Technical Report No.1374

No. 1374

Technical Trends in New-Type Voltage Source AC/DC Converters for Power Systems
- with Special Focuses on Modular Multilevel Converter (MMC) -

Investigating R&D Committee on New Voltage Source AC/DC Converters for Power Systems

Keyword :

modular multilevel converter, MMC, AC/DC converter, HVDC, power system

In Japan, after the Great East Japan Earthquake, reinforcement and further utilization of power system tie-lines have been discussed. There are growing interests in application of voltage source converters (VSCs) for dc interconnection between power grids. In Europe, VSCs have been applied for dc links between off-shore wind farms and main land systems. In the United States, VSCs have been applied for a dc link for bulk power transfer to a load center between weak power networks.
Compared to line-commutated converters, the VSC has advantages involving less number of constraints for operation with power systems, lower harmonic emission levels and reactive power controllability. The VSC, however, has disadvantages such as much power losses while operation, and longer restart time after dc faults because of fast decrease in dc capacitor voltages.
New type VSCs called modular multilevel converters (MMCs) are being in progress recently. The MMCs with high voltage and high power ratings are being developed, and being in service in power systems. The MMC has multiple converter units called cells in cascade connected. DC capacitors are divided in each cell and all cells show different operating behaviors. It is expected that the MMC has less power dissipations and suppress decrease in dc capacitor voltages after dc faults.
This technical report investigates circuits hardware and control systems of new type VSCs, with special focuses on MMCs. Operating principles, examples of practical MMC facilities and careful considerations required for the MMC used in power systems are summarized systematically. The report offers useful information for engineers and researchers who are engaged in planning, design, simulation analysis, manufacturing and operation of ac/dc converters for power networks.