Startup and Multilevel Voltage-Power Control for an All-DC Offshore Wind–PV Transmission System

To improve the development and utilization efficiency of renewable energy, offshore wind–photovoltaic (PV) all direct current (DC) collection and transmission systems, featuring fewer energy-conversion stages and flexible control capability, are emerging as an important solution. However, such all-DC transmission systems face converter-capacitor charging transients and control-mode transitions during startup, while renewable-power fluctuations may cause power imbalances and voltage deviations among DC buses. This paper first establishes an offshore wind–PV all-DC system configuration consisting of source-side voltage boosting, medium-voltage DC collection, centralized offshore voltage boosting, and high-voltage DC transmission, and analyzes the control strategies of its key conversion devices. Based on the startup-energy sources and capacitor-charging characteristics of the converters at different voltage levels, a startup strategy is proposed. Current-limiting pre-charging, controlled charging, ramped voltage establishment, and control-mode transitions are employed to establish the high-, medium-, and low-voltage DC buses sequentially. Furthermore, a multilevel power-voltage coordinated control method is proposed to dynamically adjust the voltage references according to the system power mismatch, bus-voltage margins, and participation factors, thereby coordinating power fluctuations among different voltage levels. Finally, case studies of a 2 GW offshore wind–PV transmission system were performed in MATLAB/Simulink R2023. The results verify the feasibility and effectiveness of the proposed startup and multilevel voltage-power coordinated control strategies.