Generation, Transmission, and Distribution Engineering
This domain tests the physical design and rating of facilities: generator heat rate and reactive capability, transmission line electrical and mechanical design, substation arrangement, and distribution voltage and loading practice.
Generation
- Plant economics turn on heat rate (Btu/kWh) and incremental heat rate; combined cycles typically sit near 6,000-7,000 Btu/kWh, simple cycles near 10,000.
- The reactive capability curve is bounded by stator heating, field heating, and end-region heating; it is not a circle.
Transmission
- Conductor selection (ACSR, ACSS, composite), sag-tension, tower spotting, and NESC clearances govern the mechanical design.
- Ampacity follows the IEEE 738 steady-state heat balance: resistive heating balanced against convective, radiative, and solar gain, with wind speed and ambient temperature assumed explicitly.
- Insulation coordination uses basic impulse level against switching and lightning surges; shielding and tower footing resistance control backflashover.
- Surge impedance loading (roughly V squared over 400 ohms) indicates the loading at which a line is electrically neutral; long EHV lines exhibit the Ferranti voltage rise and need shunt reactors.
Distribution
- Primary voltages span 4.16 to 34.5 kV, largely radial with reclosers and fuses; voltage is managed with load-tap changers, regulators, and capacitors.
- Multigrounded neutral construction and effective grounding govern fault detection and safety.
Worked example
A 12.47 kV three-phase feeder carries 400 A over 2 miles with R = 0.3 ohm/mile, X = 0.6 ohm/mile, load power factor 0.9. Voltage drop = 1.732 x I x (R cos theta + X sin theta) x length = 1.732 x 400 x (0.3 x 0.9 + 0.6 x 0.436) x 2 = 693 x 0.5316 x 2 = 737 V. As a percentage of 12,470 V that is 5.9%, above the 5% Range A guideline and a signal to reconductor, add a regulator, or split the feeder.
Traps that cost marks
- Omitting the 1.732 factor in three-phase voltage drop and fault calculations.
- Using conductor resistance at 20 C rather than at operating temperature.
- Sizing a conductor on thermal ampacity alone while ignoring voltage drop.
- Treating surge impedance loading as a thermal limit rather than an electrical reference point.
- Forgetting the Ferranti effect on long lightly loaded lines.