ABB REL670 2.2 IEC, Applications Manual

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The resultant reactance is of capacitive nature in the second case. Fault current will
for this reason lead the source voltage by 90 electrical degrees, which means that
reactive current will flow from series compensated line to the system. The system
conditions are in such case presented by equation
364
1
> +
C S L
X X X
EQUATION1936 V1 EN-US
(Equation 364)
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 202: 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
"Distance protection" for more
details), which cannot be used for the protection of series compensated lines with
possible current inversion. Equation
364 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.
1MRK 506 369-UEN B
Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 379
Application manual