ABB RET670 Applications Manual page 252

Section 3
IED application
246
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:
£ ×
RFPG 0.8 Z
load min
EQUATION1721 V2 EN
Where:
ϑ is a maximum load-impedance angle, related to the minimum load impedance conditions.
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.
é × +
2 R 1 R
× ¶ -
cos
ê
ë
× +
2 X 1 X
1MRK504116-UUS C
ù
0
× ¶
sin
ú
û
0
Application manual
(Equation 165)
(Equation 166)
(Equation 167)