一种中压直流短路故障下不间断运行的MMC-SST拓扑及控制

多端口固态变压器是多电压形态多电压等级的交直流混合电网的核心设备,模块化多电平(modular multilevel converter,MMC)型固态变压器(solid state transformer,SST)具有中压直流端口,可接入中压直流配电网,构成多区域交流配电网的柔性互联,提升区域网络间功率灵活调节能力。而采用传统的MMC-SST拓扑及控制,中压直流线路短路故障会引起低压端口供电中断。文中提出一种混合型MMC-SST的拓扑及控制,其具备中压交流、中压直流和低压交直流端口,通过控制使其具有中压直流短路故障耐受能力,同时故障期间保持中压交流和低压端口的不间断功率交互,从而提升低压用户供电可靠性。分析MMC-SST在正常运行和中压直流故障不间断运行控制下内部能量平衡机理,提出中压直流短路故障下电容电压平衡及不间断运行控制策略,实现MMC-SST中压直流短路故障时不间断稳定运行。通过理论分析,仿真与物理动模实验,验证了所提拓扑及控制的可行性及有效性。

Topology and Control Strategy for an MMC-SST With Uninterrupted Power Supply Capability Under Medium Voltage DC Short Circuit Fault

Multi-port solid state transformer is a key equipment of the AC/DC hybrid distribution system with multiple voltage levels and forms. The modular multilevel converter based solid state transformer(MMC-SST) provides a medium voltage DC(MVDC) port, which enables the flexible interconnection between distribution systems, enhancing the power regulation ability between distribution networks. However, With traditional MMC-SST topology and control, the MVDC line short-circuit fault will lead to the interruption of the low voltage power supply. Therefore, this paper proposed a hybrid MMC-SST topology and control, which has medium voltage AC/DC and low voltage AC/DC ports. Through control, it has a strong MVDC short circuit fault tolerance. Meanwhile, it is able to keep uninterrupted power interaction between medium voltage AC and low voltage DC ports under the fault, improving the reliability of low voltage power supply. Besides, this paper analyzed the internal energy balance mechanism of MMC-SST under normal operation and uninterrupted operation, and proposed a control strategy for capacitor voltage balance and uninterrupted operation under medium voltage DC short circuit fault, so as to realize the uninterrupted operation under MVDC short circuit fault. Finally, the feasibility and effectiveness of the proposed topology and control was validated by theoretical analysis, simulation and experiment results.