ABB REC670 Applications Manual page 424

Section 14
Control
418
Assuming for instance that they start out on the same tap position and that the LV
is within VSet ± DV, then a gradual increase or decrease in the load
busbar voltage V
B
would at some stage make V
would be initiated. However, the rate of change of voltage would normally be slow,
which would make one tap changer act before the other. This is unavoidable and is due
to small inequalities in measurement and so on. The one tap changer that responds first
on a low voltage condition with a raise command will be prone to always do so, and
vice versa. The situation could thus develop such that, for example T1 responds first to
a low busbar voltage with a raise command and thereby restores the voltage. When the
busbar voltage thereafter at a later stage gets high, T2 could respond with a lower
command and thereby again restore the busbar voltage to be within the inner deadband.
However, this has now caused the load tap changer for the two transformers to be 2 tap
positions apart, which in turn causes an increasing circulating current. This course of
events will then repeat with T1 initiating raise commands and T2 initiating lower
commands in order to keep the busbar voltage within VSet ± DV, but at the same time
it will drive the two tap changers to their opposite end positions. High circulating
currents and loss of control would be the result of this runaway tap situation.
Parallel control with the master-follower method
In the master-follower method, one of the transformers is selected to be master, and
will regulate the voltage in accordance with the principles for Automatic voltage
control. Selection of the master is made by activating the binary input FORCMAST in
TR8ATCC (90) function block for one of the transformers in the group.
The followers can act in two alternative ways depending on the setting of the parameter
MFMode. When this setting is Follow Cmd, raise and lower commands (VRAISE and
VLOWER) generated by the master, will initiate the corresponding command in all
follower TR8ATCCs (90) simultaneously, and consequently they will blindly follow
the master irrespective of their individual tap positions. Effectively this means that if
the tap positions of the followers were harmonized with the master from the beginning,
they would stay like that as long as all transformers in the parallel group continue to
participate in the parallel control. On the other hand for example, one transformer is
disconnected from the group and misses a one tap step operation, and thereafter is
reconnected to the group again, it will thereafter participate in the regulation but with a
one tap position offset.
If the parameter MFMode is set to Follow Tap, then the followers will read the tap
position of the master and adopt to the same tap position or to a tap position with an
offset relative to the master, and given by setting parameter TapPosOffs (positive or
negative integer value). The setting parameter tAutoMSF introduces a time delay on
VRAISE/VLOWER commands individually for each follower when setting MFMode
has the value Follow Tap.
Selecting a master is made by activating the input FORCMAST in TR8ATCC (90)
function block. Deselecting a master is made by activating the input RSTMAST. These
fall outside VSet ± DV and a raise or lower command
B
1MRK 511 401-UUS A
SEMOD159053-140 v5
Bay control REC670 2.2 ANSI
Application manual