Control strategy to improve power transmission capacity from wind farms to weak AC grids

Abstract

—With the increasing connection of large wind farms based on direct driven Permanent Magnet Synchronous Generators (PMSGs) to weak AC grids, the steady state characteristics of such wind farms, particularly those connected to very weak AC systems with a Short Circuit Ratio (SCR) of 1, are important factors for analysis with a view to improvement. In this paper, the issue is thus analysed in order to identify the factors limiting transfer capability from wind farm output. This paper thus presents a simulation model of a wind farm connected to a very weak AC grid, and a vector control strategy is used with respect to the full power converters of the wind farm in order to track current order injected from the converters into the AC system. A supplementary outer loop control is proposed to support grid voltage and to maximise transferred power into the very weak AC grid, based on a droop gain which is allowed to update the reactive power reference for reactive power control for the grid side converter based on changes to the output of the wind turbine, within grid code requirements. The simulation results prove that the proposed control system offers a promising method for addressing challenges arising in active power transmission, and thus increasing transferred active power from such wind farms to very weak grids.