Through-Fault Current Calculator: 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.
Through-Fault Current Calculator
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Where:
I = Inherent secondary-fault current, amperes
P = Transformer self-cooled three-phase base rating, kVA
E = System phase-to-phase voltage, kV
%Z = Percent transformer primary-to-secondary impedance, referred
A protective device is appropriate for the application if its secondary-fault interrupting rating is equal to or greater than the value for “I” calculated above.
EXAMPLE:
The inherent secondary-fault current for a 20/40/50-MVA, 115-kV, 8%-impedance transformer would be:
I = [(57.8) (20 000 kVA)] / [(8) (115 kV)] = 1257 A
Check out these other resources for selecting a substation transformer protective device:
“Selection Guide for S&C Substation Transformer Protective Devices”
The selector table in this publication lets you compare the features and capabilities of S&C’s substation transformer protective devices, so you can select the appropriate device for your application.
“Primary-Side Transformer Protection”.
This paper offers a complete overview of transformer protection, including typical protection schemes, a discussion of transient recovery voltage and why secondary-side faults are difficult to interrupt, and criteria for selecting a device appropriate to a particular application.