ABB Relion 670 Series Applications Manual page 373

1MRK 511 401-UUS F
V
set, adjust
VSet
S
a
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 V
TR1ATCC (90) or TR8ATCC (90) for voltage regulation instead of the original value
calculated set point voltage V
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 (90):
• 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 V
in the load 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
changers to their opposite end positions. High circulating currents and loss of control would be
the result of this runaway tap situation.
Bay control REC670
Application manual
Adjusted set voltage in per unit
Original set voltage: Base quality is V
Automatic load voltage adjustment factor, setting
Load current
Rated current, LV winding
Constant load voltage adjust. factor for active input
LVAConst1, LVAConst2, LVAConst3 and LVAConst4 )
set, adjust
© Copyright 2017 ABB. All rights reserved
n2
VRAuto
i (corresponding to
is shown on the local HMI as a service value under Main
is within VSet ± DV, then a gradual increase or decrease
B
fall outside VSet ± DV and a raise or lower command
B
VSet ± DV, but at the same time it will drive the two tap
Section 14
will be used by
set, adjust
Vset . The
SEMOD159053-135 v5
Control
367