Caracterização Técnico-Econômica da Transmissão CC no Acionamento de Motores CA a Longas Distâncias

Abstract

Since the consolidation of the IGBT as the switching device in frequency converters, motor control techniques based on pulse width modulation (PWM) have achieved excellent performance in terms of efficiency, torque and speed control, as well as the quality of the synthesized voltage. On the other hand, the presence of transient overvoltages and high-frequency parasitic currents has become a significant issue, affecting the operational life of system components. Various methods to mitigate the adverse effects of these phenomena have been proposed in the literature, and some are typically adopted in commercial converters, such as the use of filters at the inverter output. In this context, an alternative solution has recently been proposed to reduce - or even eliminate - the phenomena associated with the propagation of PWM pulses through long cables that feed the motor. This approach involves a change in the converter topology, separating the rectifier and inverter units, placing the inverter near the motor, and thus restricting the circulation of these pulses to just a few meters. As a result, power transmission through the long cables is realized in direct current (DC), which also requires one less conductor compared to the traditional topology. This topology leads to a reduction in cable costs and electrical losses, while also addressing high-frequency-related issues. The main objective of this thesis is to shed light on the technical and economic aspects of DC transmission in long-distance AC motor drives, including both low and medium voltage systems. More specifically, the harmonic content present in the DC current resulting from the rectification process is characterized, including guidelines for sizing the DC-link filter. Accordingly, the ampacity of the DC cable is quantified under different filter percentages, with results that can be applied in the design of new drive systems. The contribution of the cable itself to the mitigation of DC current ripple is also evaluated. Finally, a comprehensive study (regarding power drive and distance) is carried out to estimate the savings in cable and electrical losses, with results presented in both percentage and monetary terms. These findings result from the comparison with the traditional topology and may justify the adoption of the DC transmission drive topology in new systems.