ABB RELION 670 Series Applications Manual page 270

Section 7
Impedance protection
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
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
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 244.
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.
264
2
U
= ------ -
S
is the minimum phase-to-phase voltage in kV
is the maximum apparent power in MVA.
U
min
--------------------- -
×
3 I
max
and maximum current I
min
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.
×
load
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
1MRK 504 152-UEN B
(Equation 241)
(Equation 242)
(Equation 243)
(Equation 244)
Application manual