Active front end converter based on MMC

Modular multilevel converter (MMC)

Modular Multilevel Converters (MMCs) are a type of power electronic converter used for medium- and high- voltage direct current convertion, consisting of a multitude (up to several hundreds) of series-connected sub-modules of relatively low voltage rating. This topology creates multiple output voltage levels, providing improved efficiency and voltage control in high-power electrical systems.

MMCs are a key technology in driving the energy transition of our society, providing improved efficiency, power quality, fault-tolerance and higher power density. The ability to operate in different voltage ranges makes such technology essential for the development of both AC and DC grids. By leveraging the capabilities of MMC technology, we can shape the energy systems for a sustainable future.  

Our 500 kVA - 9 levels modular multilevel converter (MMC)
Our 500 kVA - 9 levels modular multilevel converter (MMC)

MMC technology is crucial for medium-voltage range hybrid grids. Despite the requirement for a large number of power electronic devices, ensuring a highly reliable converter is essential to enable uninterrupted operation and reduced maintenance time. Fraunhofer ISIT offers the following benefits:

  • High efficiency step-down operation: enhance inter-connectivity to hybrid grids
  • DC fault ride-trough: ensure grid stability during faults 
  • Innovative overcurrent management to support AC grids
  • Well-equipped medium-voltage laboratory for tests up to 10 kV – 1 MVA with flexible multiple low-voltage and medium-voltage MMC setups.

Due to the modularity, MMC topologies can be a promising solution for Hybrid grids. However, the increased number of cells, balancing the cell voltages at pre-requisite level becomes a challenging task specially with hybrid grids where fast dynamics has to be taken care of. Additionally, with increased number of devices the probability of a device failure can increase which can affect the system operation or can even interrupt the continuity of the service. Hence, accurate thermal management of the MMC converter becomes extremely necessary for such applications.

At Fraunhofer ISIT, we develop hardware solutions for medium-voltage connected MMCs, including half bridge, full bridge, and hybrid structures. To ensure optimal performance and reliability, we also conduct research on thermal monitoring with minimal extra-hardware. This enables us to implement active thermal control and incorporate effective overcurrent management capabilities into the converter. Additionally, we also investigate advanced control techniques, in particular for simultaneous electrical and thermal balancing, and, at system level, for grid forming to enhance the converter’s integration with the power grid.  

Offshore Wind Farms

  • High power transfer capability
  • Low power losses for high distances
  • Independent control of active and reactive power


  • Fast dynamics for voltage regulation
  • Low harmonic distortion
  • High efficiency for hot-standby solutions

Fast Charging stations

  • High controllability
  • Integration of energy storage units
  • High power density and efficiency

MVDC Distribution

  • Improved connectivity
  • Increases power capacity by up to 80% compared to AC
  • Power flow control
  • High flexibility

  • Investigation of the external connection mechanism for measuring the voltage drop of power semiconductors during operation.
  • Evaluation of different modulations in the MMC and their effects on the DC-side current and on the wiring of the converter.
  • Investigation of protection strategies using the device's temperature inference based on its TSEP.
  • Development of a methodology that correlates device temperature and circulating current.