Stability Evaluation on the Droop Controller Parameters of Multi-Terminal DC Transmission Systems Using Small-Signal Model

For a complex multi-terminal DC (MTDC) transmission system, controller parameter settings exert tremendous influence on the system dynamics and transient stability. The traditional droop control scheme, which aims to locally control the converter's power by regulating its DC voltage according to the pre-defined droop slopes, does not normally employ communications for coordination with other terminals. The droop scheme is designed for power sharing purpose and its controller gains' impact on the MTDC network voltage is usually ignored. Moreover, the droop controller parameters are selected without considering various operating states of the MTDC terminals. This paper proposes a droop controller parameters select the criterion to cater to MTDC network voltage stability. To do so, a detailed small-signal model for a four-terminal MTDC system candidate with typical network topology, considering each converter station's electrical characteristics and cascaded control loops, is established. In order to maintain the fidelity for DC network modeling, the model also takes account of the DC network electrical dynamics and losses. A time-domain simulation model is built in Matlab Simulink to verify the correctness of the small-signal model.