18-10-2010, 05:47 PM
CARRIER PWM ALGORITHM FOR MULTILEG MULTILEVEL INVERTER
Nguyen-Van Nho1 and Hong-Hee Lee2
1Faculty of EE, HCMUT, 268 Ly Thuong Kiet, District 10, Hochiminh City, Vietnam
2Department of EE, UOU, 680-749 San 29, Muger 2-dong, Ulsan, Korea
ABSTRACT
This paper presents a universal carrier based PWM method for multileg multilevel inverter on consideration of unbalanced dc voltage sources. Switching time diagram of PWM control in multilevel inverter can be transformed into a nominal switching two-level diagram. It will be shown the correlation between space vector PWM and carrier based PWM methods. As a result, space vector PWM and carrier based PWM methods are only two approaches for implementing defined switching state sequences in multileg multilevel inverters. The described generalized PWM approach for multileg multilevel inverters can be properly applied to different inverter topologies without difficulties. The proposed method is based on modifying of the modulating signals, depending on the dc-source variation. The additional common mode voltage can be designed for obtaining required PWM performance. Introduction Recently, multiphase inverters have drawn a great intention of specialists in several applications [1]. These are multiphase motor drives, which show be advantageous to three phase motor drive for reducing power demanding on each phase, having a better reliability in some applications and others. In literatures, multiphase motors are mainly supplied by two-level voltage source inverters. Three-phase four wire/leg inverters introduce another application of multiphase inverter. They have achieved a significant role in cleaning power utility. Their control theory and practical applications have been investigated for many years. The previous developed theory of PWM control can encourage to propose a universal PWM theory for multiphase multileg inverter with unbalanced dc voltage sources. The study can be useful and properly applied to individual inverter topologies. The three-phase inverter is just its particular case. Pulse Width Modulation techniques for multilevel inverters have been developed very intensively in recent years. Many carrier based and space vector PWM techniques for multilevel inverters have been properly deduced from that of two-level inverter. Someadvantages from space vector PWM can be listed as: - easy to implement using digital techniques - control flexibility because of available access to any selected switching state. - easily graphical explanation for balance three-phase load and balanced dc sources. In contrary, compared with carrier based PWM methods, the space vector PWM techniques have some disadvantages as below listed: - For high number of multilevel inverters (n>3), algorithms of space vector PWM techniques can not always be easily deduced. For multiphase inverter (m>3), the computating is so complicated. A study of 4- leg inverter has shown that the leg voltage is a unified quantity, consisting of active and zero sequence one. Therefore, a 3-D space vector method ( − − 0) with separate coordinates for zero sequence and active voltages can get a difficult [2]. Beside that, understanding the commutations in switching state sequence in 3-D vector diagrams is obviously not easy. For similar reasons, analysis of multiphase inverter (m>=5) usingcombined space vectors, would not be a convenient solution. - It can be concluded that description of reference voltages in multi-phase coordinate system (in multicarrier) presents graphically an effective way for analyzing inverter voltages and coresponding commutation process. The carrier based PWM approach can be beneficial for: • common mode voltage control and other modified PWM methods based on offset regulation. • Simple linear overmodulation using principle control between two limit trajectories. • Simple compensation of unbalanced dc voltages. • General PWM control, not depending on number of levels, number of phases, level of dc voltage unbalance and modulation modes (under-, over-modulation).
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