Transformers and Power Distribution
Transformer rating and fault level
Transformers and Power Distribution tests whether you can convert kVA ratings into primary and secondary currents, estimate short-circuit current from impedance, and arrange distribution so that faults are cleared and maintenance can be performed safely. Transformers are rated in kVA because the heating depends on voltage and current, not on the power factor of the load.
Core rules
- For a three-phase transformer, line current = kVA x 1000 / (sqrt(3) x line voltage). Use the secondary voltage for secondary current and the primary voltage for primary current.
- Percentage impedance controls the short-circuit current. Neglecting source impedance, Isc at the secondary terminals is approximately secondary full-load current divided by per-unit impedance.
- Vector groups matter for parallel operation. Transformers paralleled without matching vector groups, tap positions and impedance can circulate large currents.
- The neutral earthing arrangement determines the installation earthing system and the fault current path. Tertiary windings may be used for earthing or harmonic suppression and must not be left unconsidered.
- Inrush current can trip instantaneous protection on energisation. Coordination must allow for it without compromising fault protection.
- Temperature, altitude and harmonic load affect derating. K-factor or derated transformers are used where non-linear loads are significant.
Worked example
A 500 kVA, 11 kV / 400 V transformer has a 5 percent impedance. Secondary full-load current = 500000 / (1.732 x 400) = 722 A. Approximate secondary short-circuit current = 722 / 0.05 = 14.4 kA. The downstream switchgear must have a short-circuit rating above 14.4 kA, and the source impedance would reduce this value slightly. If the transformer impedance were 4 percent, the fault level would rise to 18.1 kA, showing why the nameplate impedance must be used rather than an assumed value.
Traps that cost marks
Using kW instead of kVA for transformer current; using the primary voltage to calculate secondary current; ignoring impedance when selecting switchgear; and paralleling transformers with different vector groups or tap settings.