Impedance-Based Stability Analysis and Grid-Forming Control of MMC-HVDC Systems for Offshore Wind Integration

Abstract

Offshore wind integration through modular multilevel converter (MMC) HVDC links places the converter rather than the synchronous machine at the center of grid stability. As inverter penetration rises and onshore short-circuit ratios fall, classical generator-based analysis no longer predicts the dominant interaction modes, and grid-forming (GFM) controls that are well behaved at one operating point can destabilize the link at another. This thesis develops the impedance-based modeling and GFM control architectures needed to characterize and reshape these dynamics, organized around an MMC-HVDC offshore-wind test system. A power-smoothing layer is first applied to the offshore source. A type-IV permanent-magnet wind turbine is modeled in PSCAD/EMTDC, and a coordinated supervisory loop trades DC- link energy and rotor kinetic energy against pitch action through a Gaussian-smoothed dispatch, suppressing short-term wind variability before it reaches the HVDC station. This thesis adapts the dual-port GFM concept to an MMC-HVDC offshore-wind link. An explicit energy synchronization controller decouples DC-link voltage regulation from submodule capacitor-energy management, enabling independent active- and reactive-power services without loss of arm-energy balance. The decoupling conditions are derived analytically and validated on a real-time digital simulator under DC-voltage drift, AC-frequency excursions, and three-phase grid faults. A perturbation-based extraction procedure then recovers the full dq-frame impedance of the GFM MMC over 1–1000 Hz from EMT data and from the RSCAD frequency-scanning tool, with an analytical small-signal model used as cross-check. The thesis closes by unifying the droop, virtual-synchronous-machine, and synchronverter families under a thirteen-state per-converter formulation and showing that the three reduce to identical eigenvalues at matched gains; a three-converter cluster on the RTDS real-time simulator confirms that the per-converter methodology survives the transition to multi-unit operation on a shared bus.