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 ground-fault overcurrent protection before its installation.
Dual side instrument transformers
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 breaker-and-a-half 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 ground fault protection.
Apparent impedances and MOV influence
Series capacitors reduce due to their character the apparent impedance measured by
distance IEDs on protected power lines. Figure
88 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_ansi.vsd
~
E
A
0%
33%
50%
66%
K
C
=
80% 33 % 33%50 %
21
100 %
80 %
X
ANSI06000612 V1 EN
Figure 206: 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
89 case K
C
=
0%.
Section 8 1MRK 506 369-UUS -
Impedance protection
408 Line distance protection REL670 2.2 ANSI
Application manual
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