Effects of DC voltage droop characteristics on contingency behaviour of AC/DC systems

High Voltage Direct Current (HVDC) systems are gaining importance in the currently planned grid reinforcements throughout Europe. The state-of-the-art Voltage Source Converter (VSC) technology allows the connection of multiple DC converters to meshed DC grids. A key topic of ongoing research in the field of DC grids is the task of maintaining the DC system´s power balance by controlling the DC voltage. In this paper, the behaviour of a multi-terminal HVDC-grid using different control characteristics for the VSC-converters after outage events is investigated. The used grid model is based on the New England Test System / New York Power System (NETS/NYPS). This HVAC grid is expanded by an HVDC overlay grid built up by 10 converter stations. Some of these converter stations are using voltage droop characteristics for DC voltage control, the others are equipped with a constant power control. Within this hybrid grid, the main problems of contingencies within the DC system are investigated. The paper focusses on post-outage steady-state power flow. The main contribution of this paper is to explain and show the effects of different DC voltage control strategies. In addition, principles are discussed, according to which DC voltage control strategies should be assessed.