Siemens SINAMICS S120 Function Manual page 557

The Vdc setpoint in p3510 must be set high enough to prevent the standby controller from
responding to line overvoltage (the response threshold of 97% can be increased if
necessary, but current and voltage harmonics will develop if the setting causes overcontrol).
In any case, the tolerance band must be set wide enough that it will not be violated should
the control factor reserve controller still respond because the measures described above
have not been implemented.
Master/slave switchover
If a power unit fails during operation, the higher-level controller can switch each infeed line
from current control (slave operation) to DC link voltage control (master operation) and vice
versa (parameter setting for master: p3513= 0, for slave: p3513 = 1).
Switching an ALM to an operational system
In an operational master-slave group, an ALM must first be switched in as a slave.
Shutting down an ALM from an operating group
Shutting down an ALM from the group should be realized in the slave state and with OFF2
(pulse inhibit). If a master fails with a fault (OFF2 response, pulse inhibit), one of the slaves
must be immediately switched as master.
Two masters must not be operated simultaneously in the infeed group.
Drive functions
Function Manual, 11/2017, 6SL3097-4AB00-0BP5
8.9 Master/slave function for Active Infeed
Function modules
557