Section 4
1MAC050144-MB C
Protection functions
342
615 series ANSI
Technical Manual
A correct scaling is determined by the number of steps and the direction of the deviation
from the nominal tap and the percentage change in voltage resulting from a deviation of
one tap step. The percentage value is set using the
Step of tap
parameter.
The operating range of the tap changer is defined by the
Min winding tap
and
Max winding
tap
parameters. The
Min winding tap
parameter tells the tap position number resulting in
the minimum effective number of winding turns on the side of the transformer where the
tap changer is connected. Correspondingly, the
Max winding tap
parameter tells the tap
position number resulting in the maximum effective number of winding turns.
The
Min winding tap
and
Max winding tap
parameters help the tap position compensation
algorithm know in which direction the compensation is being made. This ensures also that
if the current tap position information is corrupted for some reason, the automatic tap
changer position adaptation does not try to adapt to any unrealistic position values.
Figure 163:
Simplified presentation of the high voltage and medium voltage windings
with demonstration of the Max winding tap, Min winding tap and Tap
nominal parameters
The position value is available through the Monitored data view on LHMI or through other
communication tools in the tap position indication function. When the quality of the
TAP_POS
value is not good, the position information in
TAP_POS
is not used but the last
value with the good quality information is used instead. In addition, the minimum
sensitivity of the biased stage, set by the
Low trip value
setting, is automatically
desensitized with the total range of the tap position correction. The new acting low trip
value is
(Equation 40)
Second harmonic blocking
The transformer magnetizing inrush currents occur when energizing the transformer after
a period of de-energization. The inrush current can be many times the rated current and the
halving time can be up to several seconds. To the differential protection, the inrush current
represents a differential current, which would cause the differential protection to trip
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