P746/EN AP/G31
Application Notes
(AP) 6-
54
MiCOM
P746
AP
11.3
Support of IEEE C Class CTs
MiCOM Px40 series protection is compatible with ANSI/IEEE current transformers as
specified in the IEEE C57.13 standard. The applicable class for protection is class “C”, which
specifies a non air-gapped core. The CT design is identical to IEC class P, or British
Standard class X, but the rating is specified differently. The following table allows C57.13
ratings to be translated into an IEC/BS knee point voltage.
IEEE C57.13 – “C” Classification (volts)
C50 C100 C200 C400 C800
CT Ratio
RCT (ohm)
V
k
V
k
V
k
V
k
V
k
100/5 0.04 56.5
109 214 424 844
200/5 0.8 60.5
113
218 428 848
400/5 0.16 68.5
121 226 436 856
800/5 0.32 84.5
137 242 452 872
1000/5 0.4 92.5
145 250 460 880
1500/5 0.6 112.5
165 270 480 900
2000/5 0.8 132.5
185 290 500 920
3000/5 1.2 172.5
225 330 540 960
TABLE 3: IEC/BS KNEE POINT VOLTAGE V
K
OFFERED BY “C” CLASS CTS
Assumptions:
1.
For 5A CTs, the typical resistance is 0.0004 ohm secondary per primary turn (for 1A
CTs, the typical resistance is 0.0025 ohm secondary per primary turn).
2.
IEC/BS knee is typically 5% higher than ANSI/IEEE knee.
Given:
1.
IEC/BS knee is specified as an internal EMF, whereas the “C” class voltage is
specified at the CT output terminals. To convert from ANSI/IEEE to IEC/BS requires
the voltage drop across the CTs secondary winding resistance to be added.
2.
IEEE CTs are always rated at 5A secondary
3.
The rated dynamic current output of a “C” class CT (Kssc) is always 20 x I
n
V
k
= (C x 1.05) + (I
n
. R
CT
. Kssc)
Where:
V
k
= Equivalent IEC or BS knee point voltage
C
=
C
Rating
I
n
=
5A
R
CT
= CT secondary winding resistance
Kssc
=
20
times
Содержание MiCOM P746
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