87Bb Sensitive Differential Earth Fault Ct Requirements; Support Of Ieee C Class Cts - GE P741 Technical Manual

P74x/EN AP/Na7
(AP) 6-64
10.3

87BB Sensitive Differential Earth Fault CT Requirements

The Sensitive Differential Earth Fault (SDEF) shall be enabled only on High Impedance
Earthed Networks.
To enable the SDEF the CTs have to comply with the additional following requirement:
Where:
V = Required knee point voltage
k
X/R = Primary system X/R ratio (for the P74x system, X/R up to 80)
I
n =
R = CT secondary resistance
CT
R = Burden resistance (with R
B
10.4

Support of IEEE C Class CTs

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,
AP
but the rating is specified differently.
translated into an IEC/BS knee point voltage.
CT Ratio
100/5
200/5
400/5
800/5
1000/5
1500/5
2000/5
3000/5
TABLE 8: IEC/BS KNEE POINT VOLTAGE V
Assumptions:
1.
2.
Given:
1.
2.
3.
V
k
V min = 1.5 x X/R x I x (R
k n
Note: If the X/R is lower than 20, chose 20 as X/R.
nominal secondary current (1A or 5A)
RCT (ohm) C50
0.04 56.5
0.8 60.5
0.16 68.5
0.32 84.5
0.4 92.5
0.6 112.5
0.8 132.5
1.2 172.5
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)
IEC/BS knee is typically 5% higher than ANSI/IEEE knee
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.
IEEE CTs are always rated at 5A secondary
The rated dynamic current output of a "C" class CT (Kssc) is always 20 x I
= (C x 1.05) + (I . R
n
+ R )
CT B
= 2 R )
B L
The following table allows C57.13 ratings to be
IEEE C57.13 – "C" Classification (volts)
C100
V V
k k
109
113
121
137
145
165
185
225
OFFERED BY "C" CLASS CTS
K
. Kssc)
CT
Application Notes
MiCOM P741, P742, P743
C200 C400
V V
k k
214 424
218 428
226 436
242 452
250 460
270 480
290 500
330 540
n
C800
V
k
844
848
856
872
880
900
920
960