ABB RELION 650 Series Applications Manual page 125

1MRK 505 363-UEN A
The maximum permissible resistive reach for any zone must be checked 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 with equation 70.
Z
loadmin
EQUATION571 V1 EN-US
Where:
U
S
The load impedance [Ω/phase] is a function of the minimum operation voltage and the
maximum load current:
Z =
load
EQUATION574 V1 EN-US
Minimum voltage U
conditions. Minimum load impedance occurs normally under emergency conditions.
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
EQUATION792 V1 EN-US
Equation
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 73.
RFPE
EQUATION578 V4 EN-US
Where:
∂
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.
Application manual
2
U
= ------ -
S
the minimum phase-to-phase voltage in kV
the maximum apparent power in MVA.
U
min
--------------------- -
×
3 I
max
and maximum current I
min
As a safety margin, it is required to avoid load encroachment under three-
phase conditions. To guarantee correct, healthy phase IED operation under
combined heavy three-phase load and earth faults, both phase-to-phase and
phase-to-earth fault operating characteristics should be considered.
×
load
72 is applicable only when the loop characteristic angle for the single phase-to-earth
é
J
£ × × -
0.8 Z cos
ê
ë
load min
is a maximum load-impedance angle, related to the maximum load power.
are related to the same operating
max
× +
ù
2 R 1 R 0
J
×
sin
ú
û
× +
2 X 1 X 0
Section 7
Impedance protection
(Equation 70)
(Equation 71)
(Equation 72)
(Equation 73)
119