基于二极管整流器的双极型复合式海上换流站及其控制策略

Diode Rectifier Based Bi-pole Hybrid Offshore Stations and Their Control Strategy

为提高海上换流站的紧凑型性并降低换流站一次性投入成本,西门子公司提出了基于二极管整流器(diode rectifier,DR)的高压直流输电技术,在此基础上,国内外学者对其控制策略及各种串并联拓扑进行了研究,但仍存在结构复杂、可行性差、运行灵活性低等问题。为此,提出了基于二极管整流器的双极型复合式海上换流站拓扑,正负极分别采用模块化多电平换流器(modularmultilevelconverter,MMC)和DR,以充分利用MMC灵活可控和DR体积小、成本低、可靠性高的优点。为降低直流电缆输电损耗,提出了复合式换流站正负极功率平衡控制,将接地极电流控制在零附近,提高了输电效率,可将高压直流电缆所产生的损耗降低67.7%。设计了限功率控制,在发生直流非对称故障期间通过低速通信,可主动对风电场发出功率进行限制,防止了换流站过电流风险,提高了系统可靠性。最后通过PSCAD/EMTDC仿真分析对所提拓扑及其控制策略进行了可行性验证。

To make the offshore stations more compact and reduce the capital costs of the stations,the SIEMENS proposed the diode rectifier(DR)based high voltage DC(HVDC)systems for offshore wind energy transmission.The control strategies of such DR HVDC systems and various parallel/series topologies based on the DR technology have also been put forward.However,the significant issues,such as the complex configuration,the poor availability and the operation flexibility,etc.,still need to be addressed.This paper proposes a bi-pole hybrid topology for offshore stations,where a modular multilevel converter(MMC)and a DR are adopted as the positive and negative poles,respectively,to effectively utilize the flexible controllability of MMCs and the superiorities of DRs,such as the compact design,the low cost,and the high reliability.A power balance control for the positive and negative poles is then proposed to reduce power transmission losses along the HVDC cables.The proposed control effectively suppresses the neutral current around zero and the power losses of transmission cables can be reduced by 67.7%.For the asymmetrical DC fault protection,a power limiting control is designed to actively reduce the generated power of the offshore wind farms to avoid overcurrent of the HVDC stations,having higher reliability.The PSCAD/EMTDC simulations confirm the validity of the proposed topology and control.