ABB Relion REG670, Applications Manual

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CT 1 CT 2
VT1 VT 2
en06000611.vsd
IEC06000611 V1 EN
Figure 114: Possible positions of instrument transformers relative to line end
series capacitor
Bus side instrument transformers
CT1 and VT1 on figure 114 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
116
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 114 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 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
1MRK 502 071-UEN -
Section 8
Impedance protection
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 235
Application manual