Power Distribution System Overview: Generation, Transmission, Distribution
This domain checks whether you understand the overall power path and the vocabulary of distribution: generation voltage, transmission levels, primary and secondary distribution, substations, feeders, distributors and service mains. Questions often test where a component sits in the chain and why a particular voltage level is used.
Core rules
- Generation is produced at moderate voltage, stepped up for transmission to reduce current and I²R loss, then stepped down in stages.
- In India, transmission is commonly 132 kV, 220 kV or 400 kV; sub-transmission and primary distribution use 66 kV, 33 kV or 11 kV; secondary distribution is 415 V three-phase, 240 V single-phase.
- A distribution substation typically has an 11 kV incoming feeder and an 11/0.433 kV distribution transformer. The transformer no-load secondary is about 433 V so that consumers receive near 415 V at load.
- A feeder carries power from the substation to a distribution transformer or load point and should not be tapped along its length. A distributor is tapped to serve consumers. A service main connects the distributor to the consumer's installation.
- Radial distribution is simple and cheap but has poor reliability; ring and interconnected systems improve continuity but need more switching and protection.
Worked example
A 400 kVA, 11 kV/433 V distribution transformer is fully loaded. Find the HV and LV line currents.
HV current = 400000 / (√3 × 11000) = 400000 / 19052 ≈ 21.0 A. LV current = 400000 / (√3 × 433) = 400000 / 750 ≈ 533 A. The HV current sets the HV fuse and cable requirement; the LV current sets the LV breaker, busbar and outgoing distributor capacity. The calculation uses line voltage and line current because the transformer is rated in three-phase kVA.
Traps that cost marks
- Confusing a feeder with a distributor; feeders are not meant for consumer tappings.
- Using 415 V instead of the transformer's 433 V secondary when calculating full-load LV current.
- Forgetting that transmission is stepped up to reduce current, not to increase power.
- Treating a ring system as automatically more reliable without correct switching and protection.