Functions
2.5 Single-Phase Overcurrent Protection
SIPROTEC, 7SJ62/64, Manual
C53000-G1140-C207-2, Release date 01.2008
135
The internal burden is often stated in the test report of the current transformer. If not, it can be derived from a
DC measurement on the secondary winding.
Calculation Example:
CT 800/5; 5P10; 30 VA with R
i
= 0.3
Ω
or
CT 800/1; 5P10; 30 VA with R
i
= 5
Ω
Besides the CT data, the resistance of the longest connection lead between the CTs and the 7SJ62/64 device
must be known.
Stability with High-impedance Protection
The stability condition is based on the following simplified assumption: If there is an external fault,
one
of the
current transformers gets totally saturated. The other ones will continue transmitting their (partial) currents. In
theory, this is the most unfavorable case. Since, in practice, it is also the saturated transformer which supplies
current, an automatic safety margin is guaranteed.
Figure 2-43 shows a simplified equivalent circuit. CT1 and CT2 are assumed as ideal transformers with their
inner resistances R
i1
and R
i2
. R
a
are the resistances of the connecting cables between current transformers
and resistor R. They are multiplied by 2 as they have a forward and a return line. R
a2
is the resistance of the
longest connecting cable.
CT1 transmits current
I
1
. CT2 shall be saturated. Because of saturation the transformer represents a low-re-
sistance shunt which is illustrated by a dashed short-circuit line.
R >> (2R
a2
+ R
i2
) is a further prerequisite.
Figure 2-43
Simplified equivalent circuit of a circulating current system for high-impedance protection
That means
I
Nom
= 5 A (from 800/
5
)
ALF
= 10 (from 5P
10
)
P
Nom
=
30
VA
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