Power Generation, Transmission, and Distribution
This domain covers how electrical energy is produced, stepped up, moved, and delivered. Questions test generator and transformer behaviour in the network, line models, voltage regulation, and distribution layout choices.
Core rules
- Power transfer across a reactive link: P = (V_S V_R / X) sin δ, maximum at δ = 90°. This explains why voltage support and series compensation raise transfer limits.
- Line models: short lines (under about 80 km) use series R and X only; medium lines add shunt capacitance (nominal π); long lines need distributed parameters and surge impedance loading.
- Transmission uses high voltage to cut current and I²R loss for the same power; distribution steps down close to load centres.
- Distribution networks are typically radial (simple, lower fault current, lower reliability) or ring/mesh (higher reliability, more complex protection).
- Voltage regulation depends on load power factor, line X/R ratio, and tap-changer settings.
Worked example
A 138 kV line with 20 Ω series reactance connects two buses held at 138 kV. The maximum steady-state power is P_max = V²/X = (138,000)² / 20 = 952 MW at δ = 90°. At δ = 30°, P = 952 × sin 30° = 476 MW. This shows why operating angles are kept well below 90°: small angle increases buy large power gains, but stability margin shrinks.
Traps
Line calculations are per phase: use line-to-neutral voltage with per-phase impedance, then multiply by three for total power. Mixing MW and MVA, or forgetting that a transformer's impedance is on its own base, produces wrong answers. Neglecting line charging overestimates reactive loss on long cables.