Coordinated Control of Cascaded Current-Source Converter Based Offshore Wind Farm

Offshore wind farms with cascaded PWM current-source converters (CSCs) at both generator- and grid-side can eliminate the need for bulky central offshore converter platform, which is usually used in a voltage-source converter (VSC) based counterpart. This novel system structure can simplify the system configuration and operation. However, the wind speed inconsistency at each turbine causes different dc-link current requirements for each CSC. This causes a considerable challenge for systems in which each CSC shares equal dc-link current. In order to overcome the problem, a coordinated control scheme for the dc-link current regulation, which considers wind speed difference of each turbine, is proposed. This control scheme enables the system to operate at minimum dc-link current, contributing to a lower operation losses. In the meantime, the independent control capability of each generator is guaranteed (e.g., maximum power tracking to make full utilization of available wind energy). Furthermore, the whole wind farm control strategy, which consists of wind farm supervisory control (WFSC), local wind turbine control and centralized grid control, is investigated and studied, where maximum power tracking and power limitation modes can be easily achieved. Both simulation and experimental verification of the proposed system with use of two permanent-magnet synchronous generators (PMSGs) are provided.