ABB RET670 Applications Manual page 223

1MRK504116-UUS C
Application manual
The first case corresponds also to conditions on non compensated lines and in cases,
when the capacitor is bypassed either by spark gap or by the bypass switch, as shown
in phasor diagram in figure 87. 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
> +
X X X
C S L 1
EQUATION1936 V1 EN
With bypassed
capacitor
I
F
ANSI06000608 V1 EN
Figure 87: 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
cannot be used for the protection of series compensated lines with possible current
inversion. Equation 129
series compensated lines on location of series capacitors. X
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,
129
With inserted
capacitor
I
F
en06000608_ansi.vsd
"Distance protection"
shows also big dependence of possible current inversion on
Section 3
IED application
(Equation 129)
for more details), which
= 0 for faults just behind
L1
217