activation of any of the two stages a common signal PICKUP will be activated. At trip from any of
the two stages also a common signal TRIP will be activated.
To avoid instability there is a settable hysteresis in the power function. The absolute hysteresis of
the stage1(2) is
Hysteresis1(2)
= abs
(Power1(2)
+ drop-power1(2)). For generator low forward
power protection the power setting is very low, normally down to 0.02 p.u. of rated generator
power. The hysteresis should therefore be set to a smaller value. The drop-power value of stage1
can be calculated with
the Power1(2), Hysteresis1(2)
: drop-power1(2) =
Power1(2)
+
Hysteresis1(2)
For small power1 values the hysteresis1 may not be too big, because the drop-power1(2) would be
too small. In such cases, the hysteresis1 greater than (0.5 ·
Power1(2)
) is corrected to the minimal
value.
If the measured power drops under the drop-power1(2) value, the function will reset after a set
time
DropDelay1(2)
. The reset means that the pickup signal will drop out and that the timer of the
stage will reset.
8.11.7.1 Low pass filtering
SEMOD172136-39 v3
In order to minimize the influence of the noise signal on the measurement it is possible to
introduce the recursive, low pass filtering of the measured values for S (P, Q). This will make slower
measurement response to the step changes in the measured quantity. Filtering is performed in
accordance with the following recursive formula:
S TD S TD S
Old Calculated
= ⋅ + −
( )
⋅1
EQUATION1959-ANSI V1 EN-US (Equation 151)
Where
S is a new measured value to be used for the protection function
S
old
is the measured value given from the function in previous execution cycle
S
Calculated
is the new calculated value in the present execution cycle
TD is settable parameter by the end user which influence the filter properties
Default value for parameter
TD
is
0.00
. With this value the new calculated value is immediately
given out without any filtering (that is without any additional delay). When
TD
is set to value
bigger than 0, the filtering is enabled. A typical value for
TD=0.92
in case of slow operating
functions.
8.11.7.2 Calibration of analog inputs
SEMOD172136-57 v3
Measured currents and voltages used in the Power function can be calibrated to get class 0.5
measuring accuracy. This is achieved by amplitude and angle compensation at 5, 30 and 100% of
rated current and voltage. The compensation below 5% and above 100% is constant and linear in
between, see example in figure
291.
Section 8 1MRK 502 066-UUS B
Current protection
548
Technical manual
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