Power Systems: Generation, Transmission, and Distribution
What this domain tests
This domain covers per-unit modelling, line parameters, load flow, voltage regulation, stability, and distribution practice.
Core rules
- Per-unit base: Z_base = V_base^2/S_base. Change of base uses Z_new = Z_old (S_new/S_old)(V_old/V_new)^2.
- Short lines use series impedance; medium lines use nominal pi or T; long lines use distributed ABCD parameters with hyperbolic functions.
- Load-flow bus types: slack bus sets voltage magnitude and angle; PV bus sets voltage magnitude and real power; PQ bus sets real and reactive power.
- Voltage regulation = (V_no-load - V_full-load)/V_full-load x 100% for the same sending voltage.
- Stability improves with shorter lines, higher voltage, lower reactance, and faster excitation or fault clearing. The equal-area criterion compares accelerating and decelerating areas.
- Distribution is usually radial; voltage drop and fault level depend strongly on conductor impedance and transformer tap position.
Worked example
On a 100 MVA, 132 kV base, a 20 ohm transmission line is to be expressed in per unit. Z_base = 132^2/100 = 174.24 ohm. Z_pu = 20/174.24 = 0.115 pu. If the base were changed to 200 MVA at the same voltage, Z_pu would become 0.115 x 200/100 = 0.230 pu.
Traps
- Mixing per-unit values from different base voltages or power bases without conversion.
- Confusing line-to-line and line-to-neutral voltage bases.
- Treating the slack bus as a PQ bus in load flow.
- Assuming tap changers improve stability rather than mainly voltage profile.