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en06000608.vsd
I
F
U
S
U
’
M
=
H
U
L
H
U
S
With bypassed
capacitor
I
F
H
U
L
U
S
H
U
C
U
M
H
U
S
With inserted
capacitor
IEC06000608 V1 EN-US
Figure 218: Phasor diagrams of currents and voltages for the bypassed and inserted
series capacitor during current inversion
It is a common practice to call this phenomenon current inversion. Its consequences on
operation of different protections in series compensated networks depend on their operating
principle. The most known effect has current inversion on operation of distance IEDs (as
shown in section
for more details), which cannot be used for the
protection of series compensated lines with possible current inversion. Equation
shows
also big dependence of possible current inversion on series compensated lines on location of
series capacitors. X
L1
= 0 for faults just behind the capacitor when located at line IED and only
the source impedance prevents current inversion. Current inversion has been considered for
many years only a theoretical possibility due to relatively low values of source impedances (big
power plants) compared to the capacitor reactance. The possibility for current inversion in
modern networks is increasing and must be studied carefully during system preparatory
studies.
The current inversion phenomenon should not be studied only for the purposes of protection
devices measuring phase currents. Directional comparison protections, based on residual
(zero sequence) and negative sequence currents should be considered in studies as well.
Current inversion in zero sequence systems with low zero sequence source impedance (a
number of power transformers connected in parallel) must be considered as practical
possibility in many modern networks.
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
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 219: Possible positions of instrument transformers relative to line end series
capacitor
Section 7
1MRK 505 343-UEN B
Impedance protection
342
Application manual
Содержание Relion 670 series
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