ABB REC670 Applications Manual page 394

Section 14
Control
388
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 will be used by TR1ATCC or TR8ATCC for voltage regulation instead of
set, adjust
the original value USet. The calculated set point voltage U
local HMI as a service value under Main 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
DU and a 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 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.
Set, adjust
is within USet ± DU, then a gradual
B
fall outside USet ±
B
Bay control REC670 2.2 IEC
1MRK 511 401-UEN A
is shown on the
SEMOD159053-135 v5
Application manual