Control-Informed Quasi-Steady-State Modeling and AC/DC Power-Flow Analysis of LCC–SLCC HVDC Systems

Conventional quasi-steady-state models treat a self-adaptive STATCOM and line-commutated converter (SLCC) station as an LCC with an external reactive-power source, which cannot fully represent valve-side coupling. This paper develops a three-phase stationary-frame differential model for the SLCC and derives quasi-steady-state expressions for the average DC voltage and fundamental displacement angle. The non-commutation equivalent voltage is decomposed into fundamental and nonfundamental components. The fundamental component is retained in the power-flow model, while a control-informed harmonic extension evaluates the corresponding average DC-voltage correction over the tested operating domain. A positive-sequence fundamental-frequency formulation calculates the commutation overlap angle, and a first-zero diagnostic identifies control-sensitive conditions associated with the fast SVG voltage response. When the fast commutation-direction voltage reaches zero or reverses before current transfer is completed, a control-equivalent effective-area formulation provides an alternative low-order representation. The station equations are incorporated into a sequential AC/DC power-flow algorithm and validated against the engineering PSCAD/EMTDC main-circuit and control model of the Yangzhou–Zhenjiang HVDC Phase II project. Across the stable tested operating points, the phase-aware EMT-derived harmonic DC-voltage correction ranges from 0.585% to 1.245%, remaining below the adopted 2% screening threshold. The control-informed estimate follows the EMT-derived correction, whereas the phase-independent conservative bound reaches 2.128% at high controller gain. Across eight cases with available PSCAD reference values, the control-equivalent formulation reduces the mean and maximum overlap-angle errors from 0.90∘ and 1.37∘ to 0.78∘ and 1.09∘. For the benchmark power-flow cases, the maximum relative errors are 1.3% for the SLCC bridge reactive power and 1.0% for the SVG reactive-power output, and the calculation converges without sustained oscillation. A representative operating-point calculation is completed in approximately 3 s with the quasi-steady-state (QSS) formulation, compared with about 15 min for the engineering EMT benchmark.