兼具直流故障清除和局部自均压能力的新型混合MMC

It has been a big issue that the dc-side short-circuit fault should be cleared promptly and reliably in the overhead line (OHL) modular multilevel converter based HVDC system (MMC-HVDC). The traditional hybrid MMC which consists of the half-bridge sub-module (HBSM) and full-bridge sub-module (FBSM) has become popular due to its control flexibility and dc fault clearance capability, however, the parallel operation/current sharing technology of the semiconductor devices and the sub-module capacitance reduction issues need to be solved urgently. Therefore, this applicant proposes the novel hybrid MMC topology, it is composed of the double half-bridge sub-module (D-HBSM) and the paralleled full-bridge sub-module (P-FBSM). Both D-HBSM and P-FBSM are simply reconstructed from the traditional HBSM and FBSM, hence the novel hybrid MMC inherits the dc fault clearance capability, has exactly the same amount of semiconductor devices and will avoid the requirement of paralleled devices. Also, it has built-in local capacitor voltage self-balancing capability and is possible to further reduce the capacitance value. Beginning from the research achievements, this proposal will follow the regular procedure of theoretical investigation, scheme design, simulation analysis and prototype experimental tests, focus on the electromagnetic transient (EMT) equivalent modelling, coordinated control of the D-HBSMs and P-FBSMs together with the hybrid phase arm, optimization of the capacitance value, dc-side fault blocking mechanisms and the fast power recovery strategies etc. The anticipated output of this project will enhance competitiveness to MMC-HVDC in future OHL based high-voltage large-power transmission system.

采用架空线的模块化多电平换流器高压直流输电(Modular Multilevel Converter based HVDC, MMC-HVDC)亟需解决直流故障快速清除的问题。半桥和全桥子模块组成的传统混合MMC控制特性灵活、可清除直流故障，但仍面临功率器件并联均流和子模块电容容值较大的问题。为此，申请人提出由双半桥子模块和并联全桥子模块组成的新型混合MMC，它由传统混合MMC拓扑重构而来，具备直流故障清除能力，在相同的功率器件数量下避免了器件的直接并联，并且具备了局部电容自均压能力，可降低对电容容值的需求。在已有研究基础上，本项目将通过理论研究、方案设计、仿真和实验验证的思路，重点解决新型混合MMC电磁暂态等效建模、自均压协调控制、电容容值优化设计、直流故障闭锁机理以及功率快速恢复策略等核心问题。本项目的顺利开展将使MMC-HVDC在高电压、大容量、架空线输电领域更具竞争优势。

柔性直流电网可实现大规模可再生能源的广域互补送出，是未来电网发展和变革的重要方向之一。其网架结构多样，多种设备之间存在强耦合关系，故障电流传播特性复杂，由于无自然过零点，故障电流的抑制和开断面临巨大挑战，故需要新型换流器拓扑，且其对仿真效率要求很高，这直接推动了具备全工况仿真能力的换流器高速、高精度等效建模研究。项目研究内容包括新型混合MMC电磁暂态高效建模方法，新型混合MMC电容电压平衡和降低电容容值的控制方法，新型混合MMC直流故障暂态闭锁和故障恢复机理。项目开展以来，三项研究内容均已完成。项目取得丰硕成果，五年内将有望于柔性直流输电电磁暂态等效建模方面得到应用。