GE P742

P74
x/EN AP/Na7
Application Notes
(AP) 6
-62 MiCOM P74
1, P742, P743
AP
10. CT REQUIREMENTS
10.1 Notation
I
F
max fault
maximum fault current (same for all feeders) in A
I
F
max int
cont
maximum contribution from a feeder to an internal fault (depends on the
feeder) in A
I
np
CT primary rated current
I
n
nominal secondary current (1A or 5A)
R
CT
CT secondary winding Resistance in Ohms
R
B
Total external load resistance in Ohms
V
k
CT knee point voltage in Volts
S
VA
Nominal output in VA
K
SSC
Short-circuit current coefficient (generally 20)
General recommendations for the specification of protection CTs use common rules of
engineering which are not directly related to a particular protection.
10.2 87BB Phase CT Requirements
10.2.1 Feeders connected to sources of significant power (i.e. lines and generators)
The primary rated current is specified above a 1/20
th
of the maximum contribution of the
feeder to internal faults.
i.e. I
np
= I
F
max int/20
e.g. A power line likely to import electricity at 20 kA gives rated primary current I
np
as
1000 A.
This recommendation is used for the majority of line or transformer protection applications.
The CT must be sized so as not to saturate during internal faults:
For each CT, I
FeederMax
= maximum contribution of the feeder to an internal fault (could be
different for each feeder):
Vk > I
FeederMax
* (R
CT
+ R
B
)
Note: This specification is valid for internal faults.
10.2.2 Out of service feeders or those with low power contribution (low infeed)
Because of CT construction, thermal behaviour, and electrodynamics the CT primary rated
current cannot be as low as required compared to the maximum fault current. In the case of
a CT with primary bushings and not wound there is not a precise limit but a practical one.
The primary rated current could not be lower than the 1/200th of the maximum short-circuit
current crossing the CT at the time of an external fault
i.e. I
np
= I
F
max /200
This is possible using the fast overcurrent detection I>2 to distinguish between an internal or
external fault in case of CT saturation below than 2 ms
e.g. For a substation whose maximum short-circuit current would be 30 kA, the CTs on the
least powerful feeders are to be specified for a rated primary current I
np
= 150 A, even if the
normal consumption of the feeder is much lower than this value (Substation transformer
feeder)