Estudo de técnicas de modulação para conversor multiníveis em sistemas com terminais abertos

Abstract

The spread of applications with high voltage and power levels, fine dynamic response and high power quality stimulate studies and advances in the field of multilevel converters. The progress of these complex structures is closely related to the advances and adaptations of modulation strategies. Due to the number of commanded semiconductor switches and the number of states that produce the same voltage level in these topologies, modulators have greater operating flexibility but also greater implementation difficulty. This paper presents a comparative study of the performance of some modulation techniques for a multilevel connection structure, in terms of loss and power quality. Using two independent inverters, a load with open terminals is fed, thus obtaining a system with multilevel voltage. This structure is called Open-End Windings system. For this topology, a modulation with centered space vectors is implemented, which, by adding an appropriate zero sequence signal to the phase references, gives an equivalent response to an optimized vector modulation, comparing the modulating waves to carries with the same phase. In addition to calculating the reference signals, a decoder based in finity state machine is applied for defining the drive switching sequence. This strategy is able to optimize the harmonic output content while keeping losses between devices balanced. Originally the finite state machine is powered by a modulated digital signal and needs to be processed at a frequency hundreds of times higher than the drive's switching frequency. To implement the decoder in a digital signal processor that has PWM peripheral, a modification of the algorithm is proposed, so that the same command signals are obtained with processing speed equivalent to the system sampling frequency. In addition, a generalized expandable algorithm for n-level flying capacitor converters based systems is developed. Experimental results for a three-level system, combined with simulations of three and more levels are presented in order to validate the proposed implementation and evaluate the performance of the modulation technique.