Control scheme of hexagonal modular multilevel direct converter for offshore wind power integration via fractional frequency transmission system

• 作者: Bo LIU Haitao ZHANG Huiyong LUO Shuonan SHANG Xifan WANG Yongqing MENG
• 来源期刊: 现代电力系统与清洁能源学报(英文)
• 学科: 工学
• 关键词: HEXAGONAL MODULAR MULTILEVEL converter(Hexverter) OFFSHORE wind power integration Fractional frequency transmission system(FFTS) Control scheme CIRCULATING current optimization
• 年，卷（期）: 2018,（1）
• 页码范围: 168-180
• 页数: 13页
• 分类号: TU46

摘要

A fractional frequency transmission system(FFTS)is a promising solution to offshore wind power integration,for which the hexagonal modular multilevel converter(Hexverter)is an attractive choice for power conversion.The Hexverter has recently been proposed to directly connect two three-phase systems of different frequencies and voltage amplitudes,with only six branches in the FFTS in that case.This paper examines for the first time the control scheme of the Hexverter when applied to offshore wind power integration via a FFTS.Firstly,the frequency-decoupled mathematical model of the Hexverter is deduced by introducing the double dq transformation.Then the branch energy of the Hexverter is analyzed in detail and the reactive power constraint equation is obtained.The corresponding control scheme is thoroughly discussed,including the inner loop current control,the outer loop voltage control in both grid-connected mode and passive mode,and a novel optimization method to minimize the circulating current in the Hexverter.Finally,a simulation model of offshore wind power integration via a 4-terminal FFTS based on the Hexverter is built in MATALB/Simulink to verify the feasibility of Hexverter and the effectiveness of the control scheme proposed in this paper.