Areva MiCom P120, Technical Manual, Page: 300 / 668

P12x/EN AP/A96 Application Guide
Page 78/80
MiCOM P120/P121/P122/P123
34. CT REQUIREMENTS
Hereafter are presented the CT requirements for MiCOM P12x Overcurrent. Current
transformer requirements are based on a potential maximum fault current that is 50 times
the relay rated current (In) and on the setting of the instantaneous at 25 times rated current
(In). The current transformer requirements are designed to provide operation of all protection
elements.
When the criteria for a specific application are higher than the criteria described above, or
when the actual lead resistance exceeds the limiting value recommended, it may be
desirable to increase the CT requirements according to the following formula.
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
34.1 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
)
34.2 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
)
34.3 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:
The type of current transformers that are required are core balance type and with metering
class accuracy and with a limiting secondary voltage that follows the following formula:
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
)
In addition, note that phase error of the applied core balance current transformer should be
less than 90 minutes at 10% of rated current and less than 150 minutes at 1% of rated
current.