Bypass switch for high voltage dc systems

Abstract

Many DC applications switching converter modules are multilevel, series connected converters. If an individual converter cell fails, it is desirable to be able to bypass the failed cell, in order to continue operating the rest of the converter. A bypass switch is provided as a hybrid switch, including a parallel combination of a slow mechanical switch with high current conduction capability and a low power semiconductor switch with faster acting switching capabilities. When the bypass switch is activated (closed), the low power semiconductor switch is turned on first and quickly establishes the bypass within 10 μs or less, to temporarily conduct required current until the slower mechanical switch turns on. The mechanical switch is subsequently turned on with zero voltage, given the parallel-connected semiconductor switch being on. If desired, the semiconductor switch can then be turned off with zero voltage, given the presence of the mechanical switch. When the bypass switch is deactivated (opened), the semiconductor switch can first be turned on to divert part of the current from the mechanical switch. The mechanical switch can then be turned off at low voltage, since the mechanical switch is shorted by the semiconductor switch. Once the mechanical switch is off, the semiconductor switch can be quickly turned off.