Modular Multilevel Converters (MMCs) inherently possess an energy storage that can be used to help mitigate power imbalances due to perturbations in the system. Control systems use this reserve to release or absorb additional energy, from which additional power contributions are manifested based on the time derivative. However, this requires not only the controller response speeds to be sufficiently fast, but also the MMC to be capable of reaching the set point demanded from its controllers within sufficient time. Furthermore, large energy storage elements in the MMC have implications on its response speed. Detailed electromagnetic transient (EMT) simulations may be used to determine the natural MMC response speed capability, but this comes at high computational costs, especially in larger MMCs. This paper derives a closed-form equation for the response speed of an MMC. It is obtainable without the construction of waveforms or iterative calculations, which makes it computationally efficient. Parametric studies are conducted and investigate the effect of different MMC parameters on its settling time. Results obtained from the model have been compared against those from EMT simulations. The model may be used as an initial design tool, particularly when the natural response speed of the MMC is a concern.

The decreasing use of synchronous generators (SGs) by renewable energy sources (RESs) in power systems has led to an increasing vulnerability to frequency excursions. Without the rotating shafts of SGs serving as a reservoir of energy to provide a naturally sufficient inertial response, control systems have been developed and implemented to achieve a similar effect. Other energy reservoirs, such as the inherent energy storage in modular multilevel converters (MMCs), are controlled to help compensate the power imbalances introduced by system perturbations in the grid and improve the inertial response. However, the level of improvement is dependent on the amount of energy available, which is dictated by the MMC and system configurations. In this paper, studies using electromagnetic transient (EMT) simulations are conducted on a parametric basis to investigate how such factors impact the effectiveness of controlling the stored energy to improve the inertial response. These include the energy storage capacity of the MMC, the RES penetration level, magnitude of perturbation, and system inertia. The frequency response characteristics monitored are the frequency nadir and duration of the arresting period. Results show that the available energy does not necessarily improve the inertial response or in a linear manner.