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Calculation of Fault Setting (Primary Operate Current)
The required relay setting (87/50-1) Is can be calculated from:
POC = ( I
MAG TOTAL
Is = POC x T – I
MAG TOTAL
Is = 0.5 – 6(I
)
MAG
The magnetising current is dependent on the relay operate voltage (Vs), this is not finalised yet so a provisional
value (Vs prov) is chosen from the above i.e. 160 < Vs < 300
Say Vs prov = 220V. From the CT magnetising curve Imag = 24mA at 220V.
Irelay = 0.5 – 6(0.024) = 0.356A (use 0.35A)
Rstab = 220/.35 = 630 Ohms
Metrosil Specification
'C' Value
Vs = 220V (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
315
1SEC
For values > 8kW/s a 150mm Metrosil is required
Stabilising Resistor Specification
Continuous Power Rating
2
P
I
x
R
CONT
s
2
P
0.35
CONT
P
77W
CONT
©2013 Siemens Protection Devices Limited
+ Is ) / T = ( 6(I
) + Is ) / T
MAG
= 1000/2000 – I
MAG TOTAL
T
V
K
1
00
6
00
1
2kW
2000
stab
x
630
Chapter 7) 7SR23 DAD Applications Guide
Chapter 7 Page 14 of 42
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