The massive introduction of remote power electronics interfaced renewable generation units and high-voltage direct current (HVDC) interconnectors into the existing power system, with the aim of decommissioning a part of conventional centralized generation, has introduced new challenges to grid operators to maintain system stability. Since conventional balancing services rely on the availability of traditional synchronous generators and the kinetic energy stored in their rotating masses, new methods for providing ancillary services must be introduced. In this context, voltage source converters (VSCs), and particularly the modular multilevel converters (MMCs), have received a significant interest. An MMC stores energy in its distributed sub-modules and this energy can be controlled. This is a new 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 paper examines the benefits on the transmission system of including additional energy storage in an HVDC link for ancillary services (fast frequency response (FFR) and power oscillation damping (POD)), and how this can be implemented using an MMC with large buffer storage. Firstly, the study investigates the energy requirements to provide these network services, using adequate control laws, through EMTP-RV simulations. Then, the energy storage function of the converter is designed and simulations are carried out to highlight that an MMC with an embedded energy storage can provide these services. The obtained results confirm that the combination of an MMC with an energy storage system and dedicated control laws can greatly increase the stability of hybrid AC/DC power systems. However, it highlights that an FFR function inside an MMC requires a higher quantity of stored energy to provide system benefits, in comparison to a POD action. This would impact more deeply the construction and the operating limits of the converter.
Florian Errigo, J.C. Gonzalez-Torres, A.Benchaib, L.Chédot, A. Sari, P. Venet, F. Morel
Presented at 41st CIGRE International Symposium, Ljubljana