ABB Relion 670 series - Distance Protection; Underreaching and Overreaching Schemes

8.13.3.5 Distance protection
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).
8.13.3.6 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.
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 413
Application manual