ABB RELION 670 SERIES Applications Manual page 219

1MRK 506 369-UUS -
Line distance protection REL670 2.2 ANSI
Application manual
2
V
=
Z
loa d min
S
EQUATION1718 V1 EN
Where:
V 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:
V
=
min
Z
loa d
×
3 I
ma x
EQUATION1719 V1 EN
Minimum voltage Vmin 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 ground faults,
consider both: phase-to-phase and phase-to-ground fault operating
characteristics.
To avoid load encroachment for the phase-to-ground measuring elements, the set resistive
reach of any distance protection zone must be less than 80% of the minimum load
impedance.
£ ×
RFP G 0.8 Z
loa d
EQUATION1720 V1 EN
This equation is applicable only when the loop characteristic angle for the single phase-to-
ground 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:
Section 8
Impedance protection
(Equation 90)
(Equation 91)
(Equation 92)
213