ABB RET670 Applications Manual page 224

Section 3
IED application
218
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
88 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.
R jX
L L
i (t)
L
~
e (t)
G
en06000609.vsd
IEC06000609 V1 EN
Figure 88: Simplified equivalent scheme of SC network during fault conditions
We consider the instantaneous value of generator voltage following the sine wave
according to equation 130
( )
w l
= × × +
e E sin t
G G
EQUATION1904 V1 EN
The basic loop differential equation describing the circuit in figure
capacitor is presented by equation
di
× + × = ×
L L R i E
L L L G
dt
EQUATION1905 V1 EN
The solution over line current is presented by group of equations
-jX
C
F
131
( )
w l
× +
sin t
1MRK504116-UUS C
(Equation 130)
88 without series
(Equation 131)
132
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