ABB Relion REC670 Applications Manual page 310

Section 13
Control
I
L
I2Base
S
ci
It shall be noted that the adjustment factor is negative in order to decrease the load voltage
and positive in order to increase the load voltage. After this calculation U
by TR1ATCC or TR8ATCC for voltage regulation instead of the original value
calculated set point voltage U
menu/Test/Function status/Control/TransformerVoltageControl(ATCC,90)/TR1ATCC:x/
TR8ATCC:x.
Automatic control of parallel transformers
Control of parallel transformers means control of two or more power transformers connected
to the same busbar on the LV side and in most cases also on the HV side. Special measures
must be taken in order to avoid a runaway situation where the tap changers on the parallel
transformers gradually diverge and end up in opposite end positions.
Three alternative methods can be used for parallel control with the Automatic voltage control
for tap changer, single/parallel control TR8ATCC:
• master-follower method
• reverse reactance method
• circulating current method
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 LV busbar voltage U
increase or decrease in the load would at some stage make U
raise or lower 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
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
304
Load current
Rated current, LV winding
Constant load voltage adjust. factor for active input
LVAConst1, LVAConst2, LVAConst3 and LVAConst4 )
Set, adjust
USet ± DU, but at the same time it will drive the two tap changers to their
i (corresponding to
is shown on the local HMI as a service value under Main
is within
B
B
1MRK 511 358-UEN A
will be used
set, adjust
USet . The
USet ± DU, then a gradual
USet ± DU and a
fall outside
SEMOD159053-140 v5
Application manual
SEMOD159053-135 v5