Transformer Secondary Fault Level Calculation

The transformer secondary fault level is the fault current or fault apparent power available at the secondary terminals of a transformer. It is an important input for switchgear ratings, protective-device selection, protection coordination and downstream cable fault-withstand checks.

This simplified calculation uses transformer percentage impedance and assumes an infinite fault level on the transformer primary side. In other words, the upstream network impedance is taken as zero. Where the primary fault level is known and finite, the upstream source impedance should also be included.

Required input data

• Transformer rated capacity, kVA.
• Primary line-to-line voltage, V₁, if a finite primary fault level is being considered.
• Secondary line-to-line voltage, V₂.
• Transformer percentage impedance, Z%.

For the simplified infinite-primary-source calculation, the primary voltage is not used directly because the upstream network impedance is assumed to be zero.

Secondary fault current

The three-phase secondary short-circuit current can be estimated from:

Here I_sc is the transformer secondary fault current in amperes, V₂ is the secondary line-to-line voltage in volts, and Z% is the transformer percentage impedance.

Secondary fault level in MVA

The secondary fault level in MVA is then:

Example calculation

For a three-phase transformer with the following data:

The secondary fault current is:

The corresponding secondary fault level is:

For this example, the secondary three-phase fault level is therefore approximately 24.1 kA, or 16.6 MVA.

Use in cable fault calculations

The transformer secondary fault level can be used as a source fault level for downstream cable calculations. It may be converted to source impedance for IEC 60909-style calculations or used to check equipment short-circuit rating and cable thermal withstand.

For related topics, see Network Fault Level, IEC 60909 Fault Calculations, Fault Current Calculations and Cable Thermal Withstand.