Control system for offshore wind power generation system with DFIG and VSC-based HVDC transmission

Abstract

This paper proposes a fundamental and efficient control technique for introducing VSC (voltage source converter)based HVDC (high voltage direct current) transmission into a large-scale offshore wind power generation system with DFIGs (doubly-fed induction generators). The electric power from the offshore wind turbine DFIG is considerably fluctuating and unstable, and needs to be transmitted from offshore to the on-land major AC system generally accompanied with long distance cable transmission. From these points of view, VSC-based HVDC transmission is considered to be an efficient choice. In this paper, control systems for the AC/DC converter at the offshore side and the DC/AC inverter at the land side are proposed and presented in details. These controls aim at mitigating the impact of wind power fluctuation on the on-land AC system, while securing the stable operation of the HVDC circuit. Furthermore, by proper control applied to the converter, voltage and frequency of the offshore AC system at the wind power plant side is stabilized so as to facilitate the stable operation of DFIGs, which is modeled by a self-excitation type with HVDC link circuit in this study. Digital simulation studies were conducted and the results have verified the effectiveness of the proposed systems.