Last reviewed 16 Sept 2026 · 10 min read
Structure of a power system
Generation (typically at around 11–25 kV) → step-up transformers → transmission (e.g. 220, 400, 765 kV AC; HVDC links) → grid substations → sub-transmission (e.g. 66, 33 kV) → distribution substations → primary distribution (11 kV) → distribution transformers (11 kV/433 V) → secondary distribution (400 V three-phase, 230 V single-phase) → consumers.
Conventional generation
Thermal (coal-fired steam) power plant
- Works on the Rankine cycle: boiler (water → high-pressure steam using coal combustion) → turbine (steam expands, drives alternator) → condenser (exhaust steam condensed) → feed pump → boiler.
- Components: coal handling, pulverisers, boiler, superheater, economiser, air preheater, turbine, condenser, cooling tower, electrostatic precipitator (ash), chimney.
- Overall efficiency roughly in the range of 30–40% (higher for supercritical units).
- Advantages: can be located near load/coal; lower initial cost than hydro. Disadvantages: fuel cost, emissions, ash disposal, water needs.
Hydroelectric power plant
FormulaHydro power output
= 1000 kg/m³; = discharge (m³/s); = net head (m); = overall efficiency (turbine × generator, often about 0.85–0.9). In kW: .
- Components: dam/reservoir, intake, penstock, surge tank, turbine (Pelton — high head; Francis — medium; Kaplan — low head), generator, tailrace.
- Classification by head (low, medium, high), by storage (run-of-river, storage, pumped storage), by capacity (small/mini/micro hydro).
- Advantages: no fuel cost, clean, quick start (good for peak load), long life, multipurpose (irrigation, flood control). Disadvantages: high capital cost, long construction, site-dependent, environmental and rehabilitation issues.
Nuclear power plant
- Heat from nuclear fission (uranium-235) in a reactor produces steam for a turbine.
- Reactor components: fuel, moderator (slows neutrons — heavy water, graphite, light water), control rods (absorb neutrons — cadmium, boron), coolant, reflector, shielding.
- Indian programme largely uses pressurised heavy water reactors (PHWR).
- Advantages: very small fuel quantity, no CO₂ during operation, base-load capability. Disadvantages: high capital cost, radioactive waste, safety concerns.
Gas turbine and combined cycle
- Gas turbine — Brayton cycle; quick start (peaking); combined cycle uses exhaust heat in a steam cycle (heat recovery steam generator) — overall efficiency can exceed 50%.
Diesel power plant
- Small capacity, quick start — standby, emergency and isolated supply; high fuel cost.
Renewable energy generation
| Source | Principle | Notes |
|---|---|---|
| Solar photovoltaic (PV) | Semiconductor cells convert sunlight directly to DC electricity; inverters convert to AC | Rooftop and utility-scale; output depends on irradiance and temperature; module efficiencies commonly around 15–22% |
| Solar thermal | Concentrated sunlight produces steam | Large plants |
| Wind | Wind turbine rotor drives generator | Power ∝ cube of wind speed |
| Biomass | Combustion/gasification of agricultural residues, bagasse cogeneration | Dispatchable renewable |
| Small hydro | Run-of-river small plants | Hilly regions |
| Tidal, wave, geothermal | Tidal range, waves, earth's heat | Site specific |
| Green hydrogen and storage | Batteries, pumped storage balance variable renewables | Grid integration |
FormulaWind power
≈ 1.225 kg/m³; (swept area); = wind speed; = power coefficient. Betz limit: maximum theoretical (59.3%).
Load curves and factors
| Term | Definition |
|---|---|
| Connected load | Sum of ratings of all equipment connected |
| Maximum demand | Greatest demand during a period |
| Demand factor | (≤ 1) |
| Average load | Energy consumed ÷ hours |
| Load factor | (≤ 1) — higher is better (lower cost per unit) |
| Diversity factor | (≥ 1) — higher diversity reduces plant capacity needed |
| Plant capacity factor | |
| Plant use (utilisation) factor |
- Load curve — load vs time; load duration curve — loads arranged in descending order.
- Base load plants — run continuously at high load factor: large thermal, nuclear, run-of-river hydro.
- Peak load plants — quick start: storage hydro, pumped storage, gas turbines, diesel.
Transmission
Why high voltage?
For power at voltage and pf , line current ; line loss . Higher voltage → lower current → lower losses, smaller conductors, better regulation; limited by insulation cost.
AC vs HVDC transmission
| Aspect | HVAC | HVDC |
|---|---|---|
| Transformation | Easy (transformers) | Needs converter stations (costly) |
| Losses and conductors | Reactance, skin effect, charging current | No reactance or charging current; fewer conductors |
| Distance | Economical for moderate distances | Economical for very long distances, submarine cables, asynchronous interconnection |
| Stability | Stability limits over long lines | Fast power control |
Standard AC transmission voltages in India include 66, 110, 132, 220, 400 and 765 kV; HVDC links operate at voltages such as ±500 kV and ±800 kV.
Line parameters and classification
- Parameters: resistance R, inductance L, capacitance C, conductance G (leakage) — distributed along the line.
- Short lines (roughly up to about 80 km) — capacitance neglected.
- Medium lines (roughly 80–200 km) — capacitance lumped (nominal-T, nominal-π methods).
- Long lines (beyond about 200 km) — distributed parameters (ABCD constants, hyperbolic functions). (Exact boundaries vary between texts.)
Line phenomena
| Phenomenon | Description |
|---|---|
| Ferranti effect | Receiving-end voltage exceeds sending-end voltage on long lightly loaded or open-ended lines due to line capacitance (charging current) |
| Corona | Ionisation of air around conductors at high voltage — hissing sound, violet glow, ozone, power loss and radio interference; reduced by larger diameter conductors and bundled conductors |
| Skin effect | AC current concentrates near the conductor surface → higher effective resistance; greater at high frequency and large conductors |
| Proximity effect | Current redistribution due to nearby conductors |
Overhead line components
- Conductors: ACSR (aluminium conductor steel reinforced — aluminium for conductivity, steel core for strength) most common; AAAC, AAC, bundled conductors on EHV lines.
- Supports: wooden/RCC/steel poles for distribution; steel lattice towers for transmission.
- Insulators (porcelain, glass, polymer):
- Pin type — up to about 33 kV.
- Suspension (disc) type — strings of discs for higher voltages; number of discs increases with voltage.
- Strain type — at dead ends, sharp turns, river crossings.
- Shackle — low-voltage distribution.
- String efficiency — less than 100% because the disc nearest the conductor carries the most voltage; improved by guard (grading) rings, longer cross-arms, capacitance grading.
- Earth wire (shield wire) on top protects against lightning; lightning arresters at substations.
Sag
FormulaSag in overhead lines (supports at equal level)
= weight per unit length (N/m) (including ice and wind effects where applicable); = span; = horizontal tension. Sag must maintain ground clearance while limiting tension; it increases in hot weather.
Underground cables
- Used in urban areas, crossings and where overhead lines are impractical — no visual impact, less affected by weather; much higher cost, difficult fault location, charging current limits length (for AC).
- Construction: conductor, insulation (XLPE, PVC, paper), metallic sheath, bedding, armouring, outer serving.