The load impedance [Ω/phase] is a function of the minimum operation voltage and the
maximum load current:
EQUATION1781-ANSI V1 EN-US (Equation 189)
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-US (Equation 190)
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-US (Equation 191)
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 mar
gin 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.
1MRK 504 163-UUS A Section 8
Impedance protection
Transformer protection RET670 2.2 ANSI 311
Application manual
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