Electrical Power System Design and Analysis
The Electrical Power System Design and Analysis domain tests whether you can build a consistent model and use it to predict normal and abnormal performance. It covers per-unit bases, load flow, short-circuit and symmetrical components, transformer connections, protection coordination, motor starting, harmonics, and voltage-drop calculations. The exam rewards a clear base system and assumptions before arithmetic.
Per-unit and equipment models
Choose Sbase and voltage bases that are consistent through transformers. Zbase = kVbase²/MVAbase. Convert equipment impedance with Znew = Zold(Snew/Sold)(Vold/Vnew)². Generators use subtransient reactance for interrupting duty and transient/steady-state values for other studies. Transformers need winding connection, grounding, tap, and phase shift; zero-sequence networks depend on those details.
Load flow and voltage
Buses are slack, PV, or PQ. The slack bus balances losses; PV buses hold voltage with reactive limits; PQ buses specify load. Regulated voltage must be checked against compensation, tap ranges, and conductor loading. Motor starting is a voltage-dip problem: starting kVA and source impedance govern the dip.
Fault analysis and coordination
Symmetrical components separate a balanced or unbalanced fault into positive, negative, and zero sequence networks. Three-phase fault uses positive sequence only; single line-to-ground needs all three and the neutral grounding path. Protection coordination requires time-current curves, CT ratios, pickup, and time dial settings, with a margin between devices.
Worked example
A 50 MVA, 13.8/230 kV transformer has Z = 10% on its own base. On 100 MVA and 230 kV, with the 13.8 kV base unchanged: Zpu = 0.10 × (100/50) = 0.20 pu. At 230 kV, rated current is 50,000/(√3×230) = 125.5 A; a bolted low-side fault referred to the high side gives approximately 125.5/0.10 = 1,255 A, or 100,000/(√3×230×0.20) = 1,255 A. The two methods agree.
Traps
Mixing bases on the two sides of a transformer; ignoring the transformer's phase shift or zero-sequence path; using load-flow voltage as a fault-study voltage; and setting relays from nameplate current without CT ratio and margin.