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3 zone impedance protection (Z(n)RW)
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A resistance and reactance calculation with a complex power comparison is performed
between the circles according to the formula:
*
2
⋅
<
(
⋅
) I
⋅
S
=
U I
R
+
j X
set
set
(Equation 20)
A counter tracks how many times the set operate conditions are fulfilled. This is done
to eliminate the risk for unwanted/faulty operation due to transient measurement uncer-
tainty. When the total exceeds a level that corresponds to 2/3 periods, a memory ele-
ment is activated to mark that the measured impedance is low.
The slope for the directional lines is calculated in the same way, but by using memory
voltage. As the phase position of the voltage is available for a further period during the
fault, a selective determination of direction can be made, even if the fault voltage is very
low. If the fault voltage remains low and the fault current exceeds the lowest operate
current, tripping is completed if the impedance is low. If the fault is in the reverse di-
rection, the reverse directional measuring element will remain functional.
63
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