“Design and control of multiport, fault tolerant, modular multilevel converter (M3C)”, Ref. MINECO-13-ENE2013-48727-C2-1-R

Funding institution: Ministry of Economy and Competitiveness, National Plan for Research - Energy.

Participating institutions: University of Oviedo.

Dates: 1/1/2014 – 31/12/2016

Project leader: Fernando Briz

Summary and objectives:

The European Network of Transmission System Operators for Electricity (ENTSO-E) envisions the growth of a large number of HVDC links to integrate the large wind power plants projected in the North Sea and other locations in Northern Europe. In the mid and long term, these links will become meshed. Projected HVDC grids will not only be used for wind power integration, but will also contribute to the integration of the different electric markets. HVDC grids are more efficient than their AC counterparts for long distances, have less environmental and visual impact and contribute to enhance the robustness and reliability of the AC systems they interconnect.

In spite of these advantages, the control of power flow in HVDC grids is still an important technical challenge. Generally, in a meshed network, there will not be enough degrees of freedom (converter stations) to allow setting all power flows independently. CIGRE has pointed out this issue as one of the main challenges to tackle in order to enable the widespread deployment of HVDC grids.

Interfacing of emerging alternative and renewable energy sources with existing power systems, as well as control of the power in the future HVDC grids, requires the use of electronic power converters, able both to adapt voltage levels and types (e.g. DC/AC), as well as to decouple the dynamics among sources, distribution systems and loads.

Multilevel power converters are a suitable solution to achieve the high voltage levels requested for such applications, using the currently available switching devices. While some multilevel topologies, like NPC, FC and CHB, can be considered mature technologies, however, they present limitations, e.g. limited number of levels for the NPC and FC topologies and the lack of a DC link for the case of the CHB.

The Modular Multilevel Converter is a promising alternative to the previous topologies. While it shares the modular structure of the CHB, and therefore the capability to serialize a large number of cells, it also provides a high voltage DC link, being therefore valid for its use in HVDC.

Despite of its appealing features, a number of issues regarding the design and control of the MMC remain to be solved, including: control and modulation strategies; cells configuration; passive elements sizing; fault tolerance, ride through and failure recovery; new topologies capable of power flow control in meshed HVDC grids.

The main goal of the project was the development of key technologies for the integration of large amounts of renewable energy into the electric transmission system and power flow control with a high level of efficiency and reliability. These technologies include the development of new MMC topologies and control strategies.

Specific objectives of the project are:


Two different topologies of modular multilevel converters were studied based on CHB and MMC topologies respectively. Both allow the interconnection of HV and LV AC ports trough high frequency transformers. The second also provides a HV DC port, making it suitable for HVDC applications.

For the same voltage in the AC port, and same power devices voltage ratings, the MMC structure will require four times the number of cells and twice the number of devices compared to the MCHB structure. This increase would be only justified if the HVDC port is fully exploited.

Both topologies use a Dual Active Bridge (DAB) for isolation. Use of SiC devices is advantageous for the DAB, as the increase in the switching frequency can enable significant reduction of transformer size. 

Control strategies have been developed for both structures, including operation in grid forming and grid feeding, and including the case when unbalances among cells occur.

One of the main drawbacks of MMC is current blocking in the even for a short circuit in the HV DC port. A fault tolerant cell structure has been proposed., which reduces the number of power devices and losses in normal operation with respect to the state of the art.

The proposed concepts have been tested on a downscaled prototype with a cell voltage of 200 V and DC voltage of 800 V respectively. SiC devices were used for the DAB.

Schematic representation of MMC with power transfer capability

Centralized control

Distributed control

 Detailed MMC cell and DAB, includingsensors and control signals to/from drivers. “Of” stands for optical fiber.

MMC cell

Dual Active Bridge

Front view of three-pase MMC

Side view of MMC and DAB cells

Fault tolerant cell

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