Doubly fed induction generator in an offshore wind power plant operated at rated V/Hz

Abstract

This paper introduces the concept of constant volt/hertz operation of offshore wind power plants (WPPs). The deployment of offshore WPPs requires power transmission from the plant to the load center inland. Because this power transmission requires submarine cables, there is a need to use high-voltage direct current (HVDC) transmission, which is economical for distances greater than 50 km. In the concept presented here, the onshore substation was operated at 60 Hz synced with the grid, and the offshore substation was operated at variable frequency and voltage, allowing the WPP to be operated at constant volt/hertz. In this paper, a variable frequency at rated volt/hertz operation was applied to a Type 3 doubly fed induction generator (DFIG) wind turbine generator. The size of the power converter at the turbine can be significantly reduced from 30% of the rated power output in a conventional Type 3 turbine to 5% of the rated power. The DFIG allows each turbine to vary its operating speed with respect to the other turbines. Thus, small wind diversity within the WPP can be accommodated by the DFIG, and the collector system frequency can be controlled by HVDC to follow large variations in average wind speed.