Coordination of modular multilevel converter based HVDC terminals for ancillary services

This paper deals with the utilization of the energy storage capacity, inherently present in the Modular Multilevel Converter (MMC), in providing ancillary services to ac grids. The service considered in this paper is Power Oscillation Damping (POD), which requires active power injection and hence, causes distortion (oscillation) in the dc voltage. In a multi-terminal or meshed dc grid, the oscillation is compensated by active power taken from other terminal participating in dc voltage regulation. This action leads to propagation of the oscillation into other connected ac grids, which is undesirable. This can be avoided or reduced if the power injection is taken from the arm capacitors of the MMC. Such utilization of the MMC energy storage has already been proposed in literature. However, the amount of power that can be injected from one converter is very limited. Therefore, this paper proposes a method to coordinate such a functionality among multiple MMC terminals with the aim of obtaining higher capacity to accommodate more demanding services. Furthermore, communication between the terminals is not required because local measurement of the dc voltage is used as a feedback signal.