Line Differential Protection; Principle Of Differential Protection; Stabilized Differential Protection; Figure 4.4 Basic Principle Of Differential Protection - Siemens SIPROTEC 7SD600 System Manual

Numerical line differential protection SIPROTEC 7SD600 V3.0
4.3

Line differential protection

The line differential protection 7SD600 operates ac
cording to the current comparison principle. To
achieve this, a device must be installed at each end of
the protected line. The two 7SD600 devices must be
linked to one another by pilot wires. Thus, the diffe
rential protection system comprises the two 7SD600
devices installed at each line end, a summation trans
former and the pilot wires linking the devices.
4.3.1

Principle of differential protection

Differential protection is based on a current compari
son which is why it is also known as current compari
son protection. This utilizes the fact that, in the un
faulted state, a line L (see Figure 4.4) carries the
same current I (dotted line) at both ends. The current
flows in at one side of the zone under consideration
and leaves again at the other side. A current diffe
rence is a definite sign of a fault within the line. Provi
ding they have the same ratio, the secondary win
dings of the current transformers W1 and W2 at the
line ends can be linked to produce a closed electric
circuit with the secondary current i such that, in an
unfaulted state, a measuring element M in the cross
connection link carries no current. In the event of a
fault in the area defined by the current transformers,
the measuring element carries a current i
is proportional to the fault currents I
from both sides.
The simple configuration shown in Figure 4.4 there
fore ensures reliable operation of the protection in the
I
Station 1
W1
i
i
1

Figure 4.4 Basic principle of differential protection

Siemens AG January 1999
+ i which
1 2
+ I flowing in
1 2
I
1
I +I
1 2
M
Method of operation
System manual
event of a short-circuit in the protected zone if the
fault current flowing is sufficient to pick-up the mea
suring element M.
4.3.2

Stabilized differential protection

If external faults result in large currents flowing
through the protected zone, any transformation diffe
rences in the saturation region of current transfor
mers W1 and W2 will cause a differential current to
flow in the measuring element M which could cause
tripping. Stabilization prevents this type of protection
malfunction.
In the case of differential protection for protected
plants with two ends, stabilization is achieved by
using the sum of the magnitudes |I
res the formation of
a geometrical sum at each line end and the formation
of the sum of the magnitudes of the currents flowing
at the two ends. This serves to define:
a tripping or differential current
I = |I + I |
diff 1 2
and a stabilizing current
I = |I | + |I |
stab 1 2
I in the tripping sense, while I
diff
processing of these currents is described in section
4.3.4.
L
i +i
1 2
4
Order no. E50417-G1176-C069-A1
| + |I |. This requi
1 2
opposes this. The
stab
I I
2 Station 2
W2
i
i
2
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