ABB Relion 670 series - Page 412

diagram in figure 84. The resultant reactance is in this case of inductive nature and the fault
currents lags source voltage by 90 electrical degrees.
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
55
1
> +
C S L
X X X
EQUATION1936 V1 EN (Equation 365)
en06000608_ansi.vsd
I
F
V
S
V
’
M
=
H
V
L
H
V
S
With bypassed
capacitor
I
F
H
V
L
V
S
H
V
C
V
M
H
V
S
With inserted
capacitor
ANSI06000608 V1 EN
Figure 204: 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
55
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
Section 8 1MRK 506 369-UUS -
Impedance protection
406 Line distance protection REL670 2.2 ANSI
Application manual