Grid-Forming Control for Parallel Onshore Converters in Shore-to-Ship Applications: Design and Experimental Validation

Abstract

The maritime transport sector is a major source of pollutant emissions, posing severe health risks especially in port cities. While shore-to-ship (S2S) power connections are an effective solution to decarbonize ports, their implementation presents significant technical challenges. Specifically, S2S connections must comply with the IEC/IEEE 80005 standard when powering vessels with diverse power ratings, voltages, and frequencies. Consequently, supplying these shipboard microgrids requires robust grid-forming (GFM) converters ensuring accurate voltage and frequency regulation, a superior dynamic response, and a wide stability range against severe grid impedance variations. To accommodate the highly variable power demands of different vessels, onshore facilities typically employ multiple parallel converters. Therefore, this paper proposes a GFM controller for parallel onshore voltage source converters (VSCs) in S2S applications. The scheme integrates an inner voltage control loop based on state-space theory, which maximizes the stability range against different impedance values. Furthermore, an outer droop control loop is implemented, using low-pass filters to provide virtual inertia, damp transient power fluctuations, and decouple the control loop dynamics. The proposed controller is validated by simulation and experiments under a standard S2S connection sequence, which encompasses the synchronization and parallel operation of two VSCs with the ship’s generator, and the subsequent transition to islanded mode. The results demonstrate zero steady-state error in voltage and frequency, a superior dynamic response, and robust disturbance rejection, thereby ensuring smooth connections, accurate power sharing, and stability throughout the entire sequence.