Next-Generation Submodule Technology for MMC

High power Modular Multilevel Converters (MMC) have become key components for many future application fields, particularly, the efficient use of Large Drives, solar and wind power resources and improvement of grid infrastructure (i.e.: multi-terminal HVDC- and MVDC-networks), mainly on account of:
  • Its scalability
  • No need for passive filters
  • Low di/dt of the arm currents
  • Low switching frequency and
  • Direct and fast control of the AC- and DC-side.

 

In previous projects, MMC have basically been equipped with Half-Bridge Submodules due to the minimum expenditure of semiconductors and therefore low losses. The disadvantage of Half-Bridge Submodules on the other hand is no possibility for electronic DC-current limitation at converter level (regarding unipolar terminal voltage) and high volume submodule capacitors.

 

Future requirements for MMC at system level are therefore the:

  • Reduction of submodule capacitor volume
  • Applying submodule topologies with bipolar terminal voltage to enable DC-current limitation and cut-off by the MMC itself (what results in the redundancy of DC circuit breakers) and
  • Enabling electronic protection of submodules against explosion

 

The future requirements should concurrently not lead to increased converter losses.

 

The Double-Zero Submodule (DZ-SM)

By introducing a controllable DC-capacitor – realized by a reverse conducting SiC-FET – a novel control scheme becomes possible, enabling reduced on-state losses and reduced switching losses. In addition, a double submodule circuit (DZ-DC-SM) can be realized by external connection of two single DZ-SM.

 

Submodule Technology for MMC I.png

Fig. 1: Double-Zero submodule (DZ-SM) and Double-Connection of two DZ-SM (DZ-DC-SM)

 

The features of this novel topology in Double-Connection are:

  • Reduced on-state losses
  • Reduced capacitor size
  • Lossless capacitor balancing
  • Electronic protection against explosion and it is
  • Well adapted for SiC-FET.

 

The most promising submodules are investigated, in order to analyze the individual on-state power loss of the different semiconductors:

  1. Series connection of two Full-Bridge submodules (FB-SM)
  2. Series connection of two Double-Zero submodules (DZ-SM)
  3. Double Zero submodule in double-connection (DZ-DC-SM).

 

The left bar graphs in B) and C) are depicting submodules as a mixture of Si-HV-IGBT and SiC-FET semiconductor devices as a next technological step, where a fully Si-HV-IGBT-version of the conventional FB-SM is utilized as a reference in A). By comparison, fully SiC-FET equipped submodules are presented in the right bar graphs.
It can be noted, that the on-state losses of the Double-Zero submodule (B)) can be significantly reduced, where the full potential of loss reduction will be achieved with a fully SiC-FET equipped submodule. In the case of the double-connection (C)), further 25% loss reduction can be achieved –cutting the total on-state loss in half compared to the conventional Full-Bridge submodule topology.

 

Submodule Technology for MMC II.png

Fig. 2: Normalized total power losses per submodule in a typ. HVDC-MMC application


 
Contact
 
Publications
[1] C. Dahmen and R. Marquardt, “Progress of High Power Multilevel Converters: Combining Silicon and Silicon Carbide,” PCIM Europe; Intern. Exhibition and Conference for Power Electronics, Intelligent Motion, Renewable Energy and Energy Management, Nuremberg, Germany, 2017.
 
[2] C. Dahmen, F. Kapaun, and R. Marquardt, “Analytical Investigation of Efficiency and Operating Range of Different Modular Multilevel Converters,” IEEE 12th Intern. Conf. on Power Electronics and Drive Systems (PEDS), Honolulu, HI, USA, 2017.
 
[3] C. Dahmen and R. Marquardt, “Charge Balancing for Advanced MMC-Double-Submodules with Ultra-Low Loss,” IEEE 13th Intern. Conf. on Compatibility, Power Electronics and Power Engineering (CPE-POWERENG), Sonderborg, Denmark, 2019.
 
[4] C. Dahmen and R. Marquardt, “Power Losses of Advanced MMC Submodule Topologies using Si-and SiC-Semiconductors,” 21st European Conf. on Power Electronics and Applications (EPE '19 ECCE Europe), Genova, Italy, 2019.
 
[5] C. Dahmen and R. Marquardt, “Reduced Capacitor Size and On-State Losses in Advanced MMC Submodule Topologies,” 22nd European Conf. on Power Electronics and Applications (EPE'20 ECCE Europe), Lyon, France, 2020.