Análise da viabilidade e da confiabilidade de uma configuração alternativa de conversores de frequência para o acionamento de motores distantes e em ambientes e hostis

Abstract

In certain industrial applications, such as mining, steel industry, and oil extraction, the driving of motors through frequency converters involves large distances, and, due to the application of the PWM waveform in the long cable, several high-frequency problems are generated. Traditionally, these problems are reduced using filters installed at the output of the inverter bridge, with different types available for this purpose. However, despite the wide use of this type of solution, it does not represent the absolute solution to the problem, as the filters have drawbacks such as high weight, volume, and cost, in addition to electrical losses. Furthermore, topologies that operate in common mode and differential modes simultaneously are rare. In this context, a configuration of frequency converters aimed at drive systems for electrical machines powered by long cables was proposed in the literature, which, in addition to solving such high-frequency problems, presents additional advantages, such as reducing the voltage drop in the long cable and reducing the volume of copper required by the cable for power transmission. However, as in this proposal, the inverter bridge and part of the capacitor bank of the d.c. bus is connected directly to the motor terminals, which are normally found in more hostile environments, making it necessary for this configuration to be evaluated from the point of view of its feasibility and reliability, using a methodology that can bring reliable results (such as those based in the Physics of Failure (PoF)), to validate it for its commercial application. In this context, the main goal of this work is to develop a study comparing topologies and modulation strategies of multilevel converters under different aspects of feasibility and reliability, especially from the point of view of their applicability to d.c. transmission topology. In this sense, the dominant failure modes and mechanisms in frequency converter power devices were discussed, namely, silicon (Si) IGBTs modules, aluminum electrolytic capacitors (CEA), and metallized film capacitors (CFM), regarding temperature and humidity stressors. For IGBTs, concerning the evaluation of the thermomechanical state, the mechanism and failure mode of fatigue and bond wire lift-off, respectively, are considered dominant, and, about the humidity stressor, concerning the evaluation of the electrochemical state, the dominant failure mechanism is the metallic corrosion and the ions migration due to humidity and electric field. In the case of capacitors, for those made of metallized film, the dominant failure mechanism is the reduction of dielectric strength, and for aluminum electrolytic, there is vaporization of the electrolyte and the electrochemical reaction, due to the degradation of the oxide layer and drop in anode foil capacitance; both under wear failure mode. Some multilevel inverter topologies (classic and hybrid) were presented and discussed, their peculiarities and advantages and disadvantages were presented, and comparisons were made between them, in addition to a qualitative assessment of the influence of each of them on the reliability of the inverter as a whole. With this, a discussion was held regarding which topologies and strategies would present characteristics that would not be interesting for the system in the alternative configuration, which has banks of capacitors serving separately rectifiers and inverters, whose topologies and strategies were not considered in subsequent analyses. Still in this work, for a case study considering a winch system in an underground mine, the procedures, tools, and input data (mission profile), both environmental (ambient temperature) and system operation (winch operational variables) were presented, which were applied in the development of reliability and feasibility analyzes for the alternative configuration proposed in the literature. In this research, it was concluded that, in general, there was a better performance with metallized film capacitors, and more specifically with CFM10, CFM11, and CFM12 candidate capacitors, probably due to their lower values of R_th-ca and ESR, even though they present lower maximum hotspot temperatures than most of the other candidates. As for topologies, it is possible to conclude from the premises used in this case study that the topology 3L-ANPC applying the strategies PWM-1 and PWM-2 would be the most appropriate from the point of view of the reliability and feasibility analyzes carried out in this study. In any case, it is important to highlight that this work is a part of the analysis necessary to make the alternative configuration viable and that additional studies will still be necessary under several aspects, for example, from the point of view of the long cables that connect rectifiers and inverters. The results obtained and the methodology applied, which are mainly related to system reliability issues, will not only serve to evaluate the feasibility of this alternative frequency converter configuration proposed in the literature and aimed at driving distant machines, but will also provide important subsidies for any applications and studies related to the reliability of converters installed in hostile environments.