Electrical, Electronics and Telecommunication Engineering: Systems, Machines, Protection and Electronics
Electrical, Electronics and Telecommunication Engineering in IESL Part III covers power system analysis and protection, transformers and machines, power electronics, analogue and digital electronics, and communication links.
Core rules and concepts
- Per-unit analysis: choose base MVA and base kV, convert impedances with Z_pu,new = Z_pu,old × (S_base,new/S_base,old) × (V_base,old/V_base,new)². Impedances in series add; parallel branches combine as admittances.
- Three-phase quantities: S = √3·V_LL·I, and phase voltage is V_LL/√3 in star connection.
- Fault level at a bus ≈ S_base/Z_pu,total from the source to that bus, giving I_f = fault MVA/(√3·V_LL).
- Protection: grading by current and time, CT ratio and burden, and reliable discrimination between upstream and downstream devices.
- Machines: synchronous machine power-angle behaviour, induction motor slip s = (Ns − N)/Ns and torque proportional to slip at small slip.
- Electronics: op-amp ideal rules, transistor bias and small-signal gain; in communications check link budgets in dB and convert dBm to watts when required.
Worked example
A 1000 kVA, 11 kV/400 V transformer has 5% impedance and is fed from a source of negligible impedance. Fault MVA at the LV terminals = 1000/0.05 = 20 MVA. Fault current I_f = 20 × 10⁶/(√3 × 400) = 28.9 kA. Downstream switchgear must have a rated short-circuit breaking capacity above this value, and cable thermal withstand must be checked against the clearing time.
Traps that cost marks
Forgetting that per-unit base changes rescale impedances; dropping the √3 in three-phase fault and power formulas; confusing line and phase values in delta and star; using nominal motor current instead of starting current for voltage-dip checks; and adding dB values linearly.