Dual side instrument transformers
GUID-3B737D75-F1D0-45AB-92A1-E0AEA9A1CCFD v1
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
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Series capacitors reduce due to their character the apparent impedance measured by
distance IEDs on protected power lines. Figure
204 presents typical locations of
capacitor banks on 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.
en06000612.vsd
~
E
A
0%
33 %
50 %
66 %
K
C
=
80% 33% 33 %50 %
Z<
100 %
80 %
IEC06000612 V1 EN-US
Figure 204: 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
205 case K
C
= 0%.
en06000613.vsd
j
X
R
KC = 0%
KC = 80%
LOC = 0%
KC = 50%
LOC = 50%
j
X
j
X
R R
KC = 2 x 33%
LOC = 33%, 66%
KC = 80%
LOC = 100%
j
X
j
X
R R
IEC06000613 V1 EN-US
Figure 205: Apparent impedances seen by distance IED for different SC
locations and spark gaps used for overvoltage protection
1MRK 506 369-UEN B Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 381
Application manual
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