Control-Mode-Dependent dq-Impedance Modeling and Stability Analysis of Multi-Unit Alkaline Electrolysis Systems

Large alkaline-water-electrolysis plants are increasingly formed by multiple converter-fed electrolysis units operating in parallel, whose grid interaction is affected by both operating condition and Buck control mode. This paper develops a control-mode-dependent small-signal dq-impedance model for an IGBT rectifier–dc-link–Buck–alkaline-water-electrolyzer unit under constant-current (CC) and constant-power (CP) control. The complete Buck–AWE input impedances are retained, and the electrochemical operating point is recalculated with loading. Frequency-sweep simulations of all four dq-impedance elements reproduce the principal analytical magnitude and phase characteristics, validating the mode-dependent model. The unit admittances are then aggregated for arbitrary CC/CP combinations, and grid interaction is evaluated using the minimum impedance phase-difference margin at all magnitude crossings. For four 1 MW units at λ = 0.75 and SCR = 3, the margin increases from −22.95° for 4 CC to +4.42° for 4 CP. Parameter studies show that loading, grid strength, and CC/CP composition jointly determine the impedance-interaction screening margin. HIL tests further show that mode reconfiguration produces distinct coupled ac–dc–electrochemical transients. These tests are used as qualitative time-domain corroboration of the control-mode-dependent dynamics rather than as a quantitative validation of the numerical value of Mφ.