kind of investigation must consider also the maximum sensitivity and possible resistive reach
of distance protection devices, which on the other hand simplifies the problem.
Application of MOVs as non-linear elements for capacitor overvoltage protection makes simple
calculations often impossible. Different kinds of transient or dynamic network simulations are
in such cases unavoidable.
7.13.3.5
Distance protection
GUID-B8441469-8B23-494F-9BA8-D6A88DB2DC38 v2
Distance protection due to its basic characteristics, is the most used protection principle on
series compensated and adjacent lines worldwide. It has at the same time caused a lot of
challenges to protection society, especially when it comes to directional measurement and
transient overreach.
Distance IED in fact does not measure impedance or quotient between line current and
voltage. Quantity 1= Operating quantity - Restrain quantity Quantity 2= Polarizing quantity.
Typically Operating quantity is the replica impedance drop. Restraining quantity is the system
voltage Polarizing quantity shapes the characteristics in different way and is not discussed
here.
Distance IEDs comprise in their replica impedance only the replicas of line inductance and
resistance, but they do not comprise any replica of series capacitor on the protected line and
its protection circuits (spark gap and or MOV). This way they form wrong picture of the
protected line and all “solutions” related to distance protection of series 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).
7.13.3.6
Underreaching and overreaching schemes
GUID-3090702F-508B-480C-9AC7-2D5A82B9AD1E v1
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
en06000618.vsd
X
11
X
12
-
j X
C
A
B
D
A
D
B
Zone 1
A
Zone 1
B
Zone 2
A
Zone 2
B
G
IEC06000618 V1 EN-US
Figure 225: 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
. 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
1MRK 505 343-UEN B
Section 7
Impedance protection
347
Application manual
Summary of Contents for Relion 670 series
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