Technical FAQs

# What is the short circuit current and how does it affect the Sepam protection relay settings?

Issue: What is "Icc min" (or "Isc min") and how does it affect the Sepam protection relay?

Product Line: Sepam
Resolution:
The Sepam relay uses a value called Icc which is really the minimum short circuit current. "Isc min" refers to the smallest fault current that could flow in the event of a short-circuit downtream of the transformer, as opposed to the maximum current available from the upstream Utility network.The intended use of the "Isc min" setting for the ANSI 50/51 function, let us take a simple example. A network (with known values of short-circuit power and the R/X ratio) feeds a transformer with known values for Usc (short-circuit voltage) and Sn (the rated power).
Using standard methods for short-circuit current calculation, the user needs to simulate a short-circuit fault downstream of the transformer with the goal being to determine the smallest possible value of Isc (i.e., "Isc min"). The value will depend on the fault current available from the Utility, but the impedance of the transformer (and other components upstream of the relay location such as cables) will also affect the Isc min value. Typically it would be the "2 phases to ground" fault current.
So, if we have a transformer of 1000 kVA (46.3 Amps full load current = 555 Amps inrush at 12x), let us assume that Isc min has been calculated as 4000 Amps. We would enter 4000A for Isc min, and enable H2 Restraint in SFT2841. This would mean that the H2 Restraint will be activated for all faults with magnitude of less than 2000A, or 50% of Isc min. So, if the maximum inrush is expected to be 555A, then the H2 Restraint should always work as expected upon energization of this transformer.
Notes:
a) The idea of the "Isc min" parameter is that for high-magnitude faults, we need to ensure that Sepam will trip even if the currents sensors become saturated. That is why the H2 Restraint will only inhibit tripping for faults with a magnitude less than 50% of Isc min.
b) There could be a problem if the parameters of the upstream network can change significantly from time to time (i.e., there are various modes of operation). Using the above example, imagine that the network can go into a state where the "Isc min" drops from 4000A down to 1000A. If you had set the Sepam "Isc min" value to 1000A, then the H2 Restraint might not inhibit tripping during transformer energization. This could easily be overcome by connecting an input to the Sepam relay to allow from changing from one protection group to another to allow for any "Isc min" changes.