Siemens SIPROTEC 7SS60 Manual page 62

Functions
Summation current
matching
transformers
5-12
This circuit yields the following conditions in the differential measuring circuit for power
systems with earthing only on the consumer side:
Phase
L1
L2
L3
If an earth fault occurs in phase L2 within the protected zone, the fault characteristic
has thus only a slope of 0.33 instead of 1.
This circuit is thus not suitable for power systems with earthing only on the consumer
side.
For current summation, matching transformers of the types 4AM5120-3DA00-0AN2
for connection to transformers with a rated secondary current of 1 A (Figure 5-12) and
4AM5120-4DA00-0AN2 for connection to transformers with a rated secondary current
of 5 A (Figure 5-13) are used. These summation current matching transformers have
one secondary winding with a rated current of 100 mA, but seven input windings for
connection to the main current transformers. The numbers of turns in these windings
are such that they allow to compensate by appropriate connection circuits any
transformation ratio differences in the main transformers. In this way, for even
transformation ratios additional auxiliary CTs can be dispensed with, which makes the
input connection for the protection less complex. However, for odd transformation
ratios of the main transformers matching CTs must additionally be used to prevent
from false matching. To ensure the required degree of protection, the summation
current matching transformers are usually accommodated in cubicles (Figure 9-3).
They can also be installed in the proximity of the main current transformers.
Figure 5-12 Windings of the 4AM5120-3DA00-0AN2 summation current matching
transformers
Figure 5-13 Windings of the 4AM5120-4DA00-0AN2 summation current matching
transformers
Trip value Restraint value
I
d
0 + 6
–3 + 6
3 + 6
I
I / I
s
d
0 + 6 6/6 = 1.0
3 + 6 3/9 = 0.33
3 + 6 9/9 = 1.0
7SS60 Manual
E50417-G1176-C132-A2
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