MMC stores energy in its distributed sub-modules and this energy can be controlled. This is a degree of freedom that can be utilized to supply ancillary services. However, one of the major drawbacks is the limited amount of energy available, which is considerably smaller than a typical AC power plant. Nevertheless, the energy stored inside the MMC can be increased with the help of energy storage systems (ESSs). This study is related to the integration of ESSs within MMC submodules (SM) in order to provide ancillary services (mostly fast frequency response (FFR)).
Introduction of the ESS into all SMs as well as only in the selected number of SMs within MMC have been studied. The analysis dealt with the internal energy exchange to maintain the proper function of the converter by using circulating currents. This study helps to design embedded ES-SMs in the converter to provide new functions like ancillary services for power system operation.
A modular DC-DC converter (Patent Application) has been proposed as an interface between the energy storage element and the SM capacitor. It has been shown that the volume of the passive components can be reduced in comparison with interface converters known in the literature. The proposed control method provides an appropriate energy exchange without affecting the control philosophy of an MMC and it addresses balancing issues. Simulations and a downscaled prototype confirm the relevance of the proposed control.
In this work, supercapacitors are used as ESS. Ageing studies have been performed to assess different levels of redundancy and maintenance required to achieve expected performance over lifetime of the converter.