Wind Turbine Reactive Power Control in a Multiterminal DC (MTDC) Wind Generation System

This article introduces a reactive power control tailored for a hybrid generator (HG) in a wind turbine (WT) conversion system. The WT is interfaced to a medium-voltage DC (MVDC) collector grid via a passive rectification stage in a multiterminal DC (MTDC) grid. The MVDC grid collects the power from WTs and sends it to an offshore substation, where DC-DC converters step-up the voltage for transmission to a fixed-voltage high-voltage DC (HVDC) grid. The HG has a 9-phase stator winding and comprises of two rotors, a permanent magnet (PM) rotor, and a wound field (WF) rotor. Both rotors are mounted on the same shaft and rotate at the same speed. At each speed the PM rotor induces a fixed stator voltage, while the induced voltage due to WF rotor is adjustable by controlling a DC current into the WF winding. The paper introduces a reactive power control that is implemented through a dual-loop WF rotor control enabling an effective management of the HG and WT reactive power. The proposed control system is developed for individual WTs and for parallel WTs in a MTDC grid. Analytical modelling are verified by simulation results, while test results from a scale-down laboratory prototype HG validate the simulations.