Transmission and Distribution Systems
This domain tests how power moves at high voltage over long distances and then steps down to the consumer, and whether you can reason about line losses, voltage levels and distribution layouts.
Transmission hierarchy in India
400 kV and 220 kV form the backbone, with 765 kV on major corridors; state networks use 132 kV and 66 kV. Sub-transmission and primary distribution run at 33 kV and 11 kV, with 400/230 V three-phase four-wire LT for consumers. ACSR conductors are standard because the aluminium conducts and the steel core gives mechanical strength.
Line behaviour you must know
- Short lines (under 80 km): resistance only. Medium lines (80–250 km): nominal pi or T model. Long lines: distributed parameters, and the Ferranti effect raises receiving-end voltage at light load.
- Corona loss rises with voltage and falls with larger conductor diameter; bundled conductors reduce both corona and reactance.
- Skin effect pushes AC current to the conductor surface, raising effective resistance above the DC value.
Distribution practice
Radial feeders are cheap and simple but have poor reliability; ring main systems allow back-feeding. Distribution transformers step 11 kV down to 400 V, and balanced loading across phases keeps neutral current low.
Worked example: loss reduction by voltage upgrade
A feeder carries 200 A over 0.5 Ω of resistance. Copper loss = I²R = 200² × 0.5 = 20,000 W = 20 kW. If the same power is delivered at double the voltage, the current halves to 100 A and loss becomes 100² × 0.5 = 5 kW - a quarter of the original. That single relation explains why high-voltage transmission exists and why HT distribution beats LT for the same load.
Traps
Adding I²R loss as a percentage of the sending-end value when the base is the load; assuming 11 kV is a transmission voltage; forgetting that neutral sizing matters in unbalanced LT networks; treating T&D loss as purely technical when a large part is commercial (theft, unmetered supply, billing errors).