Parallel Nine-Phase Generator Control in a Medium-Voltage DC Wind System

In this article, we propose a variable voltage control strategy for parallel connected hybrid generators (HGs) in a medium-voltage dc (MVdc) array grid. In the presented scheme, the outputs of HGs are rectified and interfaced to an offshore MVdc array grid. The MVdc grid, whose voltage is varied but controlled, connects the wind turbines in the farm to an offshore substation. Using dcdc converters, the voltage is stepped up at the offshore substation and sent to a terrestrial grid via undersea high-voltage dc (HVdc) export cables. The MVdc voltage is varied in a prescribed control strategy, whereas the HVdc voltage is maintained fixed at all times. The HG in the scheme presented in this article utilizes two rotor elements, a permanent-magnet rotor with fixed excitation and a wound field (WF) rotor with controllable excitation. The proposed control strategy includes a WF control scheme local to the HGs coordinated with a wind farm supervisory control system (SCS). A simulation platform is developed to evaluate the effectiveness of the control strategy in a large wind farm. Simulation results are validated on a laboratory prototype HG system showing a good agreement.