ABB REL670 2.2 IEC, Applications Manual

Page: 264 / 908

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 181)
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 182)
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 132 . 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.
Section 8
1MRK 506 369-UEN B
Impedance protection
258 Line distance protection REL670 2.2 IEC
Application manual