EQUATION1753-ANSI V1 EN (Equation 183)
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:
EQUATION1754-ANSI V1 EN (Equation 184)
Minimum voltage V
min
and maximum current I
max
are related to the same operating
conditions. Minimum load impedance occurs normally under emergency conditions.
To avoid load encroachment for the phase-to-ground measuring elements, the set
impedance reach of any distance protection zone must be less than 80% of the minimum
load impedance.
For setting of the ground-fault loop, the following formula can be used:
ZPG
Z
LOAD
≤ ⋅
−
( )
( )
1 6
2 1
.
cos
β
EQUATION1604-ANSI V1 EN (Equation 185)
where:
Z
load
= magnitude of minimum load impedance
ZPG =
180°-2·g =180°–2(ArgPG-Q
Load
)
The formula is derived by trigonometric analyze of the figure 134. The length of the vector
from the origin O to the point F on the circle is defined by the law of cosine. The result
gives the maximum diameter (RFPE) for which the load impedance touch the circle with
the given load condition. Use an extra margin of 20% to give sufficient distance between
the calculated minimum load impedance and relay boundary.
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 275
Application manual
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