reach for any zone to ensure that there is a sufficient setting margin between the boundary
and the minimum load impedance. The minimum load impedance (Ω/phase) is calculated as:
Z
loadmin
U
2
S
-------
=
EQUATION571 V1 EN-US
(Equation 285)
Where:
U
is the minimum phase-to-phase voltage in kV
S
is the maximum apparent power in MVA.
The load impedance [Ω/phase] is a function of the minimum operation voltage and the
maximum load current:
Z
load
U
min
3 I
max
×
----------------------
=
EQUATION574 V1 EN-US
(Equation 286)
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.
RFPE 0.8 Z
load
×
£
EQUATION792 V1 EN-US
(Equation 287)
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
min
2
1
0
0.8
cos
sin
2
1
0
load
R
R
RFPE
Z
X
X
J
J
×
+
£
×
×
-
×
×
+
é
ù
ê
ú
ë
û
EQUATION578 V4 EN-US
(Equation 288)
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.
1MRK 505 343-UEN B
Section 7
Impedance protection
283
Application manual
Summary of Contents for Relion 670 series
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