基于三电平子模块的倍电平复合式MMC变换器

为了降低模块化多电平变换器MMC(modular multilevel converter)的接线复杂度和通信失效故障概率,ABB提出了采用压接式IGBT串联的级联两电平CTL(cascaded two-level)变换器。由于桥臂子模块数由数百降为几十,CTL变换器输出电压电平数较少,需要在交流侧安装滤波器。为此,提出了基于三电平TL(three-level)子模块的倍电平复合式MMC变换器VLD-HMMC(voltage-level-doubler hybrid MMC)。每个桥臂有TL、半桥HB(halfbridge)子模块混合组成,且交流侧串联有一个全桥FB(full-bridge)子模块,输出电压电平数提高了1倍,从而省去了交流滤波器。各子模块具有脉冲自主触发功能且其功率开关由压接式器件串联组成,降低了桥臂子模块数,简化了控制器与主电路之间的接线复杂度,降低了通信故障的概率。单个功率开关短路故障不会影响系统正常运行,并且可避免了子模块电容的过度放电,保护电容器和功率开关,提高变换器的可靠性及故障穿越能力。所提拓扑结构实现了直流故障穿越运行,为HVDC换流站的设计提供了技术参考。

Voltage-level-doubler Hybrid MMC Based on Three-level Submodules

The cascaded two-level (CTL) converter based on series-connected press-pack IGBTs is proposed by ABB to simplify the line connection complexity of a modular multilevel converter (MMC) and reduce the communication failure possibility. As the submodules per arm are reduced from several hundreds to several tens, the CTL converter has fewer output voltage levels and an ac-side filter is required to reduce harmonics. To tackle this problem, the three-level (TL) submodule based voltage-level-doubler hybrid MMC (VLD-HMMC) is proposed, where the TL submodules and half-bridge (HB) submodules are mixed in each arm and a full-bridge (FB) submodule is connected in series on the ac-side. The output voltage levels are doubled and thus any ac filter requirements are avoided. The submodule modulation is made semi-autonomous and the power devices are composed of series-connected press-pack semiconductors. As a result, the submodules per arm are reduced and the connection between controller and the main power circuit is simplified, yielding lower communication failure probability. When a short-circuit fault is applied across one switching device, the system can still operate normally and the over-discharge of submodule capacitors as well as the damage to capacitors and power switches are avoided. This improves the converter reliability and contributes to dc fault ride-through operation. The proposed topology with dc fault blocking capability provides an attractive approach for the development of HVDC power stations.