Distance Protection - ABB REG670 Applications Manual

Section 8
Impedance protection
8.3.3.5
240
especially if on one hand the compensated line is very long with high degree of
compensation, and the adjacent lines are, on the other hand, relatively short.
Extensive system studies are necessary before final decision is made on
implementation and location of series capacitors in network. It requires to correctly
estimate their influence on performances of (especially) existing distance IEDs. It is
possible that the costs for number of protective devices, which should be replaced by
more appropriate ones due to the effect of applied series compensation, influences the
future position of series capacitors in power network.
Possibilities for voltage inversion at remote buses should not be studied for short
circuits with zero fault resistance only. It is necessary to consider cases with higher
fault resistances, for which spark gaps or MOVs on series capacitors will not conduct
at all. At the same time this 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.

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