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.
Line voltage drop compensation for parallel control
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 279 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
Avoidance of simultaneous tapping (operation with the circulating current
method)
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.
1MRK504116-UUS C Section 3
IED application
667
Application manual
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