3846 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 6, NOVEMBER/DECEMBER 2014 Six-Phase Machine Conversion System With Three- and Single-Phase Series Converters Cursino Brandão Jacobina, Fellow, IEEE, Victor Felipe Moura Bezerra Melo, Member, IEEE, Nady Rocha, Member, IEEE, and Edison Roberto Cabral da Silva, Fellow, IEEE Abstract—This paper presents two conversion systems based on the association of a six-phase machine (SPM) with an elec- trical grid by means of three- and single-phase converters. The machine is composed of two three-phase groups. One of them provides the power to the three-phase converter, and the other provides the power to the single-phase converters, in an open-end winding connection (see Fig. 2). The first conversion system is composed of bidirectional switches and can be used with induction or permanent-magnet synchronous machines. The second system is partially reversible since the single-phase converters are also composed of diodes and is more indicated for permanent-magnet synchronous machines. Furthermore, the proposed configurations do not use transformers and generate multilevel voltage at the grid-side converter with amplitude higher than that of the ma- chine side, permitting one to reduce the transmission I 2 R losses. The complete analysis of the systems, including the pulsewidth– modulated techniques, is presented. Simulation and experimental results are also presented. Index Terms—Series-connected converters, six-phase machine, static power converters. I. I NTRODUCTION N OWADAYS, multiphase machines are used for high- power applications because of their inherent advantages [1]–[4], such as 1) reduced current processed by each leg of the inverter, 2) lower torque pulsations, 3) reduced rotor harmonic currents for induction motor drives for the same machine volume, 4) reduced harmonic content of the dc-link current, 5) fault tolerance capability, and 6) the possibility of eliminating the common-mode voltage. Manuscript received December 5, 2013; revised February 28, 2014; accepted April 13, 2014. Date of publication May 14, 2014; date of current version November 18, 2014. Paper 2013-IDC-0982.R1, presented at 2013 IEEE Energy Conversion Congress and Exposition, Denver, CO, USA, September 16–20, and approved for publication in the IEEE TRANSACTIONS ON I NDUSTRY APPLICATIONS by the Industrial Drives Committee of the IEEE Industry Applications Society. This work was supported in part by the Conselho Na- cional de Desenvolvimento Cientifico e Tecnologico (CNPq) and in part by the Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES). C. B. Jacobina is with the Department of Electrical Engineering, Federal University of Campina Grande (UFCG), Campina Grande, Brazil (e-mail: jacobina@dee.ufcg.edu.br). V. F. M. B. Melo is with the Post-Graduate Program in Electrical Engineering (PPgEE)-COPELE, Federal University of Campina Grande (UFCG), Campina Grande, Brazil (e-mail: victor_mbmelo@hotmail.com). N. Rocha is with the Federal University of Paraíba (UFPB), 58059-900 João Pessoa, Brazil (e-mail: nadyrocha@cear.ufpb.br). E. R. C. da Silva is with the Federal University of Campina Grande (UFCG), Campina Grande, Brazil, and also with the Federal University of Paraiba, João Pessoa, Brazil (e-mail: ercdasilva@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/TIA.2014.2323479 On the other hand, multilevel converters, as compared with two-level ones, present several advantages such as the enhanced quality of the output voltage at low switching frequencies, low electromagnetic interference, low-voltage stress on semicon- ductor switches, reduced voltage derivative, reduced common- mode voltage, higher efficiency, etc. [5]. Three types of multilevel converters are considered to be conventional. They are the neutral-point-clamped (NPC) converter [6]–[8], the flying-capacitor (FC) converter [7], [8], and the cascaded H-bridge (CHB) converter [7], [8]. The NPC converter is an attractive high-voltage multilevel converter because of its robustness. However, it requires a large number of clamping diodes when the number of voltage steps is high. In addition, NPC converters present a dc-link capacitor voltage balancing problem [9], [10]. The FC converter is similar to the NPC converter but makes use of many capacitors for voltage clamping. The dc-link voltage balancing is simpler than that of the NPC converter. However, it requires a complex control strategy to the floating capacitor voltages [9], [10]. On the other hand, the CHB converter does not make use of any clamping diodes or capacitors. It achieves high voltage by cascading standard low-voltage H-bridge modules. The advan- tage of this structure is its high modularity degree that allows a module to be quickly and easily replaced in case of defect. In addition, it requires a smaller number of components to achieve the same number of levels (particularly when dc-link voltages are different) when compared with other multilevel converters [9]–[11]. Then, due to its advantages, the CHB converter has been adopted in high-power wind energy converter systems based on permanent-magnet synchronous generator and motor drive applications [9], [10], [12]. Based on CHB converters, new cascaded converters have been proposed in the literature [5], [13]–[16]. In fact, series-connected converters allow the reduction of voltages across switching devices since the dc-link voltages present lower values than the one used in the conventional two-level topology. Consequently, the stress on these devices is reduced. Moreover, it also allows reducing the harmonic content of voltages and currents. That is, compared with their conventional two-level counterpart [see Fig. 1(a)], cascaded configurations permit decreasing the values of voltages and power processed by the converter switches and also reducing the harmonic distortion at the grid side. Since back-to-back converters must be used to transfer the energy from the generation source to the grid or to the load, multilevel converters are applied as the parts of the back-to- back power converter [17]. When cascaded or dual structures 0093-9994 © 2014 IEEE. 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