7.4.3.7
Load impedance limitation, without load encroachment function
SEMOD154469-91 v5
The following instruction is valid when the load encroachment function or blinder function is
not activated (
BlinderMode=Off).The load encroachment function will not be activated if
RLdFw and RLdRv is set to a value higher than expected minimal load impedance. If the load
encroachment or blinder function is to be used for all or some of the measuring zones, the
load limitation for those zones according to this chapter can be omitted. Check the maximum
permissible resistive reach for any zone to ensure that there is a sufficient setting margin
between the relay boundary and the minimum load impedance. The minimum load impedance
(Ω/phase) is calculated as:
Z
loadmin
U
2
S
-------
=
EQUATION571 V1 EN-US
(Equation 203)
Where:
U
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:
Z
load
U
min
3 I
max
×
----------------------
=
EQUATION574 V1 EN-US
(Equation 204)
Minimum voltage U
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-earth 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 earth-fault loop, the following formula can be used:
Load
Z
ZPE 1.6
2(1 cos( ))
b
£
×
-
EQUATION1604 V1 EN-US
(Equation 205)
where:
Z
load
= magnitude of minimum load impedance
ZPE
= 180°-2·
g
=180°–2(ArgPE-
Q
Load
)
The formula is derived by trigonometric analyze of the figure
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 505 343-UEN B
Section 7
Impedance protection
239
Application manual
Summary of Contents for Relion 670 series
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