Underreaching And Overreaching Schemes - ABB REG670 Applications Manual

1MRK 502 071-UEN -
8.3.3.6
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual

Underreaching and overreaching schemes

It is a basic rule that the underreaching distance protection zone should under no
circumstances overreach for the fault at the remote end bus, and the overreaching zone
should always, under all system conditions, cover the same fault. In order to obtain
section selectivity, the first distance (underreaching) protection zone must be set to a
reach less than the reactance of the compensated line in accordance with figure 120.
A
Zone 1
j X
-
A
C
X
11
G
D
A
Zone 2
B
IEC06000618 V1 EN
Figure 120: Underreaching (Zone 1) and overreaching (Zone 2) on series
compensated line
The underreaching zone will have reduced reach in cases of bypassed series capacitor,
as shown in the dashed line in figure 120. The overreaching zone (Zone 2) can this way
cover bigger portion of the protected line, but must always cover with certain margin
the remote end bus. Distance protection Zone 1 is often set to
(
= × + -
X K X X
Z 1 S 11 12
EQUATION1914 V1 EN
Here K is a safety factor, presented graphically in figure 121, which covers for
S
possible overreaching due to low frequency (sub-harmonic) oscillations. Here it
should be noted separately that compensation degree K
system reactance, inclusive line and source impedance reactance. The same setting
applies regardless MOV or spark gaps are used for capacitor overvoltage protection.
Equation 116 is applicable for the case when the VTs are located on the bus side of
series capacitor. It is possible to remove X
in line side, but it is still necessary to consider the safety factor K
If the capacitor is out of service or bypassed, the reach with these settings can be less
than 50% of protected line dependent on compensation degree and there will be a
section, G in figure 120, of the power line where no tripping occurs from either end.
Zone 2
A
B
X
12
Zone 1
B
D
B
en06000618.vsd
)
X
C
from the equation in cases of VTs installed
C
Section 8
Impedance protection
(Equation 116)
in figure 121 relates to total
C
.
S
241