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Reduced-order Modeling and Dynamic Stability Analysis of DC Systems Connected to Weak Grids in DC Voltage Control Timescale

To reveal the dynamic stability mechanism of DC systems connected to weak grids in DC voltage control timescale from the perspective of equivalent circuit, a reduced-order model is proposed in this paper. The DC voltage control unit can be modeled as an equivalent RLC parallel circuit, by which the influence of DC voltage control dynamic and its interaction with phase locked loop (PLL) control on system dynamic stability in weak grid condition can be characterized with corresponding equivalent resistors and inductors quantitatively. In addition, the equivalent circuit reduced-order model of the whole system can be obtained, and the essential cause of the system dynamic stability in DC voltage control timescale can be uncovered intuitively. Eventually, the proposed reduced-order model is verified based on a back-to-back DC system connected to a weak grid established in PSCAD/EMTDC, the impact of system parameters on system dynamic stability can be characterized by dynamic indexes (damping coefficient and damped oscillation frequency) in a straightforward approach.