Power System Analysis and Design
What this domain tests
Power System Analysis and Design tests whether you can model an Egyptian network at the right voltage level, convert data to a common base, calculate fault levels and voltage profiles, and select equipment ratings with defensible margins. Expect LV 220/380 V or 230/400 V, MV 11/22 kV, and transmission 66/220 kV contexts, with IEC 60909 short-circuit methods and load-flow reasoning.
Core rules and concepts
- Use per-unit analysis: choose base MVA and base kV, then convert every impedance to that base. Change-of-base formula: Znew = Zold × (Sbase,new/Sbase,old) × (Vbase,old/Vbase,new)².
- Base current: Ib = Sbase/(√3 × Vbase). For a three-phase fault, Isc = Ib/Zpu, with the equivalent source impedance including transformer, cable, and motor contributions.
- IEC 60909 uses a voltage factor c (commonly 1.1 for MV/HV, lower for LV depending on tolerance) and distinguishes maximum and minimum fault currents.
- Check four separate design limits: thermal withstand, voltage drop, fault rating, and mechanical/peak withstand. Breaker making capacity depends on peak current and X/R.
- Load flow confirms transformer tap position, reactive compensation, and whether motor starting causes unacceptable dip.
Worked example
A 5 MVA, 11 kV/0.4 kV transformer has 6% impedance. On a 10 MVA, 11 kV base, Zpu = 0.06 × (10/5) = 0.12 pu. Base current at 11 kV is 10,000/(√3 × 11) = 524.9 A. Ignoring cable and motor contribution, the 11 kV fault current is 524.9/0.12 = 4.37 kA. If the feeder adds 0.05 pu, the total is 0.17 pu and the fault current falls to 3.09 kA. The lesson: the transformer impedance dominates near the source, but feeder impedance cannot be ignored for downstream equipment ratings.
Traps that cost marks
Using the transformer’s own MVA as the base and forgetting to convert; omitting √3 in three-phase current; mixing 380 V and 400 V nominal values; adding motor contribution only when convenient rather than per IEC 60909; and quoting a breaker’s interrupting rating without checking making capacity and X/R. Also, a design that passes voltage drop may still fail thermal or fault-rating checks.
Exam-focused summary
State the base, show the conversion, and carry units. A clean per-unit table prevents most errors.