Stabilizing an AC-DC Micro-Grid Powered by Residual-Pressure Energy with the Additional Damping Controller of the Converter

To address low-frequency oscillations in islanded residual-pressure power generation systems caused by water-hammer dynamics and low inertia, this paper proposes a supplementary damping control strategy for the DC-voltage outer loop of a voltage-source converter (VSC), using generator-speed deviation as feedback. First, an electromechanical control model is established, and its oscillatory characteristics are examined using root-locus and frequency-domain analyses. Then, with the supplementary controller disabled, a small-step signal is applied in PSCAD. TLS-ESPRIT is used to identify an equivalent second-order model between the supplementary control signal and generator electromagnetic power, and an independent small-step record is used for validation. Damping-torque theory is subsequently employed to determine the required phase compensation. A constrained particle swarm search is used to audit controller settings against frequency- and DC-voltage-related criteria, while nonlinear verification determines whether the candidate parameters should be adopted. The implemented controller comprises a low-pass filter, a washout stage, and a single-stage lead–lag network. In a unified three-level load check, the controller maintains stable operation for the added loads of 80 and 160 kW, whereas the 240 kW case went out of the normal operating range. In a separate reduced-order hydraulic-lag sensitivity test at 160 kW, the controller maintains bounded responses when Th2 is varied from 0.36 to 0.54 s. A matched benchmark against a conventional uncompensated supplementary branch shows that the proposed phase-compensated controller reduces frequency ITAE by 18.9% and limiter residence by 22.4% relative to the selected conventional case, although its DC-voltage ITAE is 67.4% higher. The results therefore support a frequency-recovery benefit within the tested range rather than a universal performance superiority.