Avaliação e implementação de técnicas de modulação PWM para aplicação em conversor NPC/PONTE H de cinco níveis

Abstract

With the continued industrial growth, the demand for electricity grows at the same pace. In view of this, the use of high power and medium voltage multilevel converters, to meet industrial loads, is increasingly present in view of the advantages that multilevel converters present. The advantages include the minimization of voltage waveform THD and the minimization of EMI (electromagnetic interference) effects. However, even though they are raising in medium voltage applications, the control of these converters has still been a challenge: the number of measured/control variables and command signals is higher in multilevel applications, which makes the systems more complex. Pulse-width modulation techniques, PWM, applied in the command of the switches, aim to reduce impacts such as common mode voltages, reduce switching losses, and minimize the THD on the network and load sides, among other problems. All of these parameters can impact the choice of hardware used in the converter’s driver. For this reason, the correct choice of PWM modulation technique must be made to ensure proper operation of the multilevel converter. In this work the main techniques of pulse width modulation, scalar and vector PWM are compared from the point of view of these parameters. The topology chosen for the study of modulation techniques is the three-phase five-level H-bridge NPC converter. This choice is based in the fact that the converter has considerable benefits over other multilevel topologies present in the literature - such as lower dv / dt and THD, when compared to the traditional three-level NPC converter, or the lower assembly complexity and simplification of command signals when compared to the five-level NPC converter. The implementation and simulation of scalar modulation techniques are performed in Matlab / Simulink software, and the modulators are analysed through the simulation results obtained. The simulations allow us to conclude that the unipolar PD-PWM and SV-PD-PWM unipolar scalar modulation techniques are better suited to the application of electric drives studied in this work. The paper also presents the classical mathematical synthesis of a five-level vector modulator, discussing the mathematical complexity imposed by the five-level H-bridge NPC converter feature, which has redundant states. Finally, this study also presents a proposal to adapt the unipolar PD-PWM scalar modulator, aiming at the DC bus voltage balance. An intrinsic feature of multilevel topologies is the need to regulate busbar capacitor voltages, which justifies the proposed adaptation of the unipolar PD-PWM modulator in the application of electric drives studied. The adapted modulator is simulated and the voltage waveforms of the load current, capacitor voltage, CMV and THD are presented.