Power System Analysis and Protection
Power System Analysis and Protection tests per-unit modeling, fault calculation, load flow, stability, and protective relaying. Expect symmetrical components, sequence networks, relay coordination, CT/VT ratios, and fault-current interpretation.
Core rules
- Per-unit impedance: Z_pu = Z_actual / Z_base, with Z_base = V_base²/S_base.
- A three-phase fault uses the positive-sequence network only; unbalanced faults require negative- and zero-sequence networks.
- Grounding and transformer connection determine whether zero-sequence current can flow.
- Protection must be selective, fast, sensitive, reliable, and secure; CT saturation can delay or misoperate relays.
- Relay pickup is a threshold; coordination uses time-current curves and margins.
Worked example
On 11 kV, 10 MVA base, Z_base=11²/10=12.1 Ω. A source impedance of 1.21 Ω is 0.1 pu. For a bolted three-phase fault with only this source impedance, I_pu=1/0.1=10 pu. Base current I_base=10 MVA/(√3×11 kV)≈525 A, so fault current≈5.25 kA. If a transformer adds 0.05 pu, total is 0.15 pu and current falls to about 3.5 kA.
Traps
Forgetting √3 in base current is the most common error. Adding impedances from different MVA bases without conversion is invalid. In zero-sequence networks, a delta winding blocks transfer but can circulate current. Relay settings based on load current rather than fault current may never trip. CT ratio mismatch can make differential protection operate on through-faults.