converters with higher numbers of levels. The low-frequency ripple, which appears in the neutral-point voltage for some operation conditions, can be removed for an extended operating area when two converters are connected back to back to the same dc bus. As a consequence, a lower voltage is applied to the devices, and the value of the capaci- tors can also be significantly reduced. Some practical graphics are given to represent the design according to the specifications of the application. Index Terms—Active front end, back-to-back connection, multi- level, neutral-point balance, space-vector modulation, three-level. I. INTRODUCTION Fig. 1 shows the topology of the three-level neu- tral-point-clamped (NPC) converter, which in recent years has attracted attention for its potential use in high-power applica- tions. To take full advantage of this converter, the voltages of the two series-connected dc-link capacitors must be confined to one half the level of the dc-link voltage. Several publications discuss ways to solve this balance problem [1]–[5]; however, this objective cannot be achieved when operating with a high modulation index and a low power-factor (PF) load is connected to it [6]. Under these conditions, the average value of the neutral-point (NP) current calculated over a modulation period cannot be maintained at zero, and thus appears as a low-frequency ripple in the NP potential. As a result of this oscillation, the output line-to-line voltages will also contain low-frequency harmonics, and the devices and capacitors themselves must additionally support voltages that are higher than those that occur when balance is achieved. Larger ca- pacitors can attenuate the amplitude of this oscillation, but as it increases the cost of the whole system this solution is best Manuscript received February 18, 2003; revised October 9, 2003. This work was supported by the Departament d’Universitats, Recerca i Societat de la In- formació of the Generalitat de Catalunya under Grant 2001BEAI200192, the Ministerio de Ciencia y Tecnologia of Spain under Project DPI2001–2212, and the ERC Shared Facilities supported by the National Science Foundation under Award EEC-9731677. Recommended by Associate Editor F. Blaabjerg. J. Pou and R. Pindado are with the Power Quality and Renewable Energy (QuPER) Research Group, Department of Electronic Engineering, Technical University of Catalonia, Terrassa, Catalonia, Spain (e-mail: pou@eel.upc.es). D. Boroyevich is with the Center for Power Electronics Systems (CPES), The Bradley Department of Electrical and Computer Engineering, Virginia Poly- technic Institute and State University, Blacksburg, VA 24061 USA. P. Rodríguez is with the Power Quality and Renewable Energy (QuPER) Re- search Group, Department of Electrical Engineering, Technical University of Catalonia, Terrassa, Catalonia, Spain. Digital Object Identifier 10.1109/TPEL.2004.826528 Fig. 1. Topology of the three-level NPC converter. Fig. 2. Space-vector diagram. avoided. Other approaches suggest allowing the NP voltage to oscillate and then compensating for the effects of the oscillation in the output voltages with a proper feedforward modulation [7]–[9]. Nevertheless, these proposals do not reduce the voltage applied to the devices, and some instability may appear when the system operates as a rectifier [8], [9]. In many applications, two NPC converters are connected back-to-back. Some examples can be found in bi-directional motor drive applications [10]–[13], as well as in high voltage dc (HVDC) link [14]. In those cases the task of balancing the NP voltage can be distributed between the converters. Hence, 0885-8993/04$20.00 © 2004 IEEE