Generation, Transmission and Distribution
What the domain tests
This domain connects generation sources to the grid: thermal, hydro, solar and other plants, three-phase transmission, step-up and step-down transformers, line losses, and distribution safety. Calculations often compare losses at different transmission voltages.
Core rules
- Generation produces three-phase AC. Star (Y) and delta (Δ) connections relate phase and line quantities: in star, VL = √3 Vph and IL = Iph.
- Transmission uses high voltage to reduce current for the same power, because line loss is I²R per conductor. Power P = √3 VL IL cos φ for a balanced three-phase load.
- Step-up transformers at the power station reduce current; step-down transformers at substations provide distribution and utilization voltages.
- Distribution includes protection, earthing and isolation. High voltage is not automatically low risk; clearances and procedures matter.
Worked example
A 120 MW balanced load is transmitted at 400 kV. Line current: I = P/(√3 VL) = 120 × 10⁶/(1.732 × 400 × 10³) = 173 A. If each phase has 2 Ω resistance, total three-phase loss = 3I²R = 3 × 173² × 2 ≈ 180 kW. At 20 kV, current would be 3464 A and loss ≈ 72 MW. The same power at higher voltage needs far less current and loses far less energy.
Traps that cost marks
Do not forget √3 in three-phase calculations or use phase voltage where line voltage is given. Do not treat 3I²R as I²R when all three phases are included. Power factor changes current, so a low power factor increases losses.