zone 1, it is necessary to increase the reach of the overreaching zone to at least 120% of the
protected line. The zone 2 reach can be even higher if the fault infeed from adjacent lines at
remote end is considerable higher than the fault current at the IED location.
The setting shall generally not exceed 80% of the following impedances:
•
The impedance corresponding to the protected line, plus the first zone reach of the
shortest adjacent line.
•
The impedance corresponding to the protected line, plus the impedance of the maximum
number of transformers operating in parallel on the bus at the remote end of the
protected line.
If the requirements in the bullet list above gives a zone 2 reach that gives non-selectivity
between the overreaching zone and the shortest outgoing line at the remote end, the time
delay of zone 2 must be increased by approximately 200ms to avoid unwanted operation in
cases when the telecommunication for the short adjacent line at remote end is down during
faults. The zone 2 must not be reduced below 120% of the protected line section. The whole
line must be covered under all conditions.
The requirement that the zone 2 shall not reach more than 80% of the shortest adjacent line at
remote end is highlighted in the example below.
If a fault occurs at point F (see figure
, also for the explanation of all abbreviations used),
the IED at point A senses the impedance:
Z
V
I
Z
I
I
I
Z
I
I
I
I
R
Z
I
I
Z
AF
A
A
AC
A
C
A
CF
A
C
B
A
F
AC
C
A
C
=
=
+
+
⋅
+
+
+
⋅
=
+
+
⋅
1
F
F
C
B
A
F
I
I
I
R
+
+
+
⋅
1
EQUATION302 V5 EN-US
(Equation 189)
A
B
Z<
C
I
C
Z
AC
Z
CB
Z
CF
I
A
+ I
C
IEC09000256-2-en.vsd
F
I
A
I
B
IEC09000256 V2 EN-US
Figure 144: Setting of overreaching zone
7.4.3.6
Setting of reverse zone
SEMOD154469-33 v3
The reverse zone is applicable for purposes of scheme communication logic, current reversal
logic, weak-end infeed logic, and so on. The same applies to the back-up protection of the bus
bar or power transformers. It is necessary to secure, that it always covers the overreaching
zone, used at the remote line terminal for the telecommunication purposes.
Consider the possible enlarging factor that might exist due to fault infeed from adjacent lines.
Equation
can be used to calculate the reach in reverse direction when the zone is used for
blocking scheme, weak-end infeed, and so on.
1MRK 505 343-UEN B
Section 7
Impedance protection
233
Application manual
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