Power Systems (Generation, Transmission, Distribution and Protection)
Power Systems
This domain tests generation economics, transmission-line performance, distribution layout and protection coordination. Questions frequently use three-phase power, current, loss and relay-zone calculations.
Core rules
- Three-phase power P = √3 V_L I_L cos φ; line loss = 3I²R. For the same power, raising transmission voltage lowers current and reduces I²R loss.
- Short lines (under about 80 km) use a series-impedance model; medium lines use nominal π or T; long lines need distributed parameters. Lightly loaded long lines show Ferranti overvoltage.
- Surge impedance Zc = √(L/C), and surge impedance loading is V_L²/Zc.
- Distribution networks are usually radial or ring; voltage drop and diversity factor determine conductor size.
- Protection: CTs scale current and PTs scale voltage. Overcurrent relays protect feeders; distance relays use impedance zones; differential protection compares currents at two ends; Buchholz protects oil-filled transformers.
- Earth resistance must be low enough to clear faults and limit touch voltage.
Worked example
A 50 MW load at 132 kV, 0.9 power factor: I = 50×10^6/(√3 × 132×10^3 × 0.9) = 243 A. With R = 0.2 Ω per phase, line loss = 3 × 243² × 0.2 = 35.4 kW. At 33 kV the current would be four times larger, so the loss would be sixteen times larger.
Traps
Do not confuse line and phase voltage. Use consistent per-unit bases. Never open a CT secondary under load. Distance relays are directional and zone-based, not simple fuses.