Control of an HVDC Transmission System With Virtual Synchronous Machines Providing Inertia

This paper proposes a virtual synchronous machine (VSM)-based control scheme of a high voltage direct current (HVDC) system to provide inertial frequency support without a dedicated energy reservoir, based on the energy stored in the submodule capacitors of the modular multilevel converters (MMCs) and the HVDC line itself. In order to achieve this, we introduce a direct current (DC)-voltage control scheme consisting of a slow and a very fast controller that allow the cell voltages and the DC-voltages to drop or rise by up to 10% in order to provide an energy reserve for inertia emulation, while protecting the system from over-and undervoltages. We investigate this control in a setup similar to the German Suedlink project, with 4 MMC converters in a rigid bipolar (RBP) configuration with a ±525kV DC-line. Simulations show the performance of the system to provide inertia when the frequency in the power grid drops. The protection scheme is activated only when the DC voltage drops below (or rises above) predefined thresholds, and is able to prevent the DC-voltage from leaving the allowed range. The control scheme is validated in simulations and compared to a more standard control scheme, where a fast DC-voltage controller is employed at one terminal.