A Hybrid Chain-Link Push-Pull Series Connected (H-CL-P2SC) M2C with DC Fault Blocking Capability

Abstract

This paper introduces a hybrid chain-link push-pull series connected (H-CL-P2SC) modular multilevel converter for HVDC applications, especially for those considering a connection of off-shore wind farms to the grid. On the ac side, each converter phase is connected to the corresponding grid phase via a single-phase center-tapped transformer whereas on the dc side, the converter phases are connected in series. The required functionality of the chain-link (CL) inductors in other modular multilevel topologies is achieved by using the leakage inductance of each transformer. As a result, the converter does not require separate CL inductors. Since the phases are series connected on the dc side, the H-CL-P2SC requires 33.3% less cells (NCs) to withstand the same dc voltage when compared to the standard modular multilevel converter (M2C). A control strategy for the H-CL-P2SC is discussed in detail. The global and internal power balance controllers are illustrated with block diagrams. A numerical comparison between the H-CL-P2SC and other typologies is also presented. Finally, the response of the H-CL-P2SC to a dc pole-to-pole (P2P) fault is shown using PLECS simulations for a 20 MW-20 kV demonstrator.