Inertial support for an HVDC system with fast DC voltage protection

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.