A novel concept of offshore wind-power collection and transmission system based on cascaded converter topology

This paper presents a new concept for the grid integration of large-scale offshore wind farms. In the offshore wind farm, the wind turbines are integrated into the offshore DC collector grid by a cascaded converter. The high-voltage level for the high-voltage direct current (HVDC) transmission purpose is built by series connections of low voltage modules, without the use of an offshore step-up substation. The output power of each module is controlled by the switching of the related half-bridge, and a modified straightforward voltage balancing strategy is proposed to balance the storage capacitor voltages in the turbines. Supplementary balancing strategies are also presented to assist in balancing the capacitor voltages, when the differences in the wind velocities flowing to the turbines become larger. Because of the stable capacitor voltages, self-commutated voltage source converter (VSC) is adopted as the onshore inverter station connecting the HVDC link and the onshore AC grid. The VSC station not only regulates the DC-bus voltage of the HVDC link, but also provides reactive power compensation to the onshore grid. The time-domain simulation results show that the proposed offshore wind-power collection and transmission system provides a new option for integrating large-scale offshore wind farms. Copyright (C) 2012 John Wiley & Sons, Ltd.