Modular Multilevel Converter With Partially Rated Integrated Energy Storage Suitable for Frequency Support and Ancillary Service Provision

Grid scale energy storage systems (ESSs) have received significant interest in recent years due to their ability to reduce/defer investment in transmission/distribution networks, as well their ability to act as primary reserve sources and provide emergency support to the transmission system. This paper investigates the dual purposing of an HVdc-Scale modular multilevel converter (MMC), allowing it to also act as an ESS. This has potential application in primary frequency response provision and other services such as de-coupled power oscillation damping. In the proposed topology, a certain percentage of sub-modules (SM) within the MMC have their capacitor interfaced through a dc-dc converter to an energy storage element (ESE), formed of a battery or ultracapacitor. By applying appropriate control, energy can be exchanged from the ESE to the main SM capacitor of each ESE-SM, and from there to either the ac or dc bus. It was found that for some operating points, an injection of circulating current was required to facilitate exchanging energy with the ESE-SMs. Analysis shows that, for instance, an additional power injection to ac or dc terminal of 0.1 per unit (10%) is possible with only 4% of the SMs replaced by full-bridge ESE-SMs, and no additional SMs added.