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.
Low frequency transients
Series capacitors introduce in power systems oscillations in currents and voltages, which
are not common in non-compensated systems. These oscillations have frequencies lower
than the rated system frequency and may cause delayed increase of fault currents, delayed
operation of spark gaps as well as, delayed operation of protective IEDs. The most obvious
difference is generally seen in fault currents. Figure 85 presents a simplified picture of a
series compensated network with basic line parameters during fault conditions. We study
the basic performances for the same network with and without series capacitor. Possible
effects of spark gap flashing or MOV conducting are neglected. The time dependence of
fault currents and the difference between them are of interest.
en06000609.vsd
~
e
G
(t)
i
L
(t)
R
L
jX
L
-jX
C
F
IEC06000609 V1 EN
Figure 85: Simplified equivalent scheme of SC network during fault conditions
We consider the instantaneous value of generator voltage following the sine wave
according to equation 56
EQUATION1904 V1 EN (Equation 56)
The basic loop differential equation describing the circuit in figure 85 without series
capacitor is presented by equation
57
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 185
Application manual
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