Transmission and Distribution
Core concepts
Transmission questions center on modeling lines and reasoning about voltage. Short lines (under about 80 km) use series impedance only; medium lines add shunt capacitance (nominal π or T); long lines require hyperbolic ABCD parameters. Voltage regulation compares no-load and full-load receiving voltage at constant sending voltage, while efficiency compares output to input power. Series inductance causes voltage drop and limits loadability; shunt capacitance causes charging current and the Ferranti effect, where receiving voltage rises above sending voltage on lightly loaded long lines. Surge impedance Z_c = √(L/C), roughly 400 Ω for overhead lines, gives surge impedance loading (SIL = V²/Z_c), a useful loading benchmark. Distribution practice emphasizes voltage drop, feeder loading, capacitor placement, and coordination of fuses and reclosers.
Traps to expect
Candidates apply the approximate drop formula outside its validity or forget the √3 in three-phase drop. Corona loss rises with voltage and is reduced by bundling, which also lowers series reactance. Ferranti effect questions almost always describe a light load, not a heavy one. Be careful with per-unit: convert line impedance to ohms using Z_base = kV²/MVA.
Worked example
A three-phase feeder carries 300 A at 0.9 lagging over R = 0.4 Ω and X = 0.6 Ω per phase. Approximate line-to-line drop is √3·I·(R·cosθ + X·sinθ) = 1.732 × 300 × (0.4×0.9 + 0.6×0.436) = 1.732 × 300 × 0.622 ≈ 323 V, about 3.1 % of a 10.4 kV feeder - a quick check against a 5 % limit.