Functions
2.1 General
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
50
The rated accuracy limit factor n of the current transformers and the rated power P
N
are usually stated on the
rating plate of the current transformers. The information refers to nominal conditions (nominal current, nominal
burden). For example (according to VDE 0414 / Part 1 or IEC 60044)
Current transformer 10P10; 30 VA → n = 10; P
N
=30VA
Current transformer 10P20; 20 VA → n = 20; P
N
=20VA
The operational accuracy limit factor n' is derived from these rated data and the actual secondary burden P':
With
n' = operational accuracy limit factor (effective overcurrent factor)
n = rated accuracy limit factor of the current transformers (distinctive number behind P)
P
N
= rated burden of the current transformers [VA] at rated current
P
i
= internal burden of the current transformers [VA] at rated current
P' = actually connected burden (devices + secondary lines) [VA] at rated current
Usually, the internal burden of the current transformers is stated in the test report. If it is unknown, it can be
roughly calculated from the DC resistance R
i
of the secondary winding.
P
i
≈ R
i
· I
N
2
The ratio between operational accuracy limit factor and rated accuracy limit factor n'/n is set at address 251
K_ALF/K_ALF_N.
The CT error at rated current, plus a safety margin, is set at address 253 E% ALF/ALF_N. It is equal to the
„current measuring deviation for primary nominal current intensity F1“ according to VDE 0414 / Part 1 or IEC
60044. It is
– 3 % for a 5P transformer,
– 5 % for a 10P transformer.
The CT error at rated accuracy limit factor, plus a safety margin, is set at address 254 E% K_ALF_N. It is
derived from the number preceding the P of the transformer data.
Table 2-1 illustrates some usual protective current transformer types with their characteristic data and the rec-
ommended settings.
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