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1.2
58
Distance protection (ZM)
Functionality
Separate digital signal processors calculate the impedance as seen for different measuring loops
in different distance protection zones. The results are updated each millisecond, separately for
all measuring loops and each distance protection zone. Measurement of the impedance for each
loop follows the differential equation, which considers complete line replica impedance, as pre-
sented schematically in
figure
X
Δi t ( )
l
⋅
⋅
u t ( )
(
) i t ( )
R
+
R
---- -
=
+
- --- --- --- - -
ω
l
f
R
l
i(t)
u(t)
Where:
R
= line resistance
l
R
= fault resistance
f
X
= line reactance
l
ω
= 2 π f
f
= frequency
Figure 16:
Schematic presentation of impedance measuring principle.
Settings of all line parameters, such as positive sequence resistance and reactance as well as ze-
ro-sequence resistance and reactance, together with expected fault resistance for phase-to-phase
and phase-to-earth faults, are independent for each zone. The operating characteristic is thus au-
tomatically adjusted to the line characteristic angle, if the simplified operating characteristic has
not been especially requested. The earth-return compensation factor for the earth-fault measure-
ment is calculated automatically by the terminal itself.
Voltage polarization for directional measurement uses continuous calculation and updating of
the positive sequence voltage for each measuring loop separately. This secures correct direction-
ality of the protection at different evolving faults within the complex network configurations. A
memory retaining the pre-fault positive-sequence voltage secures reliable directional operation
at close-up three-phase faults.
16.
Δt
jX
l
R
f
98000063.vmf
Chapter 4
Line distance
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