ABB REL670 2.2 IEC, Applications Manual

Page: 386 / 908

Location of instrument transformers
GUID-A8283D59-622B-4EB3-8C0A-8C886C3CF9D0 v1
Location of instrument transformers relative to the line end series capacitors plays
an important role regarding the dependability and security of a complete protection
scheme. It is on the other hand necessary to point out the particular dependence of
those protection schemes, which need for their operation information on voltage in
IED point.
Protection schemes with their operating principle depending on current
measurement only, like line current differential protection are relatively
independent on CT location. Figure
203 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.vsd
IEC06000611 V1 EN-US
Figure 203: Possible positions of instrument transformers relative to line end
series capacitor
Bus side instrument transformers
GUID-B7D1F10A-5467-4F91-9BC1-AB8906357428 v1
CT1 and VT1 on figure 203 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
205 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
GUID-3E5897C4-9A15-4A51-90C2-F619D4E083A6 v1
CT2 and VT2 on figure 203 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.
Section 8
1MRK 506 369-UEN B
Impedance protection
380 Line distance protection REL670 2.2 IEC
Application manual