Discussions on Fault-Tolerant Operation of Modular Multilevel Converters Using Mechanical Bypass Switches

Reliability and availability of modular multilevel converters such as SSBC (single-star bridge-cell) and SDBC (single-delta bridge-cell) converters can be significantly improved by achieving continuous operation when a bridge cell fails. In order to achieve this, each bridge cell is equipped with a bypass switch at its ac terminals. When a bridge cell in a phase or cluster fails, the faulty bridge cell, and two healthy bridge cells in the other two phases or clusters in case of symmetrical operation, are switched OFF and their ac terminals are short-circuited by turning ON their bypass switches. The converter is then operated with the remaining healthy bridge cells. The converter thus transitions from normal operation to fault-tolerant operation without requiring to turn OFF the whole converter. However, when a mechanical switch is used as a bypass switch, the transition period, i.e. the interval between the fault occurrence and the closing of the bypass switch, can range from tens of milliseconds to over a hundred milliseconds. During this period, the concerned bridge cells including the faulty one may work as diode rectifiers, thus increasing their dc capacitor voltages and leading to dc overvoltage. Moreover, the ac output voltages of the concerned bridge cells are not zero and act as a disturbance to the current control of the converter leading to degradation in output performance. This paper presents a simple solution to avoid the possible dc overvoltage and proposes a current control method to improve the converter output performance, both during the transition period. The former is based on the attenuation of the current flowing through the converter, while the latter is based on the cancellation or minimization of the effect of voltages appearing across the switched-off bridge cells including the faulty one. Experimental and simulation results validate the effectiveness of the proposed ideas.