Functions
2.14 Differential Protection and Its Protected Objects
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
127
Table 2-6 Transformer Requirements
with
u
SC
Transformer impedance
x
d
” Direct-axis transient reactance
I
sN
Secondary CT rated current
τ
N
Power system time constant
Table 2-7 Knee-Point Voltages
with
U Knee-point voltages
K
ALF
Rated overcurrent factor
I
sN
Secondary CT rated current
R
BN
Rated load
R
Ct
Internal load
Transformer Generator
Transient dimensioning factor K
td
≥ 4
with τ
N
≤ 100 ms
> (4 to 5), with τ
N
> 100 ms
Symmetrical short circuit current I
pSSC
Example u
SC
= 0.1
n’ > 40
x
d
’’ = 0.12
n’ > (34 to 42)
Note:
Always use identical transformers
Power ≥ 10 or 15 VA
Example of network transform-
er:
10P10 10 or 15 VA
(I
sN
= 1 A or 5 A)
Note internal load!
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)
IEC British Standard ANSI
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