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Section 12
Primary injection testing
voltage regulation. Smaller values may lead to a normal state where tap changers for parallel
transformers are well out of step, and values significantly higher will cause over-sensitivity in
the voltage control function and tap changer hunting behavior. It is an important outcome of the
testing process that the compensating factor is checked for each transformer to ensure
sensitive but stable operation for circulating currents.
For example, if there are three transformers connected in parallel, and the tap changer of
transformer T1 is two steps over the tap changer of T2 and T3, the circulating current detected
by the VCTR for T1 will be the sum (with opposite sign) of the current measured at T2 and T3.
The currents measured at T2 and T3 will ideally be about the same values. If the voltage is
close to the upper limit of the UDeadband, the tap changer of T1 will try to decrease the
controlled voltage. In the opposite case, that is, the voltage is close to the lower limit of
UDeadband, the tap changer at T1 will not try to decrease the controlled voltage. The tap
changer for T2 and T3 will not operate due to the fact that the detected circulating current will
be half of the current detected at T1. The setting of the parameter Comp then might need to be
increased a little. At least one tap changer step difference between the different transformers
should be allowed in order to avoid the tap changers to operate too often. If the allowed
difference is, for example, two steps, the tap changer shall be stepped up three steps when
setting the parameter Comp.
10.
Set the Comp parameter for T2 and T3 in the same manner as for T1. According to the
described procedure when the tap changer of one transformer is two steps above the others, it
shall automatically step down. When there are only two transformers in the group either shall
one step down or the other step up depending on the voltage level at the ATCC.
12.1.4
Master Follower (MF) method
The Master Follower method requires a Master to be nominated in a parallel group. It is responsible
for measuring secondary bus voltage and executing commands to raise and lower tap changers,
which are repeated by Follower transformers in the group.
1.
Confirm the voltage blocking levels OVPartBK and UVPpartBK in Main menu/Settings /IED
Settings/Control /TransformerVoltageControl(90, U↕)/TR8ATCC(90, U↕||):x/Voltage/
Umax,Umin .
2.
Check that MFMode is set to Follow Tap or Follow Cmd in Main menu/Settings/IED Settings /
Control/TransformerVoltageControl(90, U↕) /TR8ATCC(90, U↕||):x/MFMode .
If this setting is Follow Tap, all Follower transformers shall match the actual tap setting of the
Master, while Follow Cmd requires that Follower transformers follow the RAISE and LOWER
commands executed by the Master.
3.
Confirm the maximum difference in tap positions for parallel transformers. The number of taps
is determined by the setting in the Parameter Setting tool under Main menu/Settings/IED
Settings/Control/TransformerVoltageControl(90, U↕) /TR8ATCC(90, U↕||):x /ParCtrl/
MFPosDiffLim
4.
Connect all transformers in the parallel group to the same busbar on both the primary and
secondary sides.
5.
Set the control mode to Manual for each transformer in the parallel control group
6.
Open the test display for Transformer Voltage Control on the LHMI in Main menu /Test/
Function status /Control/TransformerVoltageControl (90, U↕).
282
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1MRK 504 165-UEN Rev. K
SEMOD176327-244 v5
Transformer protection RET670
Commissioning manual
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