A Resource for Selecting a Substation Transformer Protective Device
One criterion in selecting a distribution substation transformer protective device is its ability to protect the transformer from secondary-side limited faults or “through-faults.” These faults are difficult to detect by the overcurrent relay of the line-terminal circuit breaker, because the magnitude of the fault current is relatively low — being limited by the impedance of the transformer. These faults are a challenge to clear as well, because of their high transient recovery voltage.
That’s why a stand-alone protective device such as an S&C Circuit-Switcher is typically applied at each transformer. S&C has tested each of its distribution transformer protective devices — Series 2000, Mark V, and Mark VI Circuit-Switchers, and Trans-Rupter II® Transformer Protector — specifically for this duty, and assigned a secondary-fault interrupting rating to each device.
This formula can be used to determine the secondary-fault interrupting rating required for the substation transformer protective device, to properly protect a particular size and impedance of transformer:
I = (57.8P) / [(%Z)E]
The formula assumes an infinite (zero-impedance) source.