Reduced-order modeling and dynamic stability analysis of MTDC systems in DC voltage control timescale

An equivalent source-load MTDC system including DC voltage control units, power control units and interconnected DC lines is considered in this paper, which can be regarded as a generic structure of low-voltage DC microgrids, mediumvoltage DC distribution systems or HVDC transmission systems with a common DC bus. A reduced-order model is proposed with a circuit structure of a resistor, inductor and capacitor in parallel for dynamic stability analysis of the system in DC voltage control timescale. The relationship between control parameters and physical parameters of the equivalent circuit can be found, which provides an intuitive insight into the physical meaning of control parameters. Employing this model, a second-order characteristic equation is further derived to investigate system dynamic stability mechanisms in an analytical approach. As a result, the system oscillation frequency and damping are characterized in a straight forward manner, and the role of electrical and control parameters and different system-level control strategies in system dynamic stability in DC voltage control timescale is defined. The effectiveness of the proposed reduced-order model and the correctness of the theoretical analysis are verified by simulation based on PSCAD/EMTDC and an experiment based on a hardware low-voltage MTDC system platform.