Transformers and Power Distribution Systems
Ratings and connections
Transformer capacity is expressed in kVA, not kW, because the limit is heating from volts and amps, not real power. Single-phase kVA = V × I / 1000; three-phase kVA = √3 × V_line × I_line / 1000. The turns ratio sets the voltage ratio, but in a delta-wye unit the ratio of line voltages is not simply the per-winding turns ratio - the √3 factor must be included. Wye secondaries provide a neutral for single-phase line-to-neutral loads and a low-impedance ground point; delta secondaries have no neutral unless corner-grounded or zig-zag grounded.
Impedance, fault current and protection
Percent impedance (%Z) is the voltage drop at full-load current expressed as a percentage of rated voltage, and it sets the available short-circuit current: I_sc ≈ full-load amps ÷ (%Z/100). Low %Z means high fault current and a need for higher interrupting ratings; high %Z means better fault limitation but more voltage sag on motor starting. Inrush current on energization can be 8–12 times full-load current for a few cycles, so primary protection must ride through it. Transformers require primary protection, secondary protection (or a single primary device in defined cases), and grounding/bonding of the secondary neutral per the Code.
Traps
Using kW instead of kVA; ignoring the √3 in three-phase fault calculations; forgetting that paralleled transformers must match voltage ratio, polarity, %Z, and tap settings; and confusing temperature rise/insulation class with allowable loading.
Worked example
A 500 kVA, 600 V three-phase transformer with 5% impedance. Full-load current = 500 000 / (1.732 × 600) = 481 A. Available short-circuit current ≈ 481 / 0.05 = 9 620 A, or about 9.6 kA. The secondary overcurrent device must have an interrupting rating above 9.6 kA, and equipment downstream must be rated for that fault level.