regarded similarly to the single transformer measured busbar voltage, and further control actions
taken.
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
thereafter 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 related to the
transformer no-load voltage. The sequence resets when V
Bmean
is inside the inner deadband at the
same time as the calculated no-load voltages for all transformers in the parallel group are inside
the outer deadband. The example in figure 464,is a fabricated case and not very realistic, but it
illustrates some details on how the described regulation works.
DB1
DB1
DB2
DB2
VSet
T1 No-load voltage
T1 T4T3T2
V
Bmean
ANSI06000526_2_en.vsd
ANSI06000526 V2 EN-US
Figure 464: Selection of transformer to tap
In the figure 464, voltage is considered as increasing above the line denoted
VSet
, and decreasing
below that line.
In the TR8ATCC (90) function for T1 and T4, the calculated no-load voltage for T1 and T4
respectively, is above the upper limit of DB1 and thus outside the deadband.
In the TR8ATCC (90) function for T2, the calculated no-load voltage for T2, viewed from the upper
DB1, is not outside (above) the deadband, but as viewed from the lower DB1 it is outside (below)
the deadband. However, there is a restriction in a situation like this, when the measured busbar
voltage, V
Bmean
, is on the opposite side of the
VSet
line (in figure 464), then V
Bmean
must be inside
DB1 if the calculated no-load voltage for that transformer shall qualify as a candidate for tapping.
Thus in the example above, the calculated no-load voltage for T2, although below DB1, would not
be considered for tapping in this case.
In the TR8ATCC (90) function for T3, the calculated no-load voltage for T3, is above the upper limit
of DB1 and thus outside the deadband. However, viewed from the upper limit DB1, transformers
with negative voltage deviation, V
di
, are disregarded and similarly, viewed from the lower limit
1MRK 502 066-UUS B Section 14
Control
875
Technical manual
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