ABB RELION 670 SERIES Applications Manual page 413

1MRK 506 369-UUS -
Line distance protection REL670 2.2 ANSI
Application manual
impedance (a number of power transformers connected in parallel) must be considered as
practical possibility in many modern networks.
Location of instrument transformers
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 87 shows schematically the possible locations of instrument transformers related
to the position of line-end series capacitor.
- jX
C
CT 1
VT1
ANSI06000611 V1 EN
Figure 205: Possible positions of instrument transformers relative to line end series
capacitor
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
CT 2
VT 2
en06000611_ansi.vsd
represent the case with bus side instrument transformers. The
represent the case with line side instrument transformers. The
Section 8
Impedance protection
89 LOC=0%), which requires special
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