The document specifies a method for fault handling in a converter circuit
for switching three voltage levels, in which the converter circuit has a
converter subsystem provided for each phase (R,S,T), in which a top fault
current path (A) or a bottom fault current path (B) in the converter
subsystem is detected, the top fault current path (A) running through the
first, second, third and sixth power semiconductor switches in the
converter subsystem or through the first and fifth power semiconductor
switches (S1, S5) in the converter subsystem, and the bottom fault
current path (B) running through the second, third, fourth and fifth
power semiconductor switches in the converter subsystem or through the
fourth and sixth power semiconductor switches in the converter subsystem,
and in which the power semiconductor switches are switched on the basis
of a fault switching sequence. To avoid phase shorting of all the phases
of the converter circuit in order to achieve a safe operating state for
the converter circuit in the event of a fault, the fault switching
sequence in the event of detection of the top or the bottom fault current
path (A, B) is initially followed by the detection's accompanying
switching status of each power semiconductor switch being recorded. In
addition, in the event of detection of the top fault current path (A) the
first power semiconductor switch and then the third power semiconductor
are turned off, and in the event of detection of the bottom fault current
path (B) the fourth power semiconductor switch and then the second power
semiconductor are turned off.