Online Soft Restart of a Cascaded H-Bridge Inverter fed Induction Motor Drive after Cell Faults

The Cascaded H-Bridge (CHB) multilevel inverter topology has found widespread acceptance in Medium Voltage (MV) drive applications. But a high count of semiconductor devices increases the probability of converter faults in these drives. Therefore, fault tolerant control techniques have been developed which would enable balanced operation even with internal cell faults. Upon detecting a fault, the gate pulses to the drive are interrupted for a short time to take the faulty cell out of service online using bypass contactors, and the drive then resumes operation with the remaining healthy cells. However, the reconnection of the converter to the machine after bypassing the faulty cell results in transient overshoots in the machine input current, due to lack of synchronism between the converter output voltage and the machine terminal voltage. These overshoots may even result in overcurrent trips thereby reducing the reliability of the drive. This paper proposes a method for online soft reconnection of the converter to the machine post fault clearing in a vector controlled drive, in both speed sensor based and sensorless control techniques. The proposed methods are validated by simulation for a 6600 V, 1050 kW 13-level CHB inverter fed induction motor drive, and experimentally on a 400 V, 30 kW laboratory machine using a 7-level CHB inverter.