ABB RELION RET670 Applications Manual page 776

Section 15
Control
770
= × ×
C I X
V
di i cc _ i i
EQUATION1979-ANSI V1 EN-US
where X is the short-circuit reactance for transformer i and C
i
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 will have positive values for transformers that produce circulating currents and
di
negative values for transformers that receive circulating currents.
Now the magnitude of the no-load voltage for each transformer can be approximated
with:
= +
V V
V
i Bmean di
EQUATION1980-ANSI V1 EN-US
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
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
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
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 VSet values for
individual transformers can cause the voltage regulation to be unstable. For this reason,
the mean value of VSet for parallel operating transformers can be automatically
calculated and used for the voltage regulation. This is set Enabled/Disabled by setting
parameter OperUsetPar. The calculated mean VSet 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 VSet is recommended for parallel operation with the circulating
current method, especially in cases when Load Voltage Adjustment is also used.
is inside the inner deadband at the same time as the
Bmean
1MRK 504 163-UUS A
(Equation 508)
, is a setting parameter
i
(Equation 509)
"Automatic voltage control
. The calculated no-
Bmean
Transformer protection RET670 2.2 ANSI
Application manual