ABB RELION RET670, Applications Manual

Page: 228 / 1016

Figure 98 shows schematically the possible locations of instrument transformers
related to the position of line-end series capacitor.
-jX
C
CT 1 CT 2
VT1 VT 2
en06000611_ansi.vsd
ANSI06000611 V1 EN-US
Figure 98: Possible positions of instrument transformers relative to line end series
capacitor
Bus side instrument transformers
SEMOD168320-266 v2
CT1 and VT1 on figure 98 represent the case with bus side instrument transformers.
The protection devices are in this case exposed to possible voltage and current
inversion for line faults, which decreases the required dependability. In addition to this
may series capacitor cause negative apparent impedance to distance IEDs on protected
and adjacent lines as well for close-in line faults (see also figure
100 LOC=0%), which
requires special design of distance measuring elements to cope with such phenomena.
The advantage of such installation is that the protection zone covers also the series
capacitor as a part of protected power line, so that line protection will detect and
cleared also parallel faults on series capacitor.
Line side instrument transformers
SEMOD168320-271 v2
CT2 and VT2 on figure 98 represent the case with line side instrument transformers.
The protective devices will not be exposed to voltage and current inversion for faults
on the protected line, which increases the dependability. Distance protection zone 1
may be active in most applications, which is not the case when the bus side instrument
transformers are used.
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 ground-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
Section 8
1MRK 504 163-UUS A
Impedance protection
222 Transformer protection RET670 2.2 ANSI
Application manual