Over the recent years, the number of installed voltage source converter (VSC)-based high-voltage direct current (HVDC) projects has increased with numerous ongoing and planned, many systems above 1 GW. Alongside, VSC-based multiterminal DC (MTDC) have also been developed. The modular multilevel converter (MMC) has become the prime topology for MTDC systems. Nanao and Zhoushan, currently operating in China, are the two pioneering MTDC systems based on MMCs featuring three and five terminals. The typical half-bridge submodules-based MMC suffer from lack of dc-fault blocking capability requiring costly dc-breakers. The application of commercial dc-breakers in the Zhoushan system is a major step towards fully functional dc-grids. Alternatively, dc-fault tolerant modular VSC topologies can also be utilized in order to improve the dc-fault handling capability of future dc-grids. Further research on dc-breakers and dc-fault tolerant topologies for VSC-based HVDC will assure more resilient MTDC grids in the future.
The economical and technological scale of HVDC and MTDC systems necessitates detailed validation and testing before actual implementation. Hence, development of benchmark dc systems is a paramount requirement. One of the main purposes of benchmark models is to provide a common basis for the testing and performance of research concepts and algorithms. Data availability is important for diverse types of studies ranging from high-level load flow analysis all the way to detailed semiconductor behaviour. In addition to the required controllers, dc-grid power flow analysis, interactions and coordination between ac and dc-grids, and protection strategies may also be included.
The alternate arm converter (AAC) is an emerging dc-fault tolerant modular VSC topology and a competitor for future dc-grids that will consist of diverse converter topologies. Along with the current research on the AAC, requirement for detailed AAC-based HVDC models that are compatible with widely accepted dc benchmark systems and systematical modeling approaches are paramount. This research provides following contributions:
- A fully AAC-based point-to-point HVDC transmission system compatible with existing MMC-based CIGRE B4 dc grid test system.
- Detailed steps for modeling and derivations of the AAC-based HVDC stations maintaining similar voltage, energy, and power ratings as the CIGRE B4 benchmark dc system.
- A complete set of results based on a real-time digital simulator (RTDS) that verify the performance of the proposed AAC-based HVDC transmission system under different operating scenarios.
- An openly available RTDS model that can be used for detailed studies of internal converter control, power flow, faults, and transients.
Harith Wickramasinghe, University of New South Wales