Abstract
At the time of contingency like converter outage in multi-terminal DC grid, it is critical that the working converter stations share the power imbalance/load according to their respective capacity. In this study, several concepts of designing variable droop constant are proposed. These variants are formulated out of conventional fixed droop scheme considering operational parameters of converters and desired voltage and frequency regulation. The adoption of variable droop allows converters to share the imbalance power proportional to their rating and available headroom. A droop design method to reduce the voltage deviations at converter terminals is also proposed. Further, a variable droop control scheme using voltage deviation, frequency deviation of the non-synchronous AC area connected to DC terminal and available headroom of the converters is also discussed. This scheme modifies the DC power flow in such a way that the converter terminal with frequency decrease in AC system, allocates to supply more power to the AC system and vice-versa. A five terminal MTDC system is modelled and simulated to show the effectiveness of proposed droop control schemes. The droop scheme for proportional power sharing according to available headroom adjusts the converter power cooperatively to respond to the needs of frequency regulation.
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