Electrical Engineering Division: Power Systems, Machines, and Circuit Fundamentals
Core concepts to master
A candidate must be fluent in three layers. First, circuit fundamentals: phasors, complex impedance, three-phase star/delta relationships (line voltage = √3 × phase voltage, line current = phase current in star), real/reactive/apparent power, and power factor correction. Second, machines: transformer equivalent circuits and regulation, induction motor slip and torque-speed behaviour, synchronous machine excitation and its effect on reactive power. Third, power systems: per-unit calculation, symmetrical components, fault levels, protection coordination, and transmission line parameters.
Traps that appear in questions
- Mixing RMS and peak values: a 400 V three-phase supply is an RMS line value; peak phase voltage is 400×√2/√3 ≈ 326 V.
- Forgetting that transformer regulation depends on load power factor, not just magnitude - regulation can be negative for leading loads.
- Confusing slip with efficiency in induction motors; slip is (Ns − Nr)/Ns, a speed ratio, not a power ratio.
- Per-unit conversions: base impedance scales as V²/S, so changing the MVA base changes Z by the inverse ratio.
- Assuming unity power factor when the question states a lagging load - reactive power then adds to the current.
Worked example
A 100 kVA, 11 kV/400 V transformer has 5% per-unit impedance. Fault current on the LV side for a bolted short circuit: base current = 100,000/(√3 × 400) ≈ 144.3 A; fault current = 144.3/0.05 ≈ 2887 A. Rule of thumb: fault MVA equals base MVA divided by per-unit impedance, so 100/0.05 = 2000 kVA - a quick sanity check before computing amperes.