Areva MiCOM P125, Technical Manual

Page: 271 / 362

Application Guide P12y/EN AP/D32
MiCOM P125/P126 & P127 Page 73/74
12. CT REQUIREMENTS
The CT requirements for the MiCOM P12y relays are given below.
The current transformer requirements are based on a maximum prospective fault current of
50 times the relay rated current (In) and the relay having an instantaneous setting of 25
times rated current (In). The current transformer requirements are designed to provide
operation of all protection elements.
Where the criteria for a specific application are in excess of those detailed above, or the
actual lead resistance exceeds the limiting value quoted, the CT requirements may need to
be increased according to the formulae in the following sections.
Nominal Rating Nominal
Output Accuracy
Class Accuracy Limit
Factor Limiting lead
resistance
1A 2.5VA 10P 20 1.3 ohms
5A 7.5VA 10P 20 0.11 ohms
12.1 Definite time / IDMT overcurrent & earth fault protection
Time-delayed Phase overcurrent elements :
V
K
≥
I
cp
/2 * (R
CT
+ R
L
+ R
rp
)
Time-delayed Earth Fault overcurrent elements :
V
K
≥
I
cn
/2 * (R
CT
+ 2R
L
+ R
rp
+ R
rn
)
12.2 Instantaneous overcurrent & earth fault protection
CT requirements for instantaneous phase overcurrent elements :
V
K
≥
I
sp
/2 * (R
CT
+ R
L
+ R
rp
)
CT requirements for instantaneous earth fault overcurrent elements :
V
K
≥
I
sn
/2 * (R
CT
+ 2R
L
+ R
rp
+ R
rn
)
12.3 Definite time / IDMT sensitive earth fault (SEF) protection
Time delay SEF protection :
V
K
≥
I
cn
/2 * (R
CT
+ 2R
L
+ R
rp
+ R
rn
)
SEF Protection - as fed from a core-balance CT :
Core balance current transformers of metering class accuracy are required and should have
a limiting secondary voltage satisfying the formulae given below:
Time Delayed element:
V
K
≥
I
cn
/2 * (R
CT
+ 2R
L
+ R
rp
+ R
rn
)
Instantaneous element:
V
K
≥
I
fn
/2 * (R
CT
+ 2R
L
+ R
rp
+ R
rn
)
Note that, in addition, it should be ensured that the phase error of the applied core balance
current transformer is less than 90 minutes at 10% of rated current and less than 150
minutes at 1% of rated current.