Control of DFIG-based wind farms with hybrid HVDC connection

This paper presents the control methods for large doubly fed induction generator (DFIG)-based wind farms with hybrid HVDC (high-voltage dc) connection. The sending end of the hybrid HVDC topology is a Line-Commutated Converter (LCC) with a STATCOM (static synchronous compensator) while the receiving end is a Current Source Inverter (CSI) using forced-commutated devices. The STATCOM provides stator voltage support for DFIGs, as well as commutation voltage for the LCC. Taking into account the nonlinear and coupling characteristics of each subsystem, a series of control schemes are provided. Simulations under both normal and fault conditions are carried out, which verify that, incorporating the advantages of existing LCC-HVDC and VSC (Voltage Source Converter)-HVDC technologies, the proposed system has the black-start capability, and can provide flexible reactive power support to the grid. Moreover it is able to separate the wind farm from the ac fault on the inverter side, thus enhancing the reliability of the overall wind energy system.