Modular Multilevel Converters: The Future of Hybrid AC/DC Grids

As the European energy landscape transitions toward a high penetration of renewable energy, the Modular Multilevel Converter (MMC) has emerged as the state-of-the-art technology for High-Voltage Direct Current (HVDC) systems and hybrid AC/DC grids.

Unlike traditional 2-level or 3-level Voltage Source Converters (VSCs), which use high-frequency pulse-width modulation to switch between a few voltage steps, an MMC is composed of hundreds of small power electronic building blocks called sub-modules. These sub-modules are stacked to create a nearly sinusoidal output voltage waveform with minimal harmonic distortion. This modularity offers significant advantages: it eliminates the need for bulky AC filters, reduces switching losses, and allows for easy scaling to very high voltage levels.

AC/DC converter based on MMC configuration

Advanced Control and the Energy Decoupling Advantage

The most critical difference between an MMC and a standard 2 or 3-level VSC lies in its internal dynamics. In a standard VSC, the DC-link capacitor is the only significant energy storage element. In contrast, an MMC contains a distributed set of capacitors within its sub-modules, which must be carefully managed.

While some control strategies in literature focus primarily on circulating current control to suppress internal current oscillations, the DAEDALOS project utilizes a comprehensive energy control approach. By explicitly controlling the energy stored within the converter arms, we manage the internal state of the converter independently from the power being exchanged with the grid.

MMC control scheme with energy control considered in DAEDALOS project