April 16, 2001

How to Select a Transformer Protective Device Based on the Secondary-Fault Current Rating

The real “work horse” capability needed for a primary-side substation transformer protective device is the ability to interrupt inherent secondary-fault current. Data shows that the highest percentage of faults cleared by the primary-side protective device originate on the secondary side of the transformer. Although exact percentages vary among different applications, secondary-side faults typically comprise 80% or more of the faults a primary-side device is called on to interrupt. As such, it is essential to evaluate the secondary-side fault interrupting capability of any primary-side protective device. S&C has tested all of its primary-side transformer protective devices (including Series 2000 Circuit-Switchers, Mark V Circuit-Switchers, and Trans-Rupter II® Transformer Protectors) specifically for this duty, a difficult duty as the interruption of secondary-side faults involves severe transient recovery voltages (TRV).

Assuming an infinite (zero-impedance) source, the secondary-fault current as reflected on the primary side of the transformer can be calculated using the following equation:

I = (57.8P) / [(%Z)E]

The formula assumes an infinite (zero-impedance) source.

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. 

For example, a 37.5/50/62.5 MVA transformer at 115 kV with 8% impedance will provide an inherent secondary-fault current of 2,356 amperes, calculated as follows:

I = [(57.8) (37 500 kVA)] / [(8) (115 kV)] = 2356 A

In this example, S&C Circuit-Switchers and Trans-Rupter II would be rated appropriately for the secondary-side fault current levels. For 115-kV Mark V and Series 2000 Circuit-Switchers, the secondary-side fault-current rating is 4000 A, while the 115-kV Trans-Rupter II is rated to interrupt secondary-side faults through 2600 A.

S&C Circuit-Switchers and Trans-Rupter II Transformer Protectors provide excellent protection for substation transformers, with test-proven capability to clear secondary-side faults. To learn more about these products and how secondary-side faults affect the application of primary-side transformer protective devices, contact S&C.