Rockwell Automation 7000A PowerFlex Medium Voltage Drive (A-Frame) - Classic Control - Manual (Page 282)

5-14 Functional Description
7000A-UM150F-EN-P – June 2013 7000 “A” Frame
Line Converter Feedback
The function of the line converter feedback block is to process (scale
and filter) the line side voltage and current feedback signals before
being sampled by the drive control software. It represents most of
the analog portion of the line side Signal Conditioning Board
(SCBL) and the Drive Control Board. The line converter Voltage
Feedback Board (VFB) provides a total of six voltage feedback
signals representing the three ac (V
a1
, V
b1
, V
c1
), two dc (V
dc+
, V
dc-
) and
one line side filter capacitor voltages referenced to ground. The
three line-to-ground voltages are subtracted from each other to
produce the three line to line voltages (V
ab1-out
, V
bc1-out
, V
ca1-out
). These
line voltages are filtered (V
ab1
, V
bc1
, V
ca1
) and sampled by software
for synchronization and protection. The two dc voltages are
subtracted to determine the line side dc link voltage (V
dc
), which is
used for hardware dc link over-voltage protection. In PWM drives,
the neutral point of the line filter capacitor is measured (V
n
) and used
for line side neutral over-voltage protection.
Current transformers (CT) in two of the ac input lines provide the
input line current feedback (I
a1-out
, I
c1-out
). These currents are then
filtered and processed by a variable gain stage (I
a1
, I
c1
,). Inverting
and adding the two current feedback signals reproduces the current
in the remaining phase (I
b1
). A Hall Effect Current Sensor (HECS) is
used for monitoring the dc link current and used for hardware over-
current protection. In addition the average value of the dc link
current feedback is measured using a V-f converter and used by the
dc link current controller to calculate the firing angle for the rectifier.
The preceding description applies to 6-SCR and PWM rectifier
options. For drives with the 18 pulse front-end, another VFB is daisy
chained with the first one providing additional six line-to-ground
voltages from the slave bridges. The slave 1 voltages are monitored
using (V
ab2
, V
bc2
, V
ca2
) while slave 2 voltage are monitored using
(V
ab3
, V
bc3
, V
ca3
). In addition current feedback from slave bridges I
a2
,
I
c2
and I
a3
, I
c3
are also brought in for protection. As in 6-pulse drives,
inverting and adding the two current feedback signals reproduces the
current in the remaining phase (I
b1,
I
b2
). Also for 18 pulse drives, the
three ac line to ground voltages are summed together to determine
the neutral to ground voltage (V
ng
) on the input transformer.