Power Distribution System Overview and Layout
Domain 2 - Power Distribution System Overview and Layout. This domain tests how well you can describe the chain from generation to the consumer's terminal, and why networks are laid out as they are.
Structure to remember
- Generation → EHV transmission (400/220/132 kV) → sub-transmission (66/33 kV) → primary distribution (11 kV, sometimes 6.6 or 22 kV) → distribution transformer (11/0.433 kV) → secondary distribution at 415 V three-phase and 240 V single-phase.
- A feeder carries power from the substation to distribution transformers and takes the whole downstream load. A distributor is tapped at many points, so its voltage drop rather than current decides its size. A service main belongs to the consumer.
- Rural networks are mostly radial: cheap, simple, but a fault trips the whole feeder. Urban areas use ring mains with isolators so a faulty section can be cut out while the rest stays alive.
- A 33/11 kV substation has an 11 kV bus with several outgoing feeders, each protected by a breaker or isolator, and each transformer protected by a DO fuse and an AB switch.
Voltage limits
Declared voltages are 11 kV and 415/240 V. State regulations typically allow about ±6% at the LT consumer terminal and roughly ±9% at 11 kV, which is why LT design is governed by voltage drop, not conductor heating.
Worked example
A feeder supplies 100, 250 and 400 kVA transformers. Installed capacity is 750 kVA; with a diversity factor of 0.7 the design load is 525 kVA, and feeder current = 525 000/(√3 × 11 000) = 27.6 A. Simply adding transformer capacities would overstate the current by about 43% and oversize every upstream item.
Traps
- Confusing feeder and distributor when explaining voltage drop.
- Treating transformer kVA as kW.
- Using 400 V where the system is 415 V, or writing 240 V as 220 V.
- Drawing a single-line diagram with an isolator on the load side of a breaker.