9.1 Power Plant

9.1 Power Plant

1. Structure of a Modern Electrical Power System

A modern electrical power system is an integrated network designed to generate, transmit, and distribute electricity reliably and efficiently from producers to consumers. It operates as a large, interconnected grid.

1.1 Generation

• Location: Power plants (thermal, hydro, nuclear, renewable farms).

• Function: Converts primary energy (coal, water, uranium, sunlight, wind) into electrical energy.

• Voltage Level: Typically generates at 11 kV to 33 kV.

• Output: Three-phase alternating current (AC) at standard frequency (50 Hz in Nepal/India, 60 Hz in some countries).

1.2 Transmission

• Purpose: To transport bulk electricity over long distances from generating stations to distribution substations near load centers.

• Key Principle: Power (P) = Voltage (V) × Current (I). For a given power, higher voltage reduces current, which minimizes $I^2R$ transmission losses.

• Components:

  1. Step-up Transformers: Located at generating stations. Increase voltage from generation level (e.g., 11 kV) to Extra High Voltage (EHV) levels (e.g., 132 kV, 220 kV, 400 kV).

  2. Transmission Lines: High-voltage overhead lines (on tall towers) or underground cables.

  3. Substations: House switching, protection, and control equipment.

• Voltage Levels:

  • Primary Transmission: 132 kV, 220 kV, 400 kV.

  • Secondary Transmission: 66 kV, 33 kV.

1.3 Distribution

• Purpose: To deliver electricity from transmission substations to end consumers (homes, businesses, industries).

• Process:

  1. Step-down Transformers at distribution substations reduce transmission voltage to primary distribution levels (e.g., 11 kV, 6.6 kV).

  2. Distribution Lines carry this medium voltage through feeders.

  3. Distribution Transformers (pole-mounted or ground) further step down voltage to utilization voltage (e.g., 400/230 V for three-phase/single-phase supply).

  4. Service Lines finally connect to consumer meters.

• Classification:

  • Primary Distribution: 11 kV or 33 kV lines.

  • Secondary Distribution: 415/240 V (three-phase four-wire system).

1.4 Loads (Consumption)

• The endpoint where electrical energy is converted into other forms (light, heat, motion).

• Classified as Domestic, Commercial, Industrial, Agricultural, etc.

• Load Factor = $\frac{\text{Average Load}}{\text{Peak Load}}$. A high load factor indicates efficient system utilization.

1.5 System Control and Protection

• SCADA (Supervisory Control and Data Acquisition): Monitors and controls the entire grid.

• Protective Relays and Circuit Breakers: Isolate faulty sections to maintain system stability.

• Load Dispatch Centers: Ensure economic and secure grid operation.

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2. Generation Technologies

2.1 Conventional Power Plants

(a) Thermal Power Plant

• Energy Source: Fossil fuels (Coal, Natural Gas, Diesel).

• Basic Principle: Converts heat energy from fuel combustion into mechanical energy (via a turbine), which then drives an electrical generator.

• Main Components:

  1. Boiler/Furnace: Burns fuel to produce high-pressure steam.

  2. Steam Turbine: Steam expands through turbine blades, causing rotation.

  3. Generator (Alternator): Coupled to the turbine, converts mechanical rotation into electricity via electromagnetic induction.

  4. Condenser: Cools exhaust steam back into water (condensate) for reuse.

  5. Cooling Tower: Rejects waste heat to the atmosphere.

• Key Efficiency Parameter: Thermal Efficiency, typically 30-40%. Defined as: $\eta_{thermal} = \frac{\text{Electrical Output Energy}}{\text{Heat Input from Fuel}} \times 100\%$

• Types: Steam (Rankine Cycle), Gas Turbine (Brayton Cycle), Combined Cycle Gas Turbine (CCGT - higher efficiency ~60%).

(b) Hydroelectric Power Plant

• Energy Source: Potential energy of stored water at a height.

• Basic Principle: Potential energy → Kinetic energy → Mechanical energy → Electrical energy.

• Main Components:

  1. Dam/Reservoir: Stores water, creates hydraulic head (height).

  2. Penstock: Large pipe/conduit that carries water under pressure to the turbine.

  3. Turbine (Pelton, Francis, Kaplan): Water strikes turbine blades, causing rotation.

  4. Generator: Coupled to the turbine shaft.

• Power Output Formula: $P = \eta \rho g Q H$ Where:

  • $P$ = Power output (Watts).

  • $\eta$ = Overall plant efficiency (typically 0.8-0.9).

  • $\rho$ = Density of water (~1000 kg/m³).

  • $g$ = Acceleration due to gravity (9.81 m/s²).

  • $Q$ = Water flow rate (m³/s).

  • $H$ = Effective head (height in meters).

• Types:

  • Run-of-River: Small/no reservoir, uses natural river flow.

  • Storage (Reservoir): Large dam, provides storage for continuous power and flood control.

  • Pumped Storage: Acts as a giant battery. Pumps water to a high reservoir during low demand, releases it to generate during peak demand.

(c) Nuclear Power Plant

• Energy Source: Nuclear fission of heavy elements (Uranium-235, Plutonium-239).

• Basic Principle: Controlled fission chain reaction releases immense heat, which is used to generate steam (like a thermal plant, but with a nuclear reactor as the heat source).

• Main Components:

  1. Nuclear Reactor Core: Contains fuel rods, moderator (water, graphite), and control rods (absorb neutrons to regulate reaction).

  2. Heat Exchanger/Steam Generator: Transfers heat from the reactor coolant (primary circuit) to the water in the secondary circuit, creating steam without mixing.

  3. Steam Turbine & Generator: Standard components.

  4. Containment Structure: Thick concrete and steel dome to contain radiation.

• Advantage: Very high energy density; 1 kg of U-235 ≈ 2.7 million kg of coal.

• Challenge: Radioactive waste disposal and safety.

2.2 Renewable Energy Technologies

(a) Solar Photovoltaic (PV)

• Principle: Direct conversion of sunlight (photons) into electricity using the photovoltaic effect in semiconductor materials (typically Silicon).

• Basic Unit: The PV Cell. Cells are connected to form Modules (Panels), which are grouped into Arrays.

• Key Equation (Ideal): The maximum power output from a PV cell.

• Main Components of a PV System:

  1. PV Array: Captures sunlight.

  2. Charge Controller: Regulates voltage/current from array to battery.

  3. Battery Bank (for off-grid): Stores DC electricity.

  4. Inverter: Converts DC from array/battery to AC for appliances.

• Types:

  • Grid-tied: Connected to the utility grid, no battery. Excess power can be fed back (net metering).

  • Off-grid: Standalone with battery storage.

  • Hybrid: Combines PV with another source (e.g., diesel generator).

(b) Wind Energy

• Principle: Converts kinetic energy of moving air into mechanical energy via rotor blades, which drives a generator.

• Power in the Wind: $P_{wind} = \frac{1}{2} \rho A v^3$ Where:

  • $\rho$ = Air density (~1.225 kg/m³ at sea level).

  • $A$ = Swept area of turbine blades = $\pi R^2$ (R is blade radius).

  • $v$ = Wind speed (m/s).

• Key Points:

  • Power is proportional to the cube of wind speed. Small changes in speed cause large power changes.

  • Betz Limit: Maximum theoretical efficiency for converting wind's kinetic energy to mechanical energy is 59.3%.

• Types:

  • Horizontal Axis Wind Turbine (HAWT): Common, propeller-type.

  • Vertical Axis Wind Turbine (VAWT): Omnidirectional, less efficient but simpler.

(c) Tidal Energy

• Principle: Harnesses the kinetic energy of tidal currents or the potential energy from the height difference between high and low tides.

• Methods:

  1. Tidal Barrage: Dam-like structure across a tidal estuary. Gates open to fill the basin at high tide and release water through turbines at low tide. Utilizes potential energy.

  2. Tidal Stream Generators: Underwater turbines placed in areas with fast-moving tidal currents. Similar to wind turbines, but in water. Utilizes kinetic energy.

• Advantage: Highly predictable (based on lunar cycles).

(d) Geothermal Energy

• Principle: Uses heat from the Earth's interior (from radioactive decay and primordial heat). Hot water or steam from underground reservoirs is brought to the surface to drive turbines.

• Types of Plants:

  1. Dry Steam: Uses steam directly from the ground.

  2. Flash Steam: High-pressure hot water is "flashed" into steam in a tank at lower pressure.

  3. Binary Cycle: Hot geothermal fluid heats a secondary working fluid (with lower boiling point, like isobutane) via a heat exchanger. The secondary fluid vaporizes and drives the turbine. Most common for moderate-temperature resources.

2.3 Emerging Technology: Fuel Cells

• Principle: Electrochemical device that converts the chemical energy of a fuel (hydrogen, methane) and an oxidant (oxygen from air) directly into electricity, with water and heat as byproducts. It is not a heat engine.

• Basic Operation:

  1. Hydrogen fuel is fed to the anode.

  2. Oxygen (from air) is fed to the cathode.

  3. At the anode, a catalyst splits hydrogen molecules into protons (H⁺) and electrons (e⁻).

  4. Protons pass through an electrolyte membrane to the cathode.

  5. Electrons travel through an external circuit (creating electric current) to the cathode.

  6. At the cathode, protons, electrons, and oxygen combine to form water.

• Key Advantages:

  • High efficiency (40-60%, up to 85% with cogeneration).

  • Zero emissions (if hydrogen is from a clean source).

  • Silent operation.

• Challenge: High cost, hydrogen production, storage, and infrastructure.

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3. Energy Storage Systems (ESS)

ESS are crucial for grid stability, integrating variable renewables (solar, wind), and ensuring reliable power supply.

3.1 Need for Energy Storage

• Intermittency Management: Stores excess energy when generation exceeds demand (e.g., sunny midday for solar) and discharges when demand exceeds generation (e.g., night-time).

• Load Leveling: Flattens the daily load curve by charging during off-peak hours and discharging during peak hours.

• Frequency Regulation: Provides rapid injection/absorption of power to maintain grid frequency (50/60 Hz).

• Backup Power: Ensures continuity of supply during outages.

3.2 Major Storage Technologies

(a) Pumped Hydroelectric Storage (PHS)

• Most mature and largest-capacity grid storage technology.

• Principle: During low demand/cheap electricity, water is pumped from a lower reservoir to an upper reservoir (storing energy as potential energy). During high demand, water is released back down through turbines to generate electricity.

• Efficiency: 70-85%.

(b) Battery Energy Storage Systems (BESS)

• Electrochemical storage. Becoming dominant for short to medium duration storage.

• Common Types:

  • Lithium-ion (Li-ion): High energy/power density, long cycle life. Used in EVs, grid storage.

  • Lead-Acid: Mature, low cost, but lower energy density and life. Used for backup (UPS).

  • Flow Batteries (e.g., Vanadium Redox): Energy stored in liquid electrolytes in external tanks. Power and capacity are independent. Good for long-duration storage.

• Key Parameter: Energy Density (Wh/kg or Wh/L), Power Density (W/kg), Cycle Life.

(c) Flywheel Energy Storage

• Principle: Stores energy as kinetic energy in a rotating mass (rotor). Electricity accelerates the rotor in a vacuum to very high speeds (to minimize friction). To discharge, the rotor's rotation drives a generator.

• Application: Primarily for frequency regulation and UPS due to very fast response and high power capability, but limited energy duration (minutes).

(d) Compressed Air Energy Storage (CAES)

• Principle: During off-peak times, electricity runs compressors to inject air into an underground cavern (salt dome, aquifer). During peak times, pressurized air is released, heated (often with natural gas), and expanded through a gas turbine to generate electricity.

• Application: Large-scale, long-duration storage.

(e) Thermal Energy Storage (TES)

• Principle: Stores energy in the form of heat (sensible or latent) or cold.

• Examples:

  • Molten Salt: Used in Concentrated Solar Power (CSP) plants. Stores solar heat to generate steam at night.

  • Ice Storage: Makes ice at night using cheap electricity, uses it for daytime air conditioning.

3.3 Role in a Modern Grid

ESS acts as a shock absorber and flexibility provider, enabling a higher penetration of renewable energy, improving power quality, and enhancing overall grid resilience and efficiency.