ABB REL670 2.2 IEC Applications Manual page 183

1MRK 506 369-UEN B
Line distance protection REL670 2.2 IEC
Application manual
Bus side instrument transformers
CT1 and VT1 on figure
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
87 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
CT2 and VT2 on figure
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
earth-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 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
Series capacitors reduce due to their character the apparent impedance measured by
distance IEDs on protected power lines. Figure
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
85 represent the case with bus side instrument
85 represent the case with line side instrument
86
Section 8
Impedance protection
presents typical locations of
SEMOD168320-266 v2
SEMOD168320-271 v2
SEMOD168320-277 v2
SEMOD168320-285 v2
177