Power Systems
Power Systems covers generation, transmission and distribution, protection, switchgear and utilization of electrical energy. Questions often test line models, per-unit ideas, protection schemes and three-phase power calculations.
Generation, transmission and distribution
- Thermal, hydro and nuclear plants differ in prime mover and cost structure; base load is supplied by low-running-cost plants.
- Short lines use series impedance; medium lines use nominal π or T; long lines need distributed parameters.
- Corona depends on conductor surface gradient, spacing and weather; bundled conductors reduce corona and reactance.
- Skin effect pushes AC current to the conductor surface; Ferranti effect raises receiving-end voltage on a lightly loaded long line.
Protection and switchgear
- CTs step current down and PTs step voltage down for relays and meters.
- Distance relays use impedance (Z = V/I) and are directional; differential protection compares currents at two ends.
- Circuit breakers are rated by voltage, current, breaking capacity and making capacity; arc quenching differs among air, oil, SF6 and vacuum breakers.
Worked example
A three-phase 11 kV load draws 100 A at 0.8 power factor lagging. Apparent power S = √3 × 11000 × 100 = 1.905 MVA. Active power P = S × 0.8 = 1.524 MW. Reactive power Q = S × sinφ = 1.905 × 0.6 = 1.143 MVAR.
Traps that cost marks
Using line voltage as phase voltage in star-connected equipment, forgetting √3 in three-phase power, confusing CT ratio with relay setting, and mixing MW with MVA are common. In per-unit work, convert every quantity to the same base before comparing.