ABB RET670 - Page 672

The calculated mean busbar voltage V
Bmean
is shown on the local HMI as a service
value BusVolt under Main menu/Test/Function status/Control/
TransformerVoltageControl(ATCC,90)/TR8ATCC:x.
Measured current values for the individual transformers must be communicated
between the participating TR8ATCC (90) functions, in order to calculate the
circulating current.
The calculated circulating current I
cc_i
for transformer “i” is shown on the HMI as a
service value ICIRCUL under Main menu/Test/Function status/Control/
TransformerVoltageControl(ATCC,90)/TR8ATCC:x.
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, V
di
, with equation
424:
di i cc _ i i
V C I X= × ×
EQUATION1979-ANSI V1 EN (Equation 424)
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 V
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
V V V= +
EQUATION1980-ANSI V1 EN
(Equation 425)
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 V
Bmean
. The calculated no-
load voltage will then be compared with the set voltage VSet. A steady deviation which
is outside the outer deadband will result in VLOWER or VRAISE 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 V
Bmean
is inside the inner deadband at the same time as the
Section 3 1MRK504116-UUS C
IED application
666
Application manual