ABB ret650, Руководство по применению, Страница: 206 / 392

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
USet ±
D
U, 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
SEMOD159053-140 v5
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 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
(URAISE and ULOWER) generated by the master, will initiate the corresponding
command in all follower TR8ATCCs 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 URAISE/ULOWER 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
function block. Deselecting a master is made by activating the input RSTMAST.
These two inputs are pulse activated, and the most recent activation is valid that is,
an activation of any of these two inputs overrides previous activations. If none of
these inputs has been activated, the default is that the transformer acts as a follower
(given of course that the settings are parallel control with the master follower
method).
When the selection of master or follower in parallel control, or automatic control in
single mode, is made with a three position switch in the substation, an arrangement
as in figure
74 below is arranged with application configuration.
Section 11
1MRK 504 169-UEN A
Control
200 Transformer protection RET650 2.2 IEC
Application manual