ABB RELION 670 Series Applications Manual page 330

Section 7
Impedance protection
Bus side instrument transformers
CT1 and VT1 on figure
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
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
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
power lines together with corresponding compensation degrees. Distance IED near the
feeding bus will see in different cases fault on remote end bus depending on type of
overvoltage protection used on capacitor bank (spark gap or MOV) and SC location on
protected power line.
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Figure 195: Typical locations of capacitor banks on series compensated line
Implementation of spark gaps for capacitor overvoltage protection makes the picture
relatively simple, because they either flash over or not. The apparent impedance corresponds
to the impedance of non-compensated line, as shown in figure
324
194 represent the case with bus side instrument transformers. The
194 represent the case with line side instrument transformers. The
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195 presents typical locations of capacitor banks on
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GUID-3B737D75-F1D0-45AB-92A1-E0AEA9A1CCFD v1
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