ABB RELION 670 SERIES Applications Manual page 195

1MRK 506 369-UUS -
Line distance protection REL670 2.2 ANSI
Application manual
Bus side instrument transformers
CT1 and VT1 on figure 87
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
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
CT2 and VT2 on figure 87
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
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
banks on power lines together with corresponding compensation degrees. Distance IED
represent the case with bus side instrument transformers. The
89
represent the case with line side instrument transformers. The
88 presents typical locations of capacitor
Section 8
Impedance protection
LOC=0%), which requires special
189