Siemens siprotec 7SD5

2.1 General
51
7SD5 Manual
C53000-G1176-C169-1
Current transformer 10P20; 20 VA → n = 20; P
N
=20VA
The operational accuracy limit factor n' is derived from this nominal data and the actual
secondary burden P':
with
n' = Operational accuracy limit factor (actual over-current factor)
n' = Nominal accuracy limit factor of CTs (distinctive number behind P)
P
N
= Nominal CT burden [VA] at nominal current
P
i
= Internal CT burden [VA] at rated current
P' = Actually connected burden (devices + secondary lines) [VA] at
nominal current
Usually, the internal burden of current transformers is stated in the test report. If un-
known, it can be calculated roughly from the DC resistance R
i
of the secondary wind-
ing.
P
i
≈ R
i
· I
N
2
The ratio between the operational accuracy limit factor and the nominal accuracy limit
factor n'/n is set under address 251 K_ALF/K_ALF_N.
The CT error at nominal current, plus a safety margin, is set under 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 CT 5P, and
– 5 % for a CT 10P.
The CT error at nominal accuracy limit factor, plus a safety margin, is set under
address 254 E% K_ALF_N. It is derived from the number preceding the P of the trans-
former data.
Table 2-1 illustrates some usual current transformer types with their characteristic data
and the recommended settings.
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