Series Chain-Link Modular Multilevel AC–DC Converter (SCC) for HVDC Applications

Introduction of the modular multilevel converter (MMC) has enabled the exploitation of voltage source converters (VSCs) in an increasing number of high-voltage direct current (HVdc) applications. Subsequently, some new topologies and solutions have been presented to tailor the MMC concept to specific uses. Particular attention has been paid to reduction of the converter footprint for applications where plant size is a critical economic aspect, for example, in offshore installations. This article introduces a new series-connected modular multilevel ac-dc converter, the series chain-link converter (SCC), which gives a significant reduction in the required number of submodules (SMs) and a more compact distribution of the energy storage, compared to an MMC. In this article, the operating principle of the converter and its design are discussed in detail; the SM count and energy storage requirement are also given. The basic control loops required for the practical operation of the converter are presented and designed. The SCC concept has been experimentally validated on a small-scale 450 V dc, 415 V ac, 4.5 kVA laboratory prototype, confirming the practical viability of the topology.