CT1 and VT1 on figure 90 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
92 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
90 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 ground-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 breaker-and-a-
half 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 ground 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
91 presents typical locations of
capacitor banks on power lines together with corresponding compensation degrees.
Section 3 1MRK504116-UUS C
IED application
222
Application manual
To Next Page
Loading...
