Operation of Dual-T-Type Modular Multilevel Converter for Uninterrupted Power Supply Under Bridge Failures

Abstract

Dual-T-type modular multilevel converter (DTMMC) is an alternative for existing uninterrupted power supply (UPS) due to its advantage in flexible leg reusing, multiple power ports and high operating efficiency. In this paper, a DTMMC-UPS which has series configurations on both input/output AC sides obtain high voltage grid/ load accessing interfaces is presented. In the described system, the fact that copies of T-type legs are employed makes the semiconductor failure common to happen. Moreover, both the coupling between rectifier and inverter conversions, and the confliction between inter-cell and inner-cell voltage balancing make the fault-tolerant control a tough issue. In this paper, the switch fault types are firstly clarified and diagnosed by the different impacts on the normal operating. An improved voltage vector modulation is then deduced to facilitate the planning of proper primary switching patterns, to achieve rectifier side normal operation. The secondary alternated LS-PWM is also modified to achieve inverter side normal operation. The experiment results are supplied to verify the validity of the modulation coordinated fault toleration strategy.