ABB RET670 Applications Manual page 668

Section 3
IED application
662
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
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 280 below is arranged with application configuration.
Figure 280: Principle for a three-position switch Master/Follower/Single
Parallel control with the reverse reactance method
Consider figure 281 with two parallel transformers with equal rated data and similar
tap changers. The tap positions will diverge and finally end up in a runaway tap
situation if no measures to avoid this are taken.
1MRK504116-UUS C
Application manual