An Offshore Wind Farm Featuring Differential Power Processing

Abstract

Offshore wind farms are a rapidly growing technology used to harvest wind energy on the open seas where wind speeds are significantly higher and steadier than onshore. Current wind farms located far away from shore (e.g., 50 km or more) require a large amount of equipment to be deployed in order to transport generated energy to shore most cost-effectively. In these cases, energy is transmitted to shore using High-Voltage DC (HVDC) transmission connected to wind turbines with AC voltage output. During the past decade, research has studied alternate arrangements to reduce the amount of equipment deployed offshore and increase conversion efficiency. The redesign of offshore collection systems between wind turbines from AC to DC voltages is seen as a key tool to achieve the research objectives. The presented research is focused on the design of offshore wind farms with DC collection system and series-connected wind turbines based on partial power processing converters (PPPCs). This wind farm configuration significantly improves conversion efficiency compared to AC wind farms with HVDC link, since PPPCs are only required to process output power differences among wind turbines in a wind farm to achieve maximum power point (MPP) operation, and other wind farm components are operated at variable operating points, improving low-load efficiency. Furthermore, PPPCs can be of reduced size to realize MPP operation. To find the best variable operating points, a loss minimizing HVDC link current scheduling scheme has been derived and a comprehensive sizing framework was developed to inform the best choice of PPPC ratings. The presented work addresses major design considerations at wind farm, wind turbine, and PPPC levels. An efficiency, size and economic evaluation has been conducted for a 450 MW wind farm located 100km from shore, confirming significant annual loss reductions and economic advantages compared to a conventional AC wind farm with HVDC link, as well as two other series-connected DC wind farm configurations. A generic converter sizing framework for single-string series-connected DC wind farms has been developed and applied to the 450 MW wind farm. Challenges in wind turbine startup with this configuration have been identified and schemes were developed to enable successful wind turbine startup without the need of significant adidtional hardware.