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.
Larger overreach than the mentioned 80% can often be acceptable due to fault current
infeed from other lines. This requires however analysis by means of fault calculations.
If any of the above gives a zone 2 reach less than 120%, 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
66, the IED at point A senses the impedance:
1 1
A C A C B C C B
A
AF AC CF AC CF
F F
A A A A A
V I I I I I I I I
Z Z Z R Z Z R
I I I I I
æ ö æ ö
+ + + +
= = + × + × = + + × + + ×
ç ÷ ç ÷
è ø è ø
EQUATION302 V4 EN (Equation 49)
A
B
21
C
I
A
IB
Z
AC
Z
CB
Z
CF
I
A+
I
B
ANSI05000457-2-en.vsd
F
ANSI05000457 V2 EN
Figure 66: Setting of overreaching zone
Setting of reverse zone
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 IED for the telecommunication
purposes.
1MRK 506 334-UUS A Section 6
Impedance protection
133
Application manual
To Next Page
Loading...
