Siemens siprotec SJ62 User Manual page 131

2 Functions
Stability with High-
impedance Protec-
tion
129
The rated current, rated power and accuracy limit factor are normally stated on the
rating plate of the current transformer, e.g.
Current transformer 800/5; 5P10; 30 VA
That means
= 5 A (from 800/5)
I
Nom
ALF = 10 (from 5P10)
P = 30 VA
Nom
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
or
CT 800/1; 5P10; 30 VA with R
Besides the CT data, the resistance of the longest connection lead between the CTs
and the 7SJ62/63/64 device must be known.
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 2-41 shows a simplified equivalent circuit. CT1 and CT2 are assumed as ideal
transformers with their inner resistances R
connecting cables between current transformers and resistor R. They are multiplied
by 2 as they have a forward and a return line. R
necting cable.
CT1 transmits current I
er represents a low-resistance shunt which is illustrated by a dashed short-circuit line.
R >> (2R + R ) is a further prerequisite.
a2 i2
Figure 2-41 Simplified equivalent circuit of a circulating current system for high-impedance
protection
= 0.3 Ω
i
= 5 Ω
i
and R
i1
. CT2 shall be saturated. Because of saturation the transform-
1
. R are the resistances of the
i2 a
is the resistance of the longest con-
a2
SIPROTEC 4, 7SJ62/63/64 Handbuch
C53000-G1140-C147-A, Edition 07.2015