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Z
load
U
min
3 I
max
×
----------------------
=
EQUATION574 V1 EN-US
(Equation 240)
Minimum voltage U
min
and maximum current Imax are related to the same operating
conditions. Minimum load impedance occurs normally under emergency conditions.
Because a safety margin is required to avoid load encroachment under three-
phase conditions and to guarantee correct healthy phase IED operation under
combined heavy three-phase load and earth faults, consider both: phase-to-
phase and phase-to-earth fault operating characteristics.
To avoid load encroachment for the phase-to-earth measuring elements, the set resistive
reach of any distance protection zone must be less than 80% of the minimum load impedance.
RFPE 0.8 Z
load
×
£
EQUATION792 V1 EN-US
(Equation 241)
This equation is applicable only when the loop characteristic angle for the single phase-to-
earth faults is more than three times as large as the maximum expected load-impedance
angle. More accurate calculations are necessary according to the equation below:
min
2
1
0
0.8
cos
sin
2
1
0
load
R
R
RFPE
Z
X
X
J
J
×
+
£
×
×
-
×
×
+
é
ù
ê
ú
ë
û
EQUATION578 V4 EN-US
(Equation 242)
Where:
ϑ
is a maximum load-impedance angle, related to the minimum load impedance conditions.
All this is applicable for all measuring zones when no power swing detection element is in the
protection scheme. Use an additional safety margin of approximately 20% in cases when a
power swing detection element is in the protection scheme, refer to the description of the
power swing detection (ZMRPSB) function.
7.5.3.8
Load impedance limitation, with load encroachment function activated
SEMOD154704-129 v1
The parameters for load encroachment shaping of the characteristic are found in the
description of the phase selection with load encroachment function, section
with load encroachment characteristic"
. If the characteristic for the impedance measurement
shall be shaped with the load encroachment algorithm, the parameter
OperationLdCmp in the
phase selection has to be switched
On.
7.5.3.9
Setting of minimum operating currents
SEMOD154704-132 v2
The operation of the distance function will be blocked if the magnitude of the currents is
below the set value of the parameter
IMinOpPE.
The default setting of
IMinOpPE is 20% of IBase where IBase is the chosen base current for the
analog input channels. The value have been proven in practice to be suitable in most of the
applications. However, there might be applications where it is necessary to increase the
1MRK 505 343-UEN B
Section 7
Impedance protection
259
Application manual
Summary of Contents for Relion 670 series
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