Modular Embedded Multilevel Converter for MV/HVDC Applications

Many of the renewable energy resources, like offshore wind, utility PV farms, are located far away from the load centers, which requires dc power transmission system to efficiently deliver power. Voltage-source converter solution is required in many applications due to the stability and compensation requirement. This paper presents a new voltage source converter topology for medium to high voltage dc application, named modular embedded multilevel converter (MEMC). MEMC is based on a three-level topology structure and consists of series-connected insulated-gate bipolar transistor (IGBT) bridge stacks, like the popular modular multilevel converter (MMC) and series-connected thyristor or press-pack high voltage integrated gate-commutated thyristor or IGBT stacks. The basic operation principle, control solutions, and the methods to commutate the thyristors are explained in detail. Due to the three-level structure, the total number of IGBT stacks are reduced by half compared with MMC, leading to much lower energy storage, weight, volume, and system complexity. Also by replacing part of the IGBT stack in MMC with thyristor stack, both of the system conduction loss and switching loss can be further reduced. There are other topology variations of MEMC with a similar operational principle but different characteristics.