Modular multilevel converters (MMC) inherent features are gaining more attention for dc voltage transmission systems. One of the main research paths regarding the converter performance deals with its voltage modulation. Specifically, for medium voltage applications with relatively small number of submodules, the voltage modulation techniques impact on the MMC performance needs to be studied. This work provides an extensive review of the carrier-based pulse with modulation (CB-PWM) techniques proposed to be applied on previous multilevel inverter versions. The CB-PWM methods were adapted to be compatible with an additional cell ranking and selection algorithm to ensure equal energy distribution on the arm cells. The state-of-the-art of zero sequence signals (ZSS) applied on three-phase inverters is also reviewed. The alliance between the ZSS with the CB-PWM, as well as the nearest level modulation (NLM), has an important impact on the MMC harmonic content, efficiency and voltage ripple of its cells capacitors. A 15 MW 28-cell-based MMC is used to investigate each particular combination between the modulation method and the common mode ZSS.Postprint (author's final draft

Multi-terminal high voltage dc (HVdc) grids can eventually became a feasible solution to transport energy to remote and/ or distant areas and its exploitation depend, among other things, on the performance of the converter terminals. Therefore, to optimize the power transmission strategy along such a grid, it is necessary to recognize the efficiency of all the converters in all points of operation, namely with the different load conditions. In this vision, the aim of this work is to provide the methodology to model the modular multilevel converter (MMC) efficiency by means of a mathematical expression that can describe, over a broad range of active and reactive power flow combinations, the power losses generated by the semiconductors. According to the presented methodology, a polynomial-based model with a reduced number of coefficients is deducted, in such a way that can be directly used for optimal power flow (OPF) studies. The accuracy of the proposed model is characterized by an absolute relative error, at the worst scenario, approximately equal to 3%.Postprint (author's final draft

In a futuristic vision, deeply interconnected ac and dc networks are expected, which would be restructured over the time in order to maximize the power transmission efficiency. In some grid configurations, grid-tied inverters can eventually become islanded from the dc grid by means of breakers and enhancing those converters with the static synchronous compensator (STATCOM) functionality can increase their value. In this context, there is an extra degree of freedom, which is the voltage drop that should be targeted between the dc poles of the converter. Due to the modular multilevel converter (MMC) flexibility, the converter energy storage and the average voltage generated in the converter arms can be managed to achieve some goals as the converter efficiency and ac power quality increase, conjointly with the voltage ripple reduction on the capacitor values. This work analyzes the referred degrees of freedom of the converter control and the correspondent impact on the converter operating performance.Postprint (published version

This work proposes a technique to inject a discontinuous zero sequence signal (D-ZSS) into voltage modulation of the modular multilevel converter arms. The core of the technique is the clamping of the arm voltages to a particular level and interval, in such a way that reduces the number of switching events on the instants that the load stress on the semiconductors is high, and consequently the dissipated energy. The clamping voltage and interval has some degrees of freedom which are also discussed. The description of the methodology is complemented with its simulation. Either in steady-state or in dynamic converter operation this technique is able to reduce the switching events and the correspondent total losses up to 6.6%. Hence contributing to a more efficient VSC-HVdc solution.Postprint (published version

The modular multilevel-based power converters (MMC) are very interesting solutions for high power transmission over large distances. The modularity of the converter is achieved by cascading individual and low voltage rated power converters in series, until the stack built can withstand the required dc voltage. Due to the large number of those individual components in real VSC-HVdc applications, the proper management of their states become a crucial agent when the converter efficiency is discussed. This work analyses several algorithms that are present in the literature, in what concerns the average switching frequency, magnitude of the capacitor's voltage ripple and complexity of the implementation. The converter simulation over a broad range of operating conditions is presented and discussed.Postprint (published version