Siemens SIPROTEC 7SS523 V3.2 Manual page 111

Standard
characteristic
Figure 5-3 Standard characteristic
The vectorial sum I as the tripping quantity
d
I = | I + I ... + I
|
d 1 2 n
is counterbalanced by the restraining quantity I
I = | I | + | I | ... + | I
s 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:
.
I > k I
d s, mod
The modified stabilizing quantity I
Figure 5-2, page 94.
Figure 5-3, page 95 illustrates the characteristic of a stabilized (restrained) differential
protection system. In the diagram, the abscissa represents the sum I
nitudes of all quantities flowing through the busbar while the vectorial sum I
as the ordinate. 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, I is equal to I
d
45°.
Any difference in phase relation of the fault currents leads to a (practically insignifi-
cant) lowering of the fault characteristic. Since in fault-free operation I
approximately zero, the x-axis may be referred to as the normal load line. The stabi-
lizing factors can be selected in a range of k = 0.10 to 0.80 for the bus zone-specific
busbar protection or k = 0.00 to 0.80 for the check zone. The factors are represented
as three straight lines with corresponding gradient and form the operating character-
istic. The differential protection system determines whether the total of all currents
supplied by the current transformers represents a point in the diagram above or below
the set characteristic line. If the point lies above that line, tripping is initiated.
|
is derived from I
s, mod
. The fault characteristic is a straight line inclined at
s
C53000-G1176-C182-1
Busbar Protection
s
and is illustrated in
s
s, mod
7SS52 V4 Manual
of the mag-
is plotted
d
is
d
95