Power Systems (Generation, Transmission, Distribution, Protection and Switchgear)
Power Systems tests generation economics, transmission parameters, per-unit calculations, fault levels, protection, and switchgear. Numerical questions usually reduce to per-unit or symmetrical-component reasoning.
Core rules
- Base impedance Z_b = V_b^2/S_b for three-phase using line-to-line voltage and three-phase MVA.
- A short line uses series impedance; a medium line uses nominal pi or T; a long line uses distributed parameters and hyperbolic functions. Surge impedance Z_c = sqrt(L/C).
- Per-unit values transfer across transformers by changing the voltage base; the MVA base stays fixed.
- Symmetrical three-phase fault current I_f = V_th/(Z_th + Z_f) in per unit; multiply by I_b = S_b/(sqrt(3) V_b).
- Distance protection uses impedance measurement; differential protection compares currents at zone boundaries. CT and PT ratios scale secondary quantities.
- Switchgear must interrupt fault current and withstand electrodynamic and thermal stress.
Worked example
On a 100 MVA, 11 kV base, a bus has Thevenin impedance 0.1 pu and prefault voltage 1.0 pu. Base current I_b = 100/(sqrt(3) × 11) = 5.25 kA. Fault current I_f = 1.0/0.1 = 10 pu = 52.5 kA. The reasoning is to compute the per-unit current first and convert only once at the end.
Traps
Using phase voltage with three-phase MVA; forgetting to square the voltage ratio when changing bases; mixing line-to-line and line-to-neutral values; and treating relay pickup as the same as breaker interrupting rating.