Chapter 7 - 7SR45 Applications Guide
©2018 Siemens Protection Devices Limited
Chapter 7 Page 6 of 10
Sample Calculation:
CT Ratio
:
100/1
Rated CT Burden (R
bn
)
:
2.5 VA (R
bn
= 2.5 Ω for 1A secondary current)
CT winding resistance (R
ct
)
:
0.5 Ω
Rated Secondary Current (Isr)
:
1 A
Case 1:
In the absence/loss of auxiliary power, the minimum phase current required for the relay operation in single phase
= 0.2 In (I
s
= 0.2A for rated current of 1 A)
ALF
≥
0.2 * (0.5 +28.5)
1 * (0.5 + 2.5)
R
b
= 28.5 at 0.2 A (Refer to Impedance graphs for current >1A (Nominal current, In = 1A)
ALF
≥
1.933
Case 2:
Fault current = 20 A
ALF
≥
20 * (0.5 + 0.49)
1 * (0.5 + 2.5)
R
b
= 0.49 at 20 A (Refer to Impedance graphs for current >1A (Nominal current, In=1A)
ALF
≥
6.6
The typical accuracy limit factor would be ALF = 10
The calculation shown above is an example. To select an appropriate CT, similar calculation needs to be done
with the actual parameters.
Recommended CTs:
Considering the CTs are connected to 7SR45 Argus Relay only and the distance between CTs and 7SR45 Argus
Relay (typically 3 m) is short, the following are the recommended CTs to be used:
•
2.5 VA, 5P10 or 5P20 for I
n
= 1 A
•
2.5 VA, 5P20 for I
n
= 5 A
NOTE:
For safe operation of the 7SR45 Argus Relay, the maximum power delivered to the relay is 1000 VA for 1 s.
Moreover, it is not recommended to overload the relay above 1000 VA for multiple times.
For more information, refer to
Short Time Thermal Withstand
.
