Coordinate Control of Wind Turbines in a Medium Voltage DC Grid

In this article a coordinate control strategy for wind turbines (WTs) and the collector grid in an all-DC wind generation system is proposed. The all-DC system is adopted from a previous work of the authors which consists of a high-voltage wind turbine generator (WTG) that uses a multiphase hybrid machine interfaced to a passive rectifier, a medium voltage DC (MVDC) collector grid, and a high-voltage DC (HVDC) transmission system. Two control schemes are implemented: 1) A local WTG controller that controls a maximum power point tracking (MPPT) for the hybrid generator using field current adjustment as the wind velocity varies, and 2) An aggregated controller that maintains the MVDC grid voltage corresponding to minimum wind velocity among WTs. The hybrid generator has two rotor elements which enables full control over the generator without the use of active power electronics. The hybrid generator is interfaced to the MVDC via an uncontrolled passive rectification stage. The MVDC grid collects the power from WTs and sends it to an offshore substation where DC-DC converters step-up the voltage to a HVDC grid with fixed voltage for transmission. A coordinate control of WTG and aggregated controller allows WTs to maintain their desired power via MPPT, while enabling a variable voltage MVDC grid operation. Simulation results under different conditions are performed to verify the effectiveness of the proposed control strategies. Experimental results from a scaled-down laboratory prototype 9-phase hybrid generator connected to a DC grid using a passive rectifier validate the simulation analyses.