Short Circuit Current Contribution from MTdc Grids to the ac Power System under ac System Faulted Conditions

This paper studies the improvement in voltage and rotor angle stability of a power system when voltage source converter based multi-terminal dc (MTdc) grids provide additional short-circuit current injection to the ac system under ac side faulted conditions. A five terminal offshore MTdc grid connected to the IEEE 39-bus test system is used as case study. Different control strategies and parameter sensitivities are evaluated in order to draw conclusions and provide recommendations on the design of future grid codes for MTdc grids operating next ac systems. Particularly it is investigated how the current limitation strategy applied could be properly exploited not only from ac system voltage and rotor angle stability point of view but also from the MTdc grid operation point of view. The analysis is performed both by applying analytical equations of the equivalent circuits augmented by time domain simulations on a multi-machine test system. The numerical results and the methods shown in the paper could be considered as a reference for the future design of grid code requirements related to MTdc grids.