GE P741

P74
x/EN AP/Na7
Application Notes
(AP) 6
-64 MiCOM P74
1, P742, P743
AP
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:
V
k
min = 1.5 x X/R x I
n
x (R
CT
+ R
B
)
Note: If the X/R is lower than 20, chose 20 as X/R.
Where:
V
k
= Required knee point voltage
X/R = Primary system X/R ratio (for the P74x system, X/R up to 80)
I
n =
nominal secondary current (1A or 5A)
R
CT
= CT secondary resistance
R
B
= Burden resistance (with R
B
= 2 R
L
)
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,
but the rating is specified differently. The following table allows C57.13 ratings to be
translated into an IEC/BS knee point voltage.
CT Ratio RCT (ohm)
IEEE C57.13 – “C” Classification (volts)
C50 C100 C200 C400 C800
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 8: 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)