ABB REG670 - Page 568

When the circulating current is known, it is possible to calculate a no-load voltage for
each transformer in the parallel group. To do that the magnitude of the circulating
current in each bay, is first converted to a voltage deviation, U
di
, with equation 272:
_di i cc i i
U C I X= ´ ´
EQUATION1869 V1 EN (Equation 272)
where X
i
is the short-circuit reactance for transformer i and C
i
, is a setting parameter
named Comp which serves the purpose of alternatively increasing or decreasing the
impact of the circulating current in TR8ATCC control calculations. It should be noted
that U
di
will have positive values for transformers that produce circulating currents
and negative values for transformers that receive circulating currents.
Now the magnitude of the no-load voltage for each transformer can be approximated
with:
i Bmean di
U U U= +
EQUATION1870 V1 EN (Equation 273)
This value for the no-load voltage is then simply put into the voltage control function
for single transformer. There it is treated as the measured busbar voltage, and further
control actions are taken as described previously in section
"Automatic voltage
control for a single transformer". By doing this, the overall control strategy can be
summarized as follows.
For the transformer producing/receiving the circulating current, the calculated no-
load voltage will be greater/smaller than the measured voltage U
Bmean
. The calculated
no-load voltage will then be compared with the set voltage USet. A steady deviation
which is outside the outer deadband will result in ULOWER or URAISE being
initiated alternatively. In this way the overall control action will always be correct
since the position of a tap changer is directly proportional to the transformer no-load
voltage. The sequence resets when U
Bmean
is inside the inner deadband at the same
time as the calculated no-load voltages for all transformers in the parallel group are
inside the outer deadband.
In parallel operation with the circulating current method, different USet values for
individual transformers can cause the voltage regulation to be unstable. For this
reason, the mean value of USet for parallel operating transformers can be
automatically calculated and used for the voltage regulation. This is set On/Off by
setting parameter OperUsetPar. The calculated mean USet value is shown on the local
HMI as a service value USETPAR under Main menu/Test/Function status/
Control/TransformerVoltageControl(ATCC,90)/TR8ATCC:x.
The use of mean USet is recommended for parallel operation with the circulating
current method, especially in cases when Load Voltage Adjustment is also used.
Section 15 1MRK 502 071-UEN -
Control
562 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual