基于环流注入控制的MMC型固态变压器低压直流真双极运行方案

Operation and Control of MMC-type Solid State Transformer With Bipolar LVDC Port Based on Active Circulating Current Injection

多端口固态变压器(solid state transformer,SST)是多电压等级交直流混合配电网的核心设备。模块化多电平(modular multilevel converter,MMC)型SST可构建中压交流、中压直流和低压直流等端口,其中压直流端口可接入中压直流配电网及构成多区域交流配电网的柔性互联,从而提升区域网络间功率灵活调节能力。对于低压侧,具有不平衡运行能力的真双极直流母线能够满足低压用户对多电压等级、供电可靠及用电安全等需求,而常规的MMC-SST仅能提供单极或伪双极低压直流端口。因此,文中提出一种真双极型MMC-SST拓扑及基于主动环流注入的桥臂电压平衡优化运行方案,该结构同时具备中压交流、中压直流及低压真双极直流等端口。文中的主要工作包括:简述真双极型MMC-SST连接方式,并基于能量平衡原理,探索低压真双极运行工作机理;建立基于环流的动态模型,揭示双极不平衡功率、桥臂电容电压及环流之间的耦合关系;基于环流注入的稳态模型,获得基于主动环流注入的桥臂电压平衡运行方法,并优化注入环流的幅值和相位;提出基于环流主动注入的桥臂电容电压平衡控制策略。最后,搭建2MVA的MMC-SST仿真模型及4.8kVA的MMC-SST实验样机,验证所提拓扑及控制的可行性和有效性。

Multi-port SST is the key equipment of multi-voltage level AC/DC hybrid distribution network.MMC-SST has medium voltage AC,medium voltage DC and low voltage DC ports,which can be connected to the medium voltage DC distribution network and form flexible interconnection of the multi-area AC distribution network.For low-voltage side,the bipolar DC bus with unbalanced operation ability can meet the demands of users for multi-voltage level,safe and reliable power supply,while the conventional MMC-SST can only provide unipolar low-voltage DC port.Therefore,this paper proposed a bipolar MMC-SST topology and an optimal operation scheme of voltage balance based on active circulating current injection.The main work included:analyzing topology and the operation principle of MMC-SST;establishing its dynamic model,revealing the coupling relationship between the unbalanced power,capacitance voltage and circulating current;establishing and analyzing the steady-state model;proposing a novel voltage balance control strategy and optimizing the amplitude and phase of circulating current.Finally,the feasibility and effectiveness of the proposed topology and control scheme were verified by simulation model.