ABB RET670 Applications Manual page 667

1MRK504116-UUS C
Application manual
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
will be used by TR1ATCC (90) or TR8ATCC (90) for voltage regulation instead
adjust
of the original value Vset. The calculated set point voltage V
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
Parallel control of power 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 in an IED 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
is within VSet ± DV, then a gradual increase or decrease in the load
busbar voltage V
B
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 VSet ± DV, but at the same time
it will drive the two tap changers to its opposite end positions. High circulating
currents and loss of control would be the result of this runaway tap situation.
fall outside VSet ± DV and a lower or raise command
B
Section 3
IED application
set,
is shown on the
set, adjust
661