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Calculation of Required Stability Voltage Limits
The assigned through fault current is 31.5kA.
8400
V
S
600
8400
V
S
600
V
300
K
V
S
2
2
Calculation of Fault Setting (Primary Operate Current)
The required relay setting Is can be calculated from:
POC = ( 3(I
MAGLCT
Irelay = POC x T – ( 3(I
= 0.1 – ( 3(I
MAGLCT
The magnetising current is dependent on the relay operate voltage (Vs), this is not finalised yet so a provisional
value is chosen from the above i.e. 107.1 < Vs < 150
Say Vsprov = 120V.
From CT magnetising curve: At 120V I
Irelay = 0.1 – (3(0.009) + 0.012) = 0.061A
Rstab = 120/.061 = 1967 Ohms (Say 2000 Ohms giving Vs = 2000 x 0.061 = 122V)
Metrosil Specification
'C' Value
Vs = 122V (i.e. > 100V) so a 'C' value of 1000 is chosen
Short Time Power Rating
Using the empirical formula
4
P
I
1SEC
F
4
P
8400
1SEC
For values < 8kW/s a 75mm Metrosil is used.
©2013 Siemens Protection Devices Limited
7.5
0.15
107.1V
6
0.15
86
.1V
15
0V
) + I
+ Irelay ) / T
MAGNCT
) + I
) = 60/600 – Imag total
MAGLCT
MAGNCT
) + I
)
MAGNCT
MAGLCT
T
V
K
1
350
6.24kW
600
Chapter 7) 7SR23 DAD Applications Guide
= 9mA and I
= 12mA
MAGNCT
Chapter 7 Page 17 of 42
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