For metering core, ISF is for protecting metering instruments like energy meters, transducers etc. These instruments are meant for measuring electrical quantities with high accuracy under normal operation. Measurement under fault condition is now required and insignificant due to short duration of faults. Metering instruments are designed to carry current upto a certain level.
For
example, an energy meter used with CT of 1000/1A ratio carries maximum 1A under
full load. Therefore its input current carrying paths needs to be designed for
current upto 150 to 200% of rated current (say 2A continuous) . For fault
conditions, which will prevail for short duration, it should withstand upto 5A
or 10A. But in case of severe fault of 40kA or 50kA, secondary current may go
upto 40A or 50A. If meter is to be designed for such heavy currents its size and
connections will be bulky. Its cost will go up and accuracy will go down. Therefore, CT core is
designed to saturate at such heavy currents and maximum secondary current is
specified as ISF.
ISF
is ratio of rated instrument limit primary current to rated primary current. If
ISF is specified as 5 for a 1000/1A CT, then CT metering core will saturate at 5000A primary
and current in secondary will not go above 5A. Sometimes Auxiliary reactors are
used to achieve ISF limit.
Equivalent circuit of CT:
IP = Primary current
IS = Secondary current
VS = Secondary voltage
ZE = Exciting impedance
IE = Exciting current
RS = Secondary resistance
XL = Leakage reactance
ZB = Burden impedance
Effect of burden on ISF: ISF is specified for a certain burden. If actual burden is different from rated burden, ISF will change as below:
ISF at actual burden = ISF at rated burden x (Rated burden / Actual burden)
* Internal secondary winding resistance of CT to be added for calculating burden.
For example, for a CT with rated burden 20VA and ISF 5, if actual burden is 10VA, ISF will be =5 x (20/10) =10. I.e. maximum secondary current will be 10A.
