Minimum voltage V
min
and maximum current I
max
are related to the same operating
conditions. Minimum load impedance occurs normally under emergency conditions.
As 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.
ANSI13000273 V1 EN-US (Equation 150)
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. For the case when the loop characteristic angle is less than three
times the load-impedance angle, more accurate calculations are necessary according to
equation 151.
RFFwPG Z
R R
X X
load
≤ ⋅ ⋅ ∂ −
⋅ +
⋅ +
⋅ ∂
0 8
2 1 0
2 1 0
. cos sin
min
ANSI13000274 V1 EN-US (Equation 151)
Where:
∂ is a maximum load-impedance angle, related to the maximum load power.
To avoid load encroachment for the phase-to-phase measuring elements, the set
resistive reach of any distance protection zone must be less than 160% of the minimum
load impedance.
ANSI13000275 V1 EN-US (Equation 152)
Equation 152 is applicable only when the loop characteristic angle for the phase-to-
phase faults is more than three times as large as the maximum expected load-
impedance angle. More accurate calculations are necessary according to equation 153.
1MRK 504 163-UUS A Section 8
Impedance protection
Transformer protection RET670 2.2 ANSI 281
Application manual
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