Improved variable speed control of series-connected DC wind turbines for offshore wind power collection to high-voltage direct current system

DC wind farm (DCWF) with series-connected DC wind turbines (DCWTs) is proved to be a potential solution of offshore wind power collection. The coupling behaviour of series-connected DCWTs is described in detail. Possible wind energy curtailment during the period of wind turbine voltage limitation and its key impact factors are firstly quantitatively derived. A novel variable speed control strategy is proposed under voltage limiting condition of the DCWT to improve its wind energy capture. This control algorithm can be implemented in the local DCWT controller without the communication need. The specific variable speed control strategy of the DCWT is realised in a generalised averaged model. Dynamic simulation cases under different operation conditions have been conducted to evaluate the effectiveness of the proposed control strategy. It is found that the proposed control strategy might be a solution for wind energy curtailment, which will significantly improve the performance of the series-connected DCWTs between the time scale above seconds and below minutes.