The second part in the parentheses is the error introduced to the measurement of
the line impedance.
If the current on the parallel line has negative sign compared to the current on the
protected line, that is, the current on the parallel line has an opposite direction
compared to the current on the protected line, the distance function will overreach.
If the currents have the same direction, the distance protection will underreach.
Maximum overreach will occur if the fault current infeed from remote line end is
weak. If considering a single phase-to-earth fault at 'p' unit of the line length from
A to B on the parallel line for the case when the fault current infeed from remote
line end is zero, the voltage U
A
in the faulty phase at A side as in equation
112.
U p ZI I K I K I
A
L ph N Nm p
= ⋅ + ⋅ + ⋅
( )
3 3
0 0
IECEQUATION1278 V2 EN-US (Equation 112)
One can also notice that the following relationship exists between the zero
sequence currents:
3 0 3 0 0 2
0
I Z I Z p
L p L
⋅ = ⋅ −
( )
EQUATION1279 V3 EN-US (Equation 113)
Simplification of equation 113, solving it for 3I0p and substitution of the result into
equation
112 gives that the voltage can be drawn as:
U p ZI I K I K
I p
p
A
L ph N Nm
= ⋅ + ⋅ + ⋅
⋅
−
3
3
2
0
0
IECEQUATION1280 V2 EN-US (Equation 114)
If we finally divide equation 114 with equation 109 we can draw the impedance
present to the IED as
Z p ZI
I KN I KN
I p
p
I I KN
L
ph m
ph
= ⋅
+ ⋅ + ⋅
⋅
−
+ ⋅
3
3
2
3
0
0
0
EQUATION1379 V3 EN-US (Equation 115)
Calculation for a 400 kV line, where we for simplicity have excluded the
resistance, gives with X1L=0.303 Ω/km, X0L=0.88 Ω/km, zone 1 reach is set to
90% of the line reactance p=71% that is, the protection is underreaching with
approximately 20%.
The zero sequence mutual coupling can reduce the reach of distance protection on
the protected circuit when the parallel line is in normal operation. The reduction of
Section 8 1MRK 506 369-UEN B
Impedance protection
220 Line distance protection REL670 2.2 IEC
Application manual
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