ABB RET670 Applications Manual page 285

1MRK504116-UUS C
Application manual
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:
Z
£ ×
Load
ZPE 1.6
- b
2(1 cos( ))
EQUATION1604 V1 EN
where:
Z = magnitude of minimum load impedance
load
= 180°-2·g =180°–2(ArgPE-Q
jPE
The formula is derived by trigonometric analyze of the figure 125. 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.
jX
rc
?
ß
O
IEC06000406 V1 EN
Figure 125: Definition of the setting condition to avoid load encroachment for
ground-fault loop
)
Load
ZPE/2
(Ref)
φ
F
ArgLd
Load
IED application
R
Ohm/phase
en06000406.vsd
Section 3
(Equation 196)
279