ABB RELION 670 SERIES Applications Manual page 201

1MRK 506 369-UUS -
Line distance protection REL670 2.2 ANSI
Application manual
compensated and adjacent lines are concentrated on finding some parallel ways, which
may help eliminating the basic reason for wrong measurement. The most known of them
are decrease of the reach due to presence of series capacitor, which apparently decreases
the line reactance, and introduction of permanent memory voltage in directional
measurement.
Series compensated and adjacent lines are often the more important links in a transmission
networks and delayed fault clearance is undesirable. This makes it necessary to install
distance protection in combination with telecommunication. The most common is
distance protection in Permissive Overreaching Transfer Trip mode (POTT).
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 93.
A
j X Zone 1
-
A
C
X
11
G
D
A
Zone 2
B
IEC06000618 V1 EN
Figure 93: 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 93. 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 94, which covers for possible
S
overreaching due to low frequency (sub-harmonic) oscillations. Here it should be noted
separately that compensation degree K
inclusive line and source impedance reactance. The same setting applies regardless MOV
or spark gaps are used for capacitor overvoltage protection.
Zone 2
A
B
X
12
Zone 1
B
D
B
en06000618.vsd
)
X
C
in figure
C
Impedance protection
94 relates to total system reactance,
Section 8
(Equation 66)
195