High voltage direct current (HVDC) transmission


Modern transmission grids are predominantly based on high voltage alternating current (HVAC) scheme due to the superior performance and low cost of AC generators and transformers. However, high voltage direct current (HVDC) transmission scheme also has some distinct advantages, such as 1) lower cost for long distance bulk power transmission, especially when utilizing cables, 2) interconnecting two asynchronous power systems, 3) better use of right-of-way, 4) AC system support and etc.

On the other hand, HVDC lines are embedded in HVAC grids and require power electronics converters and other associated station equipment, including filters, communications and special transformers. The high cost of converter stations makes the HVDC a niche, albeit important technology in today’s transmission grid. With new advances in power electronics, the growth of renewable energy resources in remote and offshore locations, and concerns on environmental impacts, HVDC is getting more attentions and expected to play a more significant role in future power grids.


Basic structure of modular multilevel converter (MMC)

Two types of HVDC converters are used, the current source converters (CSC) and the voltage source converters (VSC). VSC-HVDC has the advantages of low harmonics, black start capability and independent active and reactive power control, which make it suitable for the growing renewable energy resources integration applications, such as offshore wind farm. The state-of-the-art VSC-HVDC converter topology is the modular multilevel converter (MMC), as shown in the figure. MMC exhibits many advantages, such as no direct series of power switches due to the modular structure with a series connection of power electronics building blocks (i.e. submodules), and high efficiency and low harmonics due to the multilevel structure.

However, MMC requires large submodule capacitors due to the single-phase nature of the submodules and low switching frequency. It also has an inherently uneven power loss distribution between the two devices in the submodule, which may lead to overdesign of the cooling system. These shortcomings of MMC may hinder their applications where size and weight are important, such as for offshore wind.

This project aims to increase the MMC power density, by proposing methods to reduce the capacitor voltage ripples and deal with the uneven power loss distribution in submodules.

How WBG Can Help

WBG devices offer much lower losses (on-state and switching) than their Si counterparts. Using WBG devices can directly improve the efficiency and reduce the cooling requirement. Furthermore, it provides MMC much fast switching capability without compromising efficiency. Considering the capacitor voltage balancing control in MMC is highly dependent on switching frequency, it is conceivable that faster switching can result in lower ripple voltage and reduced capacitor need.

Personnel Involved

  • Shuoting Zhang
  • Siqi Ji
  • Xiaojie Shi
  • Yalong Li