Distance IEDs are exposed especially to voltage inversion for close-in reverse faults, which
decreases the security. The effect of negative apparent reactance must be studied seriously in
case of reverse directed distance protection zones used by distance IEDs for teleprotection
schemes. Series capacitors located between the voltage instruments transformers and the
buses reduce the apparent zero sequence source impedance and may cause voltage as well as
current inversion in zero sequence equivalent networks for line faults. It is for this reason
absolutely necessary to study the possible effect on operation of zero sequence directional
earth-fault overcurrent protection before its installation.
Dual side instrument transformers
SEMOD168320-277 v2
Installations with line side CT2 and bus side VT1 are not very common. More common are
installations with line side VT2 and bus side CT1. They appear as de facto installations also in
switchyards with double-bus double-breaker and 1½ breaker arrangement. The advantage of
such schemes is that the unit protections cover also for shunt faults in series capacitors and at
the same time the voltage inversion does not appear for faults on the protected line.
Many installations with line-end series capacitors have available voltage instrument
transformers on both sides. In such case it is recommended to use the VTs for each particular
protection function to best suit its specific characteristics and expectations on dependability
and security. The line side VT can for example be used by the distance protection and the bus
side VT by the directional residual OC earth fault protection.
Apparent impedances and MOV influence
SEMOD168320-285 v2
Series capacitors reduce due to their character the apparent impedance measured by distance
IEDs on protected power lines. Figure
presents typical locations of capacitor banks on
power lines together with corresponding compensation degrees. Distance IED near the
feeding bus will see in different cases fault on remote end bus depending on type of
overvoltage protection used on capacitor bank (spark gap or MOV) and SC location on
protected power line.
en06000612.vsd
~
E
A
0%
33 %
50 %
66 %
K
C =
80%
33%
33 %
50 %
Z<
100 %
80 %
IEC06000612 V1 EN-US
Figure 101: Typical locations of capacitor banks on series compensated line
Implementation of spark gaps for capacitor overvoltage protection makes the picture
relatively simple, because they either flash over or not. The apparent impedance corresponds
to the impedance of non-compensated line, as shown in figure
case K
C
= 0%.
en06000613.vsd
jX
R
KC = 0%
KC = 80%
LOC = 0%
KC = 50%
LOC = 50%
jX
jX
R
R
KC = 2 x 33%
LOC = 33%, 66%
KC = 80%
LOC = 100%
jX
jX
R
R
IEC06000613 V1 EN-US
Figure 102: Apparent impedances seen by distance IED for different SC locations and
spark gaps used for overvoltage protection
Section 7
1MRK 505 343-UEN B
Impedance protection
170
Application manual
Содержание Relion 670 series
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