I
rated
Secondary rated current of the current transformer
n Rated overcurrent factor
Rated current, rated power, and overcurrent factor are found on the name plate of the transformer.
EXAMPLE
Current transformer with the following data on the name plate: 800/5; 5P10; 30 VA
You can read the following transformer data with this data:
I
rated
= 5 A (out of 800/5)
n = 10 (out of 5P10)
P
rated
= 30 VA
The internal resistance is frequently to be found in the test report of the transformer. If it is not known, it can
be approximately determined by a direct current measurement at the secondary winding.
EXAMPLE
Calculation of the knee-point voltage
Current transformer 800/5; 5P10; 30 VA with R
i
= 0.3 Ω
[foukp5aw-310113-01.tif, 1, en_US]
Current transformer 800/1; 5P10; 30 VA with R
i
= 5 Ω
[foukp1aw-310113-01.tif, 1, en_US]
Besides the current-transformer data, the resistance of the longest connection line between transformer and
device must be known.
Stability Consideration for High-Impedance Restricted Ground-Fault Protection
The stability condition is based on the simplified assumption that one current transformer is completely satu-
rated and the others transfer their partial currents proportionately in the case of an external fault. This is theo-
retically the worst case. A safety margin is automatically provided, since, in practice, even the saturated trans-
former still delivers some current.
Figure 6-107 shows an equivalent circuit of this simplification. CT1 and CT2 are assumed to be ideal trans-
formers with their internal resistances R
i1
and R
i2
. R
a
are the core resistances of the connection lines between
transformer and resistance R; they are used doubled (forward line and return line). R
a2
is the resistance of the
longest connection line.
CT1 transmits the current I1. CT2 is assumed to be saturated. This is indicated by the dotted short-circuit line.
The transformer thus represents a low-impedance shunt by its saturation.
A further prerequisite is R >> (2R
a2
+ R
i2
).
Protection and Automation Functions
6.12 Overcurrent Protection, 1-Phase
538 SIPROTEC 5, Overcurrent Protection, Manual
C53000-G5040-C017-8, Edition 07.2017
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