Minimum voltage Umin 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.
EQUATION792 V2 EN-US (Equation 89)
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
21 0
0.8 cos sin
21 0
load
RZx R Zx
RFPEZx Z
XZx X Zx
EQUATION578 V5 EN-US (Equation 90)
Where:
ϑ is a maximum load-impedance angle, related to the minimum load impedance conditions.
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.
EQUATION579 V3 EN-US (Equation 91)
Equation 91 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
92.
RFPP 1.6 Z
loadmin
cos
J
R1PP
X1PP
---------------–
sin
J×× ×£
EQUATION580 V1 EN-US
(Equation 92)
Section 8 1MRK 506 369-UEN B
Impedance protection
200 Line distance protection REL670 2.2 IEC
Application manual
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