Abstract
The modular multilevel converter is capable to reach high-voltage levels with high flexibility, high reliability, and high power quality as it became the standard solution for high-power high-voltage applications that operate with fixed frequency. However, in machine-drive applications, the modular multilevel converter shows critical problems since an extremely high submodule-capacitor voltage ripple occurs in the machine start-up and at low-speed operation, which can damage the converter. Recently, a new converter solution named modular multilevel series converter was proposed as a promising alternative for high-power machine-drive applications since it presented many important structural and operational advantages in relation to the modular multilevel converter such as the reduced number of submodule capacitors and the low submodule-capacitor voltage ripple at low frequencies. Even though the modular multilevel series converter presented a reduced number of capacitors, the size of these capacitors was not analyzed. This paper presents a detailed comparison analysis of the performance of the modular multilevel converter and the modular multilevel series converter at variable-frequency operation, which is based on the proposed analytical description of the submodule-capacitor voltage ripple in such topologies. This analysis concludes that the new modular multilevel series converter can be designed with smaller capacitors in comparison to the modular multilevel converter if these converters are used to drive electrical machines that operate within a range of low-frequency values. In other words, the modular multilevel series converter experiences extremely low submodule-capacitor voltage ripple at very low frequencies, which means that this converter solution presents high performance in the electrical machine start-up and at low-speed operation.
Highlights
In order to keep up with the development of the modern industry, many high-power electrical applications have been emerging such as the high-voltage-direct-current (HVDC)transmission systems, upscaled wind turbines [1,2,3,4,5], pumped-hydro-storage systems [6,7,8]and heavy industrial motor drives
Let us consider that this machine driven by either the modular multilevel converter (MMC) or the modular multilevel series converter (MMSC) is connected to a grid with frequency equal to f i = 50 Hz
It is clear that the highest voltage-ripple value for the MMC case would occur for f o = 1 Hz while the highest voltage-ripple value for the MMSC case would occur for f o = 45 Hz
Summary
In order to keep up with the development of the modern industry, many high-power electrical applications have been emerging such as the high-voltage-direct-current (HVDC). One way to solve this problem is to build a MMC with bigger capacitors to limit the high voltage ripple at low frequencies but with the downside of increasing the cost, size and weight Another approach is to modulate an extra common-mode-voltage component in combination with the injection of circulating-current components [10,14] to reduce the MMC submodule-capacitor voltage ripple at low-speed operation. In this approach, huge overcurrents occur in the MMC arms due to the high peak values of the circulating-current components injected to compensate for the high submodule-capacitor voltage ripple in the machine start-up This control technique can avoid the need of bigger submodule capacitors but, due to the overcurrents in the MMC arms, semiconductor devices with higher current ratings might be required, which would result in higher costs. The analytical model proposed in this paper is a helpful tool that can be used in the design stage of the MMSC in order to define the size of the submodule capacitors for a given application with a given frequency range of operation
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