The condition for operation at phase-to-phase fault is that the angle β between the two
compensated voltages Ucomp1 and Ucomp2 is greater than or equal to 90° (figure
67). The angle will be 90° for fault location on the boundary of the circle.
The angle β for L1-to-L2 fault can be defined according to equation
38.
L1L2
L1L2
-I ZPP
arg
U-(-I ZRevPP)
U ×
×
b =
æ ö
ç ÷
è ø
EQUATION1792 V1 EN (Equation 38)
where
EQUATION1800 V1 EN
is the U
L1L2
voltage
ZRevPP is the positive sequence impedance setting for phase-to-phase fault in reverse direction
ZPPI 2L1L •
L1L2
comp1 U=U
ß
IL1L2jX
IL1L2R
-
ZPPI
2L1L
•
vPPReZ2L1LI •
U
U2Ucomp =
=
-
IF•ZF=UL1L2
en07000110.vsd
IEC07000110 V1 EN
Figure 67: Simplified offset mho characteristic and voltage vectors for phase L1-
to-L2 fault.
Operation occurs if 90≤β≤270.
Offset mho, forward direction
When forward direction has been selected for the offset mho, an extra criteria beside
the one for offset mho (90<β<270) is introduced, that is the angle φ between the
Section 7 1MRK502052-UEN B
Impedance protection
186
Technical manual
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