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Computer Science and Electronics for Digital Assistant · Chapter 11

Power System Generation and Protection

What to remember

  • Power moves in four stages: generation, transmission, distribution, utilisation. High voltage is used for long-distance transmission to cut losses.
  • Power stations use steam, water, nuclear fuel, sun, wind or gas. The generator (alternator) is turned by a prime mover.
  • Protection uses fuses, circuit breakers and relays. Earthing keeps people and equipment safe from faults.

Structure of a power system

Electric power is generated in power stations at about 11 kV to 25 kV, stepped up by transformers for transmission, carried over long lines, stepped down at substations and delivered to consumers. In India, the frequency is 50 Hz. Standard transmission levels include 132 kV, 220 kV, 400 kV and 765 kV. Primary distribution is at 11 kV, and the consumer gets 230 V single phase or 415 V three phase.

StageTypical voltage levelEquipment
Generation11 kV to 25 kVAlternator, turbine
Transmission132 kV and aboveStep-up transformer, towers, insulators
Sub-transmission / primary distribution33 kV, 11 kVSubstation, step-down transformer
Secondary distribution415 V / 230 VDistribution transformer, poles, meters

Why high voltage for transmission: for the same power P = V I, a higher voltage gives lower current, and line loss I²R falls with the square of the current. Example: if the voltage is raised 10 times, the current falls to one-tenth and line loss falls to one-hundredth.

Conventional generation

Thermal (steam) power station: coal, oil or gas heats water into high-pressure steam. The steam turns a turbine coupled to the alternator. The main parts are coal handling, boiler, superheater, turbine, condenser, cooling tower, and ash handling. The Rankine cycle is the basic working cycle. Efficiency is limited (about one-third of the fuel energy becomes electricity). Fly ash and carbon dioxide are the environmental concerns.

Hydro power station: water stored in a dam falls through a penstock and turns a turbine. Power depends on head (height) and flow: P = ρ g Q H η, which is about 9.81 × Q × H × η kW when Q is in m³/s and H is in metres. Example: Q = 10 m³/s, H = 50 m, efficiency 0.9 gives 9.81 × 10 × 50 × 0.9 = 4414.5 kW. Turbines: Pelton wheel (high head, impulse), Francis (medium head, reaction) and Kaplan (low head, reaction). Hydro stations start quickly and have low running cost, but they depend on rain and need a large dam. Pumped storage stores energy by pumping water uphill when demand is low. The Srisailam project on the Krishna river is a well-known hydro station of the Telugu states, and Polavaram is the multipurpose project on the Godavari (check the latest official release for status).

Nuclear power station: fission of uranium heats a coolant to make steam. The reactor has fuel, moderator (slows neutrons), control rods (absorb neutrons) and coolant. A small amount of fuel gives very large energy and there is no smoke, but waste disposal and safety need great care.

Gas turbine and diesel stations: quick to start, used for peak and standby. A combined cycle plant uses the exhaust heat of a gas turbine to make steam and gets higher efficiency.

Renewable generation

Solar photovoltaic (PV) cells convert sunlight directly to DC electricity, and an inverter changes it to AC. Solar thermal uses mirrors to heat a fluid. Wind turbines turn a generator; power is proportional to the cube of the wind speed. Biomass, small hydro and tidal energy are other sources. Renewable sources are clean but depend on weather, so storage and grid support are used. Andhra Pradesh has high potential for solar and wind and also has solar parks (check the latest official release for names and capacities). Rooftop solar can feed surplus power to the grid under net metering.

Load, demand and tariff terms

TermMeaning
Connected loadSum of ratings of all devices connected
Maximum demandHighest load in a given period
Demand factorMaximum demand / connected load
Load factorAverage load / maximum demand
Plant capacity factorActual energy produced / energy at full rating
Diversity factorSum of individual maximum demands / combined maximum demand

Base load is the minimum steady demand; peak load is the high demand for a short time. A higher load factor means better use of the plant and a lower cost per unit. Worked example: a plant has maximum demand 100 MW and a daily energy of 1,800 MWh. Average load = 1800/24 = 75 MW, load factor = 75/100 = 0.75.

Billing: energy is charged per kWh (unit). Tariffs may include a fixed charge, an energy charge, and a power factor penalty. Meters record kWh. Power factor is cos φ; a low power factor raises current for the same real power, so capacitors are used to improve it.

Transmission and distribution

Overhead lines use tower, conductors (such as ACSR, aluminium conductor steel reinforced), insulators (porcelain, glass) and cross arms. Underground cables are safer in cities but costly. Insulator types: pin, suspension (disc) and strain. Line parameters are resistance, inductance and capacitance. Corona is the glow and loss around conductors at very high voltage. Distribution systems may be radial (simple, one path), ring main (more reliable) or interconnected. HVDC is used for very long lines and for joining separate grids. The national grid joins regional grids, and load despatch centres balance supply and demand (check the latest official release for names of state agencies).

A substation changes voltage and has transformers, busbars, isolators, circuit breakers, lightning arresters, current and voltage transformers, and relays. An isolator is operated without load only. A circuit breaker can open under load and fault.

Faults and protection

Common faults are short circuit (between phases or phase to earth), open circuit and overload. A short circuit gives a very large current. Protection devices:

  • Fuse: a thin wire that melts when current is too high; cheap and used on small circuits. It must be replaced after it operates.
  • MCB (miniature circuit breaker): resets after a trip; protects against overload and short circuit in homes.
  • MCCB / ELCB / RCCB: moulded case breaker; earth leakage and residual current breakers cut power when current leaks to earth and protect people from shock.
  • Circuit breakers: oil, air blast, SF6 gas and vacuum types. Arc quenching is the main job.
  • Relay: senses a fault and sends a trip signal to the breaker. Types include overcurrent, differential, distance and Buchholz relays. The Buchholz relay protects oil-filled transformers against internal faults.
  • Lightning arrester: diverts surge voltage to the earth.
  • Surge protection and earthing reduce damage from over-voltages.

Earthing: connecting the metal body of equipment to the general mass of earth through an electrode (plate, pipe or rod). It makes sure that the fault current finds a safe path and the body voltage stays near zero. The earth resistance should be as low as practical. Pipe and plate earthing are common.

Safety

Switch off before work, use insulated tools and rubber mats, never use water on an electrical fire (use CO₂ or dry powder extinguishers), and give first aid for shock by cutting power first. Always treat a line as live until it is tested and proven dead.

Quick facts on plants

PlantFuel or sourcePrime moverMain advantageMain drawback
ThermalCoal, gas or oilSteam turbineSteady large outputPollution, fuel cost
HydroStored waterWater turbineCheap, quick startDepends on rain, large dam
NuclearUraniumSteam turbineLarge energy from little fuelWaste and safety
Solar PVSunlightNone (direct DC)Clean, modularOnly in daytime
WindWindWind turbineCleanDepends on wind

Plant efficiency is output energy divided by input energy. Worked example: a plant burns fuel giving 300 MW of heat and delivers 90 MW of electricity; its efficiency is 90/300 = 30%. Worked example: a load of 2 kW running for 5 hours uses 10 kWh; at a rate of 5 rupees per unit the cost would be 50 rupees (the rate is only an example). Distribution losses can be cut by using higher voltages, correct conductor size and power factor correction.

Exam traps

  • Fuse and MCB: a fuse is replaced after use; an MCB is reset.
  • Isolator and circuit breaker: an isolator is not for breaking load current.
  • Relay and breaker: the relay senses; the breaker opens.
  • Load factor and demand factor: load factor uses average load; demand factor uses connected load.
  • Pelton, Francis and Kaplan: high, medium and low head in that order.
  • Moderator and control rod: a moderator slows neutrons; a control rod absorbs them.
  • Line loss: it depends on the square of the current, not the voltage directly.
  • Earthing and neutral: they are different conductors with different jobs.
  • Fuse in the neutral: a fuse or switch should be in the live wire.
  • kW and kVA: real power in kW, apparent power in kVA.

One-liners

  • 1. India's grid frequency is 50 Hz.
  • 2. Stepping up voltage cuts transmission loss.
  • 3. A penstock carries water to a hydro turbine.
  • 4. The Pelton wheel suits high head.
  • 5. The Kaplan turbine suits low head.
  • 6. Control rods absorb neutrons in a reactor.
  • 7. Load factor = average load / maximum demand.
  • 8. A lightning arrester protects against surges.
  • 9. An MCB can be reset after tripping.
  • 10. The Buchholz relay protects transformers.
  • 11. Earthing gives a safe path for fault current.
  • 12. Pumped storage uses surplus power to lift water.

Practice questions

  1. A plant has a maximum demand of 100 MW and supplies 1800 MWh in a day. The load factor is

    1. 1.33
    2. 0.5
    3. 0.25
    4. 0.75
    Answer

    D. 0.75

    Average load = 1800/24 = 75 MW; load factor = 75/100 = 0.75.

  2. A plant has an average load of 50 MW and a maximum demand of 100 MW. The load factor is

    1. 2
    2. 0.25
    3. 0.5
    4. 0.75
    Answer

    C. 0.5

    Load factor = average load / maximum demand = 0.5.

  3. A consumer has a connected load of 100 kW and a maximum demand of 60 kW. The demand factor is

    1. 1.67
    2. 0.6
    3. 0.4
    4. 6
    Answer

    B. 0.6

    Demand factor = maximum demand/connected load = 0.6.

  4. The sum of individual maximum demands is 150 MW and the combined maximum demand is 100 MW. The diversity factor is

    1. 0.67
    2. 50
    3. 2.5
    4. 1.5
    Answer

    D. 1.5

    Diversity factor = 150/100 = 1.5.

  5. A 50 MW unit generates 600 MWh in a day. Its capacity factor is

    1. 0.25
    2. 0.12
    3. 12
    4. 0.5
    Answer

    D. 0.5

    600/(50 x 24) = 0.5.

  6. If the transmission voltage is raised 10 times for the same power, the line loss becomes

    1. one-hundredth
    2. ten times
    3. one-tenth
    4. unchanged
    Answer

    A. one-hundredth

    Current falls to one-tenth, and loss I^2 R falls to one-hundredth.

  7. A line of resistance 5 ohm carries 10 A. The line loss is

    1. 5000 W
    2. 500 W
    3. 50 W
    4. 15 W
    Answer

    B. 500 W

    Loss = I^2 R = 100 x 5 = 500 W.

  8. A hydro plant has a flow of 10 m^3/s, a head of 50 m and an overall efficiency of 0.9. The power is about

    1. 4905 kW
    2. 491 kW
    3. 44,145 kW
    4. 4414 kW
    Answer

    D. 4414 kW

    P = 9.81 x 10 x 50 x 0.9 = 4414.5 kW.

  9. A plant receives 300 MW of heat and delivers 90 MW of electricity. Its efficiency is

    1. 90%
    2. 30%
    3. 3.3%
    4. 33%
    Answer

    B. 30%

    90/300 = 0.30.

  10. A 2 kW load runs for 5 hours. At 5 rupees per unit (an example rate) the cost is

    1. 25 rupees
    2. 10 rupees
    3. 50 rupees
    4. 100 rupees
    Answer

    C. 50 rupees

    Energy = 10 kWh; cost = 10 x 5 = 50 rupees.

  11. A 10 kVA load has a power factor of 0.8. The real power is

    1. 8 kW
    2. 12.5 kW
    3. 10 kW
    4. 6 kW
    Answer

    A. 8 kW

    P = kVA x power factor = 10 x 0.8 = 8 kW.

  12. If the wind speed doubles, the power available in the wind becomes

    1. 2 times
    2. 8 times
    3. 4 times
    4. 16 times
    Answer

    B. 8 times

    Wind power is proportional to the cube of speed.

  13. Consumer energy is billed in units, where one unit equals

    1. 1 MW
    2. 1 kW
    3. 1 kWh
    4. 1 kVA
    Answer

    C. 1 kWh

    A unit of electricity is one kilowatt-hour.

  14. The frequency of the grid supply in India is

    1. 50 Hz
    2. 25 Hz
    3. 100 Hz
    4. 60 Hz
    Answer

    A. 50 Hz

    India's grid operates at 50 Hz.

  15. The pipe that carries water from a dam to the turbine of a hydro station is the

    1. Surge arrester
    2. Draft tube
    3. Cooling tower
    4. Penstock
    Answer

    D. Penstock

    A penstock is the high-pressure pipe feeding the turbine.

  16. Which turbine is used for high heads in hydro stations?

    1. Propeller turbine
    2. Francis turbine
    3. Kaplan turbine
    4. Pelton wheel
    Answer

    D. Pelton wheel

    Pelton is an impulse turbine for high heads.

  17. Which turbine is used for low heads and large flows?

    1. Kaplan turbine
    2. Steam turbine
    3. Pelton wheel
    4. Gas turbine
    Answer

    A. Kaplan turbine

    Kaplan is a reaction turbine with adjustable blades.

  18. In a nuclear reactor, the material that absorbs neutrons to control the reaction is the

    1. coolant
    2. control rod
    3. fuel pellet
    4. moderator
    Answer

    B. control rod

    Control rods absorb neutrons; the moderator slows them.

  19. A moderator in a nuclear reactor is used to

    1. absorb all neutrons
    2. cool the turbine
    3. slow down neutrons
    4. generate AC
    Answer

    C. slow down neutrons

    Slow neutrons are better at causing fission in uranium-235.

  20. Which device converts sunlight directly into DC electricity?

    1. Transformer
    2. Solar PV cell
    3. Alternator
    4. Boiler
    Answer

    B. Solar PV cell

    The photovoltaic effect produces DC.

  21. A device used on a solar PV system to change DC to AC is the

    1. relay
    2. fuse
    3. inverter
    4. rectifier
    Answer

    C. inverter

    An inverter converts DC to AC.

  22. Which device protects equipment from lightning and surge voltages?

    1. Lightning arrester
    2. Isolator
    3. Fuse
    4. Wattmeter
    Answer

    A. Lightning arrester

    It diverts surge current to the earth.

  23. Which device can be reset after tripping and is used in homes?

    1. Rewirable fuse
    2. Isolator
    3. Bus bar
    4. MCB
    Answer

    D. MCB

    A miniature circuit breaker is reset by hand.

  24. Which relay protects oil-filled transformers against internal faults?

    1. Thermal relay
    2. Overcurrent relay
    3. Buchholz relay
    4. Distance relay
    Answer

    C. Buchholz relay

    The Buchholz relay detects gas from internal faults.

  25. Which device is operated only when there is no load current through it?

    1. Isolator
    2. Fuse
    3. MCB
    4. Circuit breaker
    Answer

    A. Isolator

    An isolator provides a visible gap and is not meant to break load.

  26. The main job of earthing is to

    1. raise the line voltage
    2. reduce frequency
    3. increase the load factor
    4. give fault current a safe path to earth
    Answer

    D. give fault current a safe path to earth

    Earthing keeps the body of equipment near earth potential.

  27. Which fire extinguisher is safe on an electrical fire?

    1. Water
    2. CO2
    3. Bucket of sand and water
    4. Foam with water
    Answer

    B. CO2

    CO2 and dry powder do not conduct electricity.

  28. Which conductor is commonly used in overhead transmission lines?

    1. ACSR
    2. Tungsten
    3. Brass
    4. Nichrome
    Answer

    A. ACSR

    Aluminium conductor steel reinforced combines light weight and strength.

  29. Which insulator type is used for high-voltage lines as a string of discs?

    1. Shackle insulator
    2. Post pin bushing
    3. Suspension insulator
    4. Pin insulator
    Answer

    C. Suspension insulator

    Disc strings hang from the cross arm.

  30. The glow and power loss around conductors at very high voltage is called

    1. surge
    2. corona
    3. arc quenching
    4. flashover
    Answer

    B. corona

    Corona results from ionisation of the air.

  31. The device that senses a fault and sends a trip signal to the circuit breaker is a

    1. busbar
    2. fuse
    3. isolator
    4. relay
    Answer

    D. relay

    The relay senses; the breaker opens.

  32. Which of these is a renewable source of electricity?

    1. Natural gas
    2. Wind
    3. Diesel
    4. Coal
    Answer

    B. Wind

    Wind, solar and small hydro are renewable.

  33. A pumped storage plant

    1. burns coal at night only
    2. pumps water uphill when demand is low
    3. stores heat in the boiler
    4. stores charge in a battery bank only
    Answer

    B. pumps water uphill when demand is low

    The stored water drives turbines at peak demand.

  34. Statement 1: A fuse is replaced after it operates. Statement 2: An MCB can be reset after tripping. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

  35. Statement 1: Raising the transmission voltage reduces line current for the same power. Statement 2: Line loss depends on the square of the current. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

  36. Statement 1: A circuit breaker can open a circuit under fault. Statement 2: An isolator is used to break heavy load current. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    An isolator is not meant for breaking load current.

  37. Statement 1: Load factor equals average load divided by maximum demand. Statement 2: Demand factor equals average load divided by connected load. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Demand factor = maximum demand/connected load.

  38. Statement 1: Thermal power stations use steam turbines. Statement 2: Hydro stations depend on rainfall and storage. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

  39. Statement 1: A moderator absorbs neutrons. Statement 2: Control rods slow down neutrons. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    D. Neither 1 nor 2

    Both roles are swapped: the moderator slows neutrons and control rods absorb them.

  40. Statement 1: Overload and short circuit are common faults. Statement 2: A short circuit gives a very large current. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

  41. Statement 1: Rooftop solar can feed surplus power to the grid under net metering. Statement 2: Solar PV output is the same at night as in daytime. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Solar PV gives no output at night.

  42. Statement 1: An ELCB or RCCB cuts the supply when current leaks to earth. Statement 2: A fuse in a circuit should be placed in the live wire. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

  43. Statement 1: Pelton wheel is a reaction turbine for low heads. Statement 2: Kaplan turbine is suited to low heads. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Pelton is an impulse turbine for high heads.

  44. Statement 1: In combined cycle plants, exhaust heat of a gas turbine makes steam. Statement 2: Combined cycle plants have higher efficiency than a simple gas turbine plant. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

  45. Statement 1: Capacitors are used to improve a low power factor. Statement 2: A low power factor increases current for the same real power. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

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