An aggregation scheme for offshore wind farms with VSC-based HVDC collection system
This paper proposes a new aggregation scheme for large-scale offshore wind farms. The aggregation scheme is based on direct-drive, variable-speed, wind-power units which are interfaced with a High-Voltage DC (HVDC) collector substation. Each wind-power unit employs a diode-bridge rectifier cascaded with a Sinusoidal Pulse-Width Modulated (SPWM), Voltage-Sourced Converter (VSC). The use of the diode-rectifier is expected to bring about higher efficiency, lower converter cost, and higher reliability. The offshore HVDC collector employs a three-level Neutral-Point Clamped (NPC) converter which adopts a single-pulse switching strategy. Therefore, a high voltage level is permitted while the switching losses are minimized. Moreover, the pulse width of the HVDC AC voltage is chosen such that a twelve-pulse waveform is approximated, thereby, avoiding phase-shifting transformers in the interest of lower weight and footprint. The feasibility and effectiveness of the proposed aggregation scheme and its controls are demonstrated by simulation studies in the PSCAD/EMTDC software environment.