Siemens SIPROTEC 7SS60 Manual page 58

Functions
Figure 5-5 Block diagram of the 7SS60 system with summation current transformer
Increased earth
current sensitivity
5-8
The following sections deal with circuit examples for the version with summation
current transformers.
The input current transformer ("summation current transformer") has several primary
windings and one secondary winding. The primary windings have different numbers
of turns, which allows to connect the three phase currents with a ratio of e.g. 2 : 1 : 3.
Depending on the station operating conditions and the requirements that the
protection must meet, there are two different circuits for connecting the summation
current transformer to the main transformers.
Figure 5-6 shows that the windings with the ratio of turns 2 ("ratio-2 winding") and 1
("ratio-1 winding") are each connected to one of the three phase currents, and the third
winding (the "ratio-3 winding") is connected to the neutral conductor.
This is the most frequently used circuit. It is used in power systems where system
faults involving earth currents are likely to induce relatively small fault currents. One
of their characteristics is that the fault detection thresholds for 2-phase short-circuits
have a ratio of 1 : 2. With any other winding ratio than 1 : 2 between the windings in
the phases, the fault detection thresholds would differ by more than the ratio 1 : 2. This
ratio is thus an optimum. In comparison with the fault-free, symmetrical 3-phase
current, the ratio is 2 / √3 = 1.15.
A second characteristic of this connection circuit is its increased sensitivity to earth
faults, which is due to the connection of the "ratio-3 winding" to the earth current path.
The ratio of turns 2 : 1 : 3 (sequence of the currents L1, L3, N) yields, in the connection
circuit shown in Figure 5-6, the effective number of turns and fault detection thresholds
listed in the following table.
Differential Current
Restraint Current
R
E50417-G1176-C132-A2
7SS60 Manual