Electrical Power Systems Analysis and Design
Power-system analysis rests on three pillars: per-unit representation, the load-flow problem, and fault-level estimation. Master per-unit first - converting every quantity to a common base removes transformer ratios from calculations and lets you compare voltages and impedances across voltage levels. Know that base impedance scales as the square of base voltage, so changing bases on a per-unit impedance requires the (V_old/V_new)² and (S_new/S_old) correction factors.
For load flow, understand the bus classification: slack (specifies voltage magnitude and angle), PV (generation bus, specifies P and |V|), and PQ (load bus, specifies P and Q). Questions often test which quantities are known versus solved at each bus. Know that Newton-Raphson converges quadratically but needs a Jacobian, while Gauss-Seidel is simple but slow.
For faults, distinguish symmetrical three-phase faults from unbalanced faults requiring symmetrical components (positive, negative, zero sequence). The classic trap: forgetting that zero-sequence impedance depends on transformer winding connections and earthing - a delta winding blocks zero-sequence current from passing through.
Worked rule of thumb: To convert a per-unit impedance from an old base to a new base: Z_pu,new = Z_pu,old × (S_new/S_old) × (V_old/V_new)². If a generator rated 100 MVA, 13.8 kV has X = 0.15 pu on its own base, on a 50 MVA, 13.8 kV base it becomes 0.15 × (50/100) = 0.075 pu. Always check voltage bases match before applying the ratio.
Watch for questions mixing line-to-line and line-to-neutral voltages, or asking for fault current in amperes rather than per unit - divide by √3 for three-phase quantities.