The load impedance [Ω/phase] is a function of the minimum operation voltage and the
maximum load current:
EQUATION1719 V1 EN (Equation 165)
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.
EQUATION1720 V1 EN (Equation 166)
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:
min
2 1 0
0.8 cos sin
2 1 0
load
R R
RFPG Z
X X
× +
£ × × ¶ - × ¶
× +
é ù
ê ú
ë û
EQUATION1721 V2 EN (Equation 167)
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.
Section 3 1MRK504116-UUS C
IED application
246
Application manual
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