Cost-effective Cells for High-power Modular Multilevel Converters

Abstract: The modular multilevel converter (MMC) topology was introduced in 2003and has since been receiving considerable attention from industry and academia.Notable benefits compared to two- or three-level voltage source convertersare its scalability, low switching losses, low output filter requirementsand ease of adding redundancy combined with fail-safe operation. There area number of disadvantages, such as the use of large bus capacitors and complexityof control. However, with currently predominant switch technology forhigh power levels, which are bipolar silicon devices, the benefits outweigh thedisadvantages. The use of integrated gate commutated thyristors (IGCTs) inan MMC is an attractive option compared to using IGBTs due to their robustnessand low conduction losses. Switching losses are of lesser importance,but not negligible.The objectives of this work are to contribute to a reduction in equipmentcost and to a decrease power loss in HVDC and STATCOM installations.The approach to achieve those objectives is by using the auxiliary resonantcommutated pole topology in IGCT-based MMC. In case thyristors are usedas auxiliary switches, the total amount of needed hardware does not differsignificantly from a hard-switched solution which needs a di/dt reactor and asnubber. The reduced switching losses in the main switches (IGCTs) lead toan efficiency increase, but also to the possibility of using higher-voltage devicesthat would otherwise be impracticable. This in turn can lead to a reduction inhardware cost due to a lower number of cells and lower conduction losses. Inaddition, the improved switching conditions allow for an increase in turn-offcurrent capability. Practical aspects that are also treated in this work are theuse of snubber circuits for the auxiliary switch, and the behaviour under ashoot-through. The latter aspect is essential for practical use in high-powerMMCs. In this work a fault-tolerant soft-switching cell is presented, whichallows for a current limiting series inductance between the main switches andthe dc-bus capacitor without causing a problematic voltage overshoot.