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 (Equation 145)
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
146.
RFFwPG Z
R R
X X
load
≤ ⋅ ⋅ ∂ −
⋅ +
⋅ +
⋅ ∂
0 8
2 1 0
2 1 0
. cos sin
min
ANSI13000274 V1 EN (Equation 146)
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 (Equation 147)
Equation 147 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
148.
RFFwPP Z
R
X
load
≤ ⋅ ⋅ ∂ − ⋅ ∂
1 6
1
1
. cos sin
min
IEC13000276 V1 EN (Equation 148)
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 247
Application manual
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