Siemens 7SR45 Argus User Manual page 64

Section 1: Current Transformer Requirements
7SR45 self powered overcurrent and earth fault 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 the CT winding resistance, the connecting lead resistance, the
impedance of any auxiliary CTs, and the impedance of the connected relays and meters.
After the ohmic burden is determined, the next step in predicting CT performance is to determine the required CT
excitation voltage or secondary limiting voltage by multiplying the calculated total ohmic burden by the accuracy
limit factor (ALF) and rated secondary current.
The secondary limiting excitation voltage (E
E = ALF * I * (R + R
ALF sr ct
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
the CT may get saturated.
To select a proper CT, the accuracy limit factor (ALF) at a given burden needs to be considered. The ALF needs
to be checked for entire range of currents the relay is expected to operate. For proper operation of the CT, the
following condition needs to be fulfilled.
So,
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 7SR45 relay depends on the input current. The impedance of the relay is inversely proportional to
the input current. Based on the field conditions, it is recommended to select the primary CT turns ratio should be
at least 10 %xIn to reduce the dependency of the internal battery.
The following graphs show the impedance of the relay for different currents when the relay is powered through a
single phase current input.
NOTE:
When all the three phases are available, the burden on each CT reduces.
©2017 Siemens Protection Devices Limited
) is given by the following expression:
ALF
)
bn
) calculated exceeds the knee point voltage of the connected CT,
ALF
ALF * I * (R
sr
ALF = Is *(R
) ≥ I
+ R * (R + R )
ct bn s ct b
+ R )
ct b
Isr *(R +R )
ct bn
) and resistance (R
rel
7SR45 Applications Guide
) of the cable
l
Chapter 6 Page 3 of 6