ABB REG670 Applications Manual page 563

1MRK 502 071-UEN -
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
In order to realize the need for special measures to be taken when controlling
transformers in parallel, consider first two parallel transformers which are supposed
to be equal with similar tap changers. If they would each be in automatic voltage
control for single transformer that is, each of them regulating the voltage on the LV
busbar individually without any further measures taken, then the following could
happen. Assuming for instance that they start out on the same tap position and that the
is within USet ± DU, then a gradual increase or decrease in the
LV busbar voltage U
B
load would at some stage make U
command 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 USet ± DU,
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 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.
fall outside USet ± DU and a raise or lower
B
Section 15
Control
557