Power Systems and Distribution
Power Systems and Distribution covers the path from generation to the LT service, distribution layouts, diversity and transformer sizing.
Core rules
- Generation is typically at 11–25 kV, transmitted at 132, 220 or 400 kV, distributed at 11 kV and supplied at 415 V three-phase four-wire, giving 230 V phase-to-neutral.
- High transmission voltage reduces current for the same power, so I²R losses and conductor size fall; it does not reduce the power delivered.
- Radial distribution is simple and cheap but has poor reliability; ring mains allow two paths and are common in domestic wiring.
- Diversity factor = sum of individual maximum demands / maximum demand on the system; it is greater than 1. Load factor = average load / maximum demand.
- ACSR conductors combine aluminium conductivity with steel strength; disc insulators are graded for the line voltage.
Worked example
Five houses each have a maximum demand of 4 kW. The sum is 20 kW. With a diversity factor of 1.25, the distributor maximum demand = 20/1.25 = 16 kW. At a power factor of 0.8, the required apparent power = 16/0.8 = 20 kVA. A 25 kVA distribution transformer gives a reasonable margin. If the load factor is 0.4, average load = 0.4 × 16 = 6.4 kW, which is useful for energy-loss estimation.
Traps
Diversity factor is not the same as load factor; it is usually greater than 1, not less. Do not multiply by √3 for a single-phase 230 V load, and do not claim that raising voltage lowers the power; it lowers current and losses for the same power.