A.7 Current Transformer Requirements
505
7UM62 Manual
C53000-G1176-C149-3
with
u
SC
Short circuit voltage
x
d
’’
Subtransient reactance
I
sN
Rated secondary ct current
τ
N
Network time constant
with
U
Knee-point voltage
K
ALF
Rated accuracy limiting factor
I
sN
Rated secondary ct current
R
BN
Rated burden resistance
R
Ct
Secondary winding resistance
Symmetrical short circuit cur-
rent
I
pSC
Example
u
SC
= 0.1
n’ > 40
x
d
’’ = 0.12
n’ > (34 to 42)
Note:
Always use identical trans-
formers
Power
≥
10 or15 VA
Example of network
transformer:
10P10 10 or15 VA
(
I
sN
= 1 A or 5 A)
Note the internal burden!
Example:
I
N, G
approx. 1000 to 2000 A
5P15 15 VA
(
I
sN
=1 A or 5 A)
I
N, G
> 5000 A
5P20 30 VA
(
I
sN
=1 A or 5 A)
Table A-4
Transformer Requirements
Transformer
Generator
1
u
SC
---------- I
pN, Tr
⋅
≈
1
x
d
″
-------- I
pN, G
⋅
≈
Table A-5
Knee-Point Voltages
IEC
British Standard
ANSI
U
K
AL F
R
Ct
R
BN
+
(
)
I
sN
⋅
⋅
=
U
R
C t
R
BN
+
(
)
I
sN
⋅
1.3
---------------------------------------------
K
ALF
⋅
=
U
20
I
sN
R
Ct
R
BN
+
(
)
K
ALF
20
-------------
⋅
⋅
⋅
=
I
sN
= 5 A (typical value)