Power-dependent droop-based control strategy for multi-terminal HVDC transmission grids

The concept of voltage source converter based multi-terminal high-voltage direct current (MTDC) grids, represents both challenges and opportunities for the future of large power transfer and integration of renewable energy sources. For this kind of grids, the control aspect is of great importance, with voltage-droop based methods considered as one of the most attractive solutions. All existing strategies are designed to maintain the level of voltage in the MTDC grid constant during unexpected events, thus sacrificing the power flow. The aim of this study is to propose a new droop controller structure that maintains the dc-grid voltage close to the nominal value and at the same time tries to preserve the power flow in the dc grid, following events such as faults or disconnection of stations. The control scheme is presented and simulations are carried out in a four- and five-terminal MTDC grid, proving the validity of the concept.