Electrical Engineering - Power Systems
Power Systems in the TSGENCO AE exam tests network modelling, per-unit analysis, load flow, symmetrical faults, protection, stability and transmission-line performance. Numerical questions usually combine a one-line diagram with base quantities, so set the base MVA and base kV first.
Core rules
- Per-unit impedance: Z_pu = Z_actual/Z_base, where Z_base = kV_base²/MVA_base.
- For a three-phase fault, use the positive-sequence Thevenin reactance only: I_f(pu) = 1/X_th(pu).
- Fault MVA = MVA_base/X_th(pu); line current = fault MVA/(√3 × kV).
- Surge impedance loading, SIL = V_L²/Z_c; roughly 1 MW per kV for a 1 kΩ surge impedance.
- Protection: overcurrent for distribution, distance for long lines, differential for transformers and generators.
Worked example
A 100 MVA, 11/132 kV system has generator X = 0.2 pu on its own 100 MVA base and transformer X = 0.1 pu. At the 132 kV bus, X_th = 0.3 pu. Fault MVA = 100/0.3 = 333.3 MVA. Fault current at 132 kV = 333.3/(1.732 × 132) = 1.46 kA. If the transformer were rated 50 MVA, its 0.1 pu would become 0.2 pu on a 100 MVA base before adding.
Traps
Mixing line-to-line and phase voltage; forgetting to convert all impedances to one base; using subtransient reactance for steady-state fault current; applying distance protection without considering infeed; and treating surge impedance loading as a thermal limit.