EQUATION1781-ANSI V1 EN (Equation 220)
Minimum voltage V
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 ground faults,
consider both: phase-to-phase and phase-to-ground fault operating
characteristics.
To avoid load encroachment for the phase-to-ground measuring elements, the set resistive
reach of any distance protection zone must be less than 80% of the minimum load
impedance.
EQUATION792 V2 EN (Equation 221)
This equation is applicable only when the loop characteristic angle for the single phase-to-
ground 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
21 0
0.8 cos sin
21 0
load
RZx R Zx
RFPEZx Z
XZx X Zx
EQUATION578 V5 EN (Equation 222)
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, 78) function.
8.5.3.8 Load impedance limitation, with load encroachment function activated
The parameters for load encroachment shaping of the characteristic are found in the
description of the phase selection with load encroachment function, section "Resistive
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 299
Application manual
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