Abstract-This paper presents a control scheme that utilizes energy storage of the Modular Multilevel Converter (MMC) for Power Oscillation Damping (POD) service. Such a service is used to enhance grid stability by providing damping power in the electromechanical (0.2-2Hz) dynamics range. The scheme uses compensated modulation with average energy control. The aforementioned service comes at the cost of additional energy storage requirement for the MMC. A worst case estimate of this additional capacity, together with potential options to obtain it, is also addressed in this paper. The scheme is validated by simulation studies on a four terminal HVDC test grid.

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.

Abstract-The Modular Multilevel Converter (MMC), being a complex system, requires a number of controllers to function properly. These range from low level switching pattern generation to high level control loops. Among these controllers are modulation techniques that calculate the insertion indices, control inputs that decide the number of sub-modules inserted in a given arm. One such technique is compensated modulation which divides the reference voltages, generated by high level controllers, by the respective arm voltages (sum of capacitor voltages) when calculating the insertion indices. This prevents the arm voltage ripple from affecting the generated output voltage. For this purpose, the arm voltages can be measured (closedloop approach) or estimated (open-loop approach). This paper presents an arm voltage estimation technique for compensated modulation of MMCs. The proposed technique combines the benefits of the open-loop approach with that of the closed loop one.

Power oscillation damping (POD) is one of the ancillary services expected from high-voltage direct current (HVDC) converters. When providing POD to the ac side, converters draw power from the dc side, which can cause distortion to the dc voltage especially in the case of limited dc capacitance. In meshed/multi-terminal HVDC grids, where dc voltage regulation is distributed using a dc voltage droop control strategy, the distortion due to the POD controller action is propagated to other connected ac grids because of the droop action. This propagation can be reduced by using the inherent energy storage capability of the modular multilevel converter (MMC), which is a common converter topology for HVDC. Different methods to utilise the energy stored in the MMC for the purpose of POD have been proposed in the literature. This study presents a detailed analysis and experimental validation of one of these methods, referred to as virtual capacitance support, which increases the effective dc grid capacitance by using the stored energy of multiple MMCs connected in the same grid. The experiments, which were carried out using power hardware in the loop setup, demonstrated the effectiveness of the method.

Abstract-The Multi-terminal DC grid is expected to be built incrementally by interconnecting existing HVDC installations. In doing so, an enabling component is the DC-DC converter which also plays the role of a power flow controller. A number of DC-DC converter topologies, targeting different applications, have been proposed in literature. However, detailed simulation models for system level studies are not developed, yet. This paper will focus on reduced order modelling and control of the Front-to-Front MMC based DC-DC converter for system level studies. The developed model is validated against a full detail average model. A simple approach to investigate poorly damped oscillations is also proposed. The approach is then used to investigate a poorly damped mode in the leg energy state that was excited by a transient in ac power.