Power Systems: Generation, Transmission, Distribution, Protection
Core concepts
Cover per-unit calculations, transmission-line models (short, medium pi/T, long), surge impedance, and voltage regulation. Know generation economics: load factor, diversity factor, tariff structures, and economic load dispatch. For distribution, understand radial vs ring feeders and voltage-drop limits. For protection, master relays (overcurrent, directional, distance, differential), CT/VT ratios, coordination, and circuit-breaker types. Symmetrical components and fault analysis (three-phase, line-to-ground) are essential.
Traps examiners set
- Per-unit values differ by base; always convert to a common base before combining.
- Surge impedance loading is not the same as maximum power transfer.
- Distance relays are affected by arc resistance and infeed; differential protection is best for transformers and busbars.
- Load factor is average load divided by maximum demand, never the reverse.
- Fault current is highest for a three-phase bolted fault in most transmission studies.
Worked rule of thumb
Per-unit conversion: Z_pu_new = Z_pu_old * (S_base_new/S_base_old) * (V_base_old/V_base_new)^2. Example: a 0.1 pu impedance on 100 MVA, 11 kV moved to 50 MVA, 11 kV becomes 0.1 * (50/100) * 1 = 0.05 pu. Always state the base with the answer.
Practice strategy
Sketch the single-line diagram with bases labelled before any calculation.