'(& )( )'& &) ( &!& ")& &$(( $# Ć %&( $# #)" &&#$
4 - 6
Siemens AG ⋅ May 1998
e-function with time constant = 64 ms
mod
t
Fault characteristic
k = 0.80
k = 0.65
k = 0.50
Pick-up characterĆ
istics
Normal load line
10
n
10
n
0
|āā|
The vectorial sum
=
1
+
2
... +
n
as the tripping quantity is counterbalanced, by the
restraining quantity, whereby
| |=
1
+
2
... +
n
which is the arithmetic sum of the magnitudes of
each current.
The criterion for a short-circuit on the busbar is thus:
1
+
2
... +
n
>k@ | |āā
mod
The modified stabilizing quantity | |āā
mod
is derived
from | |āā and is illustrated in Fig 4.4.
Fig 4.5 illustrates the characteristic of a stabilized (reĆ
strained) differential protection system. In the diaĆ
gram, the abscissa represents the sum | |ofthe
magnitudes of all quantities flowing through the busĆ
bar while the vectorial sum is plotted as the ordiĆ
nate. Both axes use rated current as the unit and both
have the same scale. If a short-circuit occurs on the
busbars whereby the same phase relation applies to
all infeeding currents, then = | |. The fault charĆ
acteristic is a straight line inclined at 45_.
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