Minimum voltage U
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 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 264)
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. 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
265.
min
21 0
0.8 cos sin
21 0
load
RZx R Zx
RFPEZx Z
XZx X Zx
EQUATION578 V5 EN-US (Equation 265)
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.
EQUATION579 V3 EN-US
(Equation 266)
Equation 266 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
267.
load min
R1Zx
RFPPZx 1.6 Z cos sin
X1Zx
IECEQUATION2307 V2 EN-US
(Equation 267)
1MRK 506 369-UEN B Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 311
Application manual
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