INSTRUMENT SECURITY FACTOR OF CT
Instrument security factor, ISF is the times of rated current above which the CT metering core will saturate. The ratio of the saturating primary current to the CT-rated current is known as the Instrument security factor.
Table of Contents
The multi-functioning meters or energy meters connected to the secondary of the metering core CT are designed for the low amplitude of the current. Also, the connection leads can handle a small current above the rated secondary current of the metering core.
In case of a fault, the primary current of the CT becomes very high which is also translated in the secondary winding obeying the transformation ratio. Now this high current in the primary if transformed fully into the secondary, the instruments connected to the secondary circuits will get damaged as they cannot handle such high currents.
In order to save the instruments connected, the Current transformer’s metering core is so designed that it gets saturated when the current in the secondary gets high enough to cause damage.
Therefore, the Instrument security factor of the CT is an essential factor that provides safety to the connected instruments in the secondary of the metering core of the CT by saturating the core of CT as it crosses the threshold. On the stage of saturation of CT, the core loses the excitation and thus the output secondary current becomes constant or zero as the primary current increases beyond saturation limits.
Example
A CT of 400/1 ratio having ISF 10 means that when the primary rated current crosses 10*400 = 4000A the core will undergo saturation thus saving the connected instruments on the secondary side of it.
It may be noted that ISF is only defined for the metering core of the CT and not the protection core.
As per IEC-61869-2, the standard ISF values are 5,10.
The lower the ISF value, the more security to the instruments, hence lower ISF values are preferred.
COMPENSATING DEVICE:
A compensating device is also used in the secondary side of the metering core for the purpose of achieving security if the CT core does not saturate above the ISF.
It is basically a high resistance connected in parallel to the connected instruments like meters. As shown in the figure, the connection of the metering core 3S1 is shorted with the S’ terminal which is the compensating device/ resistance. In case of high current in the secondary and the CT core does not saturate, the current will be shorted in the shunted path via S’ and the connected instruments between 3S1 and 3S2/3S3/3S4 will thus be saved. It may be noted that the S terminal will have to be connected to 3S2/3S3/3S4 whichever CT ratio is preferred.
FAQ’s
What is the formula for the Instrument Security Factor?
The formula for ISF is
= Saturating primary current (Isat) / Rated primary current (IR)
Why is the Instrument Security Factor only limited to the metering core of the CT?
ISF is only limited to the metering core because the metering core has to be saturated so that the meters connected to the secondary do not get damaged from the high secondary current. However, the protection core has to translate the high fault current to the secondary winding accurately otherwise the relays connected to the secondary will malfunction.
For this reason, the ISF is only limited to the metering core of the CT, and this principle is not used in the protection core of the current transformer.
