ABB RELION RET670, Applications Manual

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Line voltage drop compensation for parallel control
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The line voltage drop compensation for a single transformer is described in section
"Line voltage drop" . The same principle is used for parallel control with the circulating
current method and with the master – follower method, except that the total load
current, I
L
, is used in the calculation instead of the individual transformer current. (See
figure 365 for details). The same values for the parameters Rline and Xline shall be set
in all IEDs in the same parallel group. There is no automatic change of these
parameters due to changes in the substation topology, thus they should be changed
manually if needed.
Avoidance of simultaneous tapping
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Avoidance of simultaneous tapping (operation with the circulating current
method)
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For some types of tap changers, especially older designs, an unexpected interruption of
the auxiliary voltage in the middle of a tap manoeuvre, can jam the tap changer. In
order not to expose more than one tap changer at a time, simultaneous tapping of
parallel transformers (regulated with the circulating current method) can be avoided.
This is done by setting parameter
OperSimTap to On . Simultaneous tapping is then
avoided at the same time as tapping actions (in the long term) are distributed evenly
amongst the parallel transformers.
The algorithm in Automatic voltage control for tap changer, parallel control TR8ATCC
(90) will select the transformer with the greatest voltage deviation V
di
to tap first. That
transformer will then start timing, and after time delay t1 the appropriate VRAISE or
VLOWER command will be initiated. If now further tapping is required to bring the
busbar voltage inside
VDeadbandInner , the process will be repeated, and the
transformer with the then greatest value of V
di
amongst the remaining transformers in
the group will tap after a further time delay t2 , and so on. This is made possible as the
calculation of I
cc
is cyclically updated with the most recent measured values. If two
transformers have equal magnitude of V
di
then there is a predetermined order
governing which one is going to tap first.
Avoidance of simultaneous tapping (operation with the master follower method)
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A time delay for the follower in relation to the command given from the master can be
set when the setting
MFMode is Follow Tap that is, when the follower follows the tap
position (with or without an offset) of the master. The setting parameter tAutoMSF then
introduces a time delay on VRAISE/VLOWER commands individually for each
follower, and effectively this can be used to avoid simultaneous tapping.
Homing
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Homing (operation with the circulating current method)
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This function can be used with parallel operation of power transformers using the
circulating current method. It makes possible to keep a transformer energized from the
1MRK 504 163-UUS A
Section 15
Control
Transformer protection RET670 2.2 ANSI 771
Application manual