EQUATION571 V1 EN (Equation 72)
Where:
U the minimum phase-to-phase voltage in kV
S the maximum apparent power in MVA.
The load impedance [Ω/phase] is a function of the minimum operation voltage and the
maximum load current:
Z
load
U
min
3 I
max
×
----------------------
=
EQUATION574 V1 EN (Equation 73)
Minimum voltage U
min
and maximum current I
max
are related to the same operating
conditions. Minimum load impedance occurs normally under emergency conditions.
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.
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.
EQUATION792 V2 EN (Equation 74)
Equation 74 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
75.
min
21 0
0.8 cos sin
21 0
load
RZx R Zx
RFPEZx Z
XZx XZx
EQUATION578 V5 EN
(Equation 75)
Where:
∂ is a maximum load-impedance angle, related to the maximum load power.
Section 8 1MRK 502 071-UEN -
Impedance protection
208 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
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