scheider electric MiCOM P746, Technical Manual

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P746/EN AP/G31 Application Notes
(AP) 6-
52
MiCOM P746
AP
11. CT REQUIREMENTS
11.1 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.
11.2 87BB Phase CT Requirements
11.2.1 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.
In any case the maximum peak current shall be less than 90 In (90A for 1A input and 450A
for 5A Input) i.e. 32 In RMS fully offset.
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.