Electrical Power Engineering Fundamentals
Electrical Power Engineering Fundamentals tests AC circuit quantities, three-phase power, transformers, rotating machines, and per-unit reasoning. Expect phasors, impedance, power factor, efficiency, and basic machine equivalent circuits.
Core rules
- In a balanced Y load, line voltage is √3 times phase voltage; line current equals phase current. In Δ, line current is √3 times phase current; line voltage equals phase voltage.
- Complex power S=P+jQ=VI*; P=VI cosφ, Q=VI sinφ. Lagging power factor means inductive current.
- Total three-phase power is 3× per-phase power, or √3 V_L I_L cosφ.
- Transformer turns ratio sets voltage ratio; power is approximately conserved, losses are copper and core.
- Per-unit values remove voltage-level differences and simplify fault and load-flow calculations.
Worked example
A balanced Y load draws 10 A per phase at 400 V line-line, pf 0.8 lagging. Phase voltage = 400/√3 ≈ 231 V. Per-phase P = 231×10×0.8 ≈ 1.85 kW; total P = 3×1.85 ≈ 5.54 kW, equivalently √3×400×10×0.8 ≈ 5.54 kW. Q totals about 4.16 kvar.
Traps
Using line voltage as phase voltage in a Y load overstates power by 3. Mixing leading and lagging signs flips Q. Transformer efficiency is not 100%; copper loss varies with current squared. In per-unit, changing the base voltage changes impedance by the square of the base ratio. Do not add phase and line quantities directly.