Current-Transformer Requirements And Sample Calculations; Current-Transformer Requirements - Siemens Reyrolle 7SR45 Argus Device Manual

5.2

Current-Transformer Requirements and Sample Calculations

Current-Transformer Requirements

5.2.1
The 7SR45 Argus relay uses the current inputs to energize the relay.
Higher ohmic burdens in the current transformer (CT) secondary circuit results in greater saturation of the
core and introduces larger errors in the secondary current waveform. For a given secondary current, it requires
more voltage from the CT for a higher burden.
The total burden on the CT is the vector sum of
•
CT winding resistance
•
Connecting lead resistance
•
Impedance of any auxiliary CTs
•
Impedance of the connected relays and meters
After the ohmic burden is determined, the next step is to predict the CT performance. This is done by deter-
mining the required CT excitation voltage or secondary limit voltage. The calculated total ohmic burden is
mutiplied by the accuracy limit factor (ALF) and rated secondary current to measure this value.
The following expression shows the secondary limiting excitation voltage (E
E = ALF ⋅ I ⋅ (R
ALF sr
Where
ALF – Accuracy limiting factor
I – Rated secondary current of CT (A)
sr
R – CT winding resistance (Ω)
ct
R – Rated burden of CT (Ω)
bn
If the secondary limiting excitation voltage (E
CT, the CT gets saturated.
To select a proper CT, consider the ALF at a given burden. Check the ALF for the entire range of currents the
relay is expected to operate.
For proper operation of the CT, fulfill the following condition:
[fo_accuracy-limiting-factor_V1, 1, --_--]
So,
[fo_accuracy-limiting-factor_V2, 1, --_--]
Where
I – Actual secondary current of CT (A)
s
R – Actual secondary burden of CT (Ω) (includes burden of the relay (R
b
between CT and relay.
The burden of the 7SR45 relay depends on the input current. The impedance of the relay is inversely propor-
tional to the input current. Based on the field conditions, Siemens recommends selecting the primary CT turns
ratio that is at least 10 % ⋅ I
The following graphs show the impedance of the relay for different currents when the relay is powered
through a 1-phase current input.
Reyrolle, 7SR45, Device Manual
C53000-G7040-C101-2, Edition 09.2020
+ R )
ct bn
to reduce the dependency of the internal battery.
rated

5.2 Current-Transformer Requirements and Sample Calculations

) calculated exceeds the knee-point voltage of the connected
ALF
rel
):
ALF
) and resistance (R ) of the cable
l
Applications
91