Coordinate Control of Wind Turbines in a Medium Voltage DC Grid

In this paper 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 taken from a previous work of the authors and consists of a high voltage wind turbine generator (WTG) system, a medium voltage DC (MVDC) collector grid, and a high voltage DC (HVDC) transmission grid. Two controllers are implemented in the all-DC system: 1) A local controller for WTG that controls a maximum power point tracking (MPPT) as the wind velocity varies, and 2) An aggregated controller that maintains the MVDC grid voltage to a desired value corresponding to minimum wind velocity among WTs. In the scheme under study the WTG uses a hybrid generator that has two rotor elements which enables full control over the generator without 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 fixed HVDC for transmission. A coordinate control of WTG and aggregated controller allows wind turbines 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 coordinate control strategy.