Rockwell Automation 7000L PowerFlex Medium Voltage AC Drive (C Frame) - Classic Control - Manual (Page 331)

Functional Description 5-23
7000 “C” Frame 7000L-UM300I-EN-P – June 2013
On-line detection of Failed SCRs/SGCTs
When the gating is enabled for both converters, the feedback from
the gate drivers is constantly switching on and off, usually several
times per cycle. The diagnostics feedback signals from each device
are monitored and the protective measures are performed.

SCR On-Line Diagnostics
For SCR rectifier drives, the drive detects both open and shorted
devices while running. Due to notching and interaction with the other
phases, the SCR feedback diagnostic changes state many times per
cycle, although it is only valid just before and after firing the device.
Just before firing a device, the drive takes several samples of the
diagnostic feedback from the SCR. If every sample indicates that the
device was on before it was fired, the drive considers that the device
may be shorted, and starts a timer. When this timer exceeds the
number of line cycles specified by the parameter Rec Dvc Diag Dly
(266) the drive generates an Online SC fault. Each device has its own
timer. A delay of zero will generate a fault immediately. A delay of 2
will generate a fault after 2 cycles which indicates that the fault has
been seen three times in a row.
Shortly after the drive fires an SCR it checks the feedback from the
gate driver boards. If the feedback shows that the device did not fire
the drive considers that the device may be open-circuited and starts a
timer. If the fault persists for 6 cycles, the drive generates an Online
OC fault. As with the short circuit fault, each device has its own
timer, however the delay is not adjustable.
Both on-line device diagnostics are not available at all modes of
operation due to the nature of the feedback from the gate driver. No
diagnostics are done when the rectifier firing angle is less than 15
degrees. No diagnostics are done when the dc current is
discontinuous.

SGCT on-line diagnostics
The PWM rectifier and inverter generate only one type of on-line
diagnostic fault. Due to the intelligent gate driver board the drive is
able to check the status of every SGCT in a bridge any time a device
in the bridge is fired. The drive takes a sample of every device’s
feedback before and after firing the bridge. If both samples indicate
that the device is not functioning correctly the drive starts a timer for
that device. When this timer reaches the value specified by parameter
Rec Dvc Diag Dly (266) for the PWM rectifier, or Inv Dvc Diag Dly
(268) for the inverter the drive generates an Online Flt. The actual
time to trip will vary with the switching frequency of the bridge in
question. A bridge changes state at three times the switching
frequency. For a PWM rectifier switching at 420 Hz (7pulse at 60 Hz),
the bridge changes state at 1260 Hz. This means the delay is in
multiples of around 0.8 ms.