Power Transmission and Distribution

Electricity is often generated far from the homes, schools, businesses, and factories that use it. An electrical grid must therefore transport enormous amounts of electrical energy over long distances while minimizing energy losses.

The key idea is:

For the same transmitted power, increasing voltage allows current to decrease.

Because heating losses in transmission wires depend strongly on current, electricity is transmitted over long distances at very high voltage.

The general pathway is:

power station → step-up transformer → high-voltage transmission → substations → step-down transformers → local distribution → consumers

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The Electrical Grid

An electrical grid is a large interconnected system that generates, transmits, distributes, and controls electrical power.

It includes:

• generating stations
• transformers
• transmission lines
• substations
• distribution lines
• control systems
• consumers

A grid must continually balance the electricity being produced with the electricity being used.

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Generation

Electrical energy can be generated using many different energy sources.

Examples include:

• hydroelectricity
• wind
• solar
• nuclear energy
• natural gas
• coal
• geothermal energy
• biomass

Many large generators produce AC because alternating voltage can be transformed efficiently to different voltage levels.

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From Generator to Consumer

Electricity typically passes through several stages before reaching the consumer:

Generation

↓

Voltage increased

↓

Long-distance transmission

↓

Voltage reduced at substations

↓

Local distribution

↓

Further voltage reduction

↓

Homes and businesses

Different grids use different voltage levels, but the basic principle is similar.

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Electrical Power

Electrical power is calculated using:

P = VI

where:

• P = power in watts
• V = voltage in volts
• I = current in amperes

Rearranging:

I = P/V

For a particular amount of transmitted power:

higher voltage → lower current

This is the central reason high-voltage transmission is used.

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Example: Transmitting at Low Voltage

Suppose a transmission system needs to carry:

P = 1,000,000 W

at:

V = 10,000 V

Current:

I = P/V

I = 1,000,000/10,000

I = 100 A

Now consider transmitting the same power at a much higher voltage.

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Example: Transmitting at High Voltage

Suppose the same:

P = 1,000,000 W

is transmitted at:

V = 100,000 V

Then:

I = P/V

I = 1,000,000/100,000

I = 10 A

Increasing voltage by a factor of 10 reduced current by a factor of 10.

That reduction in current has a major effect on energy losses.

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Resistance in Transmission Lines

Transmission wires are made from conducting materials, but they still have electrical resistance.

When current flows through resistance, electrical energy is converted into thermal energy.

The power lost through resistance is:

P_loss = I²R

where:

• P_loss = power lost as heat
• I = current
• R = resistance of the transmission line

The important feature is the square of the current.

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Why Current Matters So Much

Because:

P_loss = I²R

doubling current produces:

2² = 4 times

the heating loss.

Tripling current produces:

3² = 9 times

the heating loss.

Reducing current to one-tenth produces:

(1/10)² = 1/100

of the original heating loss.

This is why reducing transmission current is so important.

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Why High Voltage Reduces Transmission Losses

For transmitted power:

P = VI

Therefore:

I = P/V

Increasing V allows I to decrease.

Since:

P_loss = I²R

a lower current means dramatically lower resistive heating losses.

So:

high voltage → low current → lower I²R losses → greater transmission efficiency

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Worked Example: Comparing Transmission Losses

Suppose a transmission line has:

R = 5.0 Ω

First consider:

I = 100 A

P_loss = I²R

P_loss = (100)²(5.0)

P_loss = 50,000 W

Now suppose higher transmission voltage allows the current to decrease to:

I = 10 A

Then:

P_loss = (10)²(5.0)

P_loss = 500 W

The current decreased by a factor of 10.

The heating loss decreased by a factor of:

100

This illustrates the enormous advantage of high-voltage transmission.

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Transformers Make High-Voltage Transmission Practical

Transformers allow AC voltage to be changed efficiently.

Near the generating station, a step-up transformer increases voltage.

For an ideal transformer:

V_pI_p = V_sI_s

If voltage increases, current decreases.

This allows electrical power to enter the transmission network at high voltage and relatively low current.

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Step-Up Transformers

A step-up transformer has:

N_s > N_p

and therefore:

V_s > V_p

The transformer increases voltage before long-distance transmission.

The resulting reduction in current decreases heating losses in the transmission lines.

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Why Not Generate Electricity at the Final Voltage?

Different parts of the electrical system have different requirements.

Extremely high transmission voltages are useful for transporting energy but are not appropriate for:

• many generators
• homes
• ordinary appliances
• most local electrical equipment

Transformers allow engineers to choose suitable voltages for each part of the system.

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High-Voltage Transmission Lines

After voltage is increased, electrical power travels through transmission lines.

These lines can carry large amounts of power over long distances.

Transmission conductors are commonly suspended high above the ground using towers or poles.

High voltage requires significant electrical insulation and safe clearances.

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Why Transmission Towers Are Large

High-voltage systems require substantial separation between:

• conductors
• towers
• ground
• nearby structures

Air normally acts as an electrical insulator.

However, sufficiently large electric fields can cause electrical breakdown and arcing.

Large towers and long insulators help maintain safe separation.

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Transmission vs Distribution

Transmission generally refers to moving large amounts of electrical power over relatively long distances at high voltage.

Distribution refers to delivering electrical power from substations to local consumers.

A simplified comparison is:

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Electrical Substations

A substation is an important point in the electrical grid where power can be:

• transformed to another voltage
• switched between circuits
• monitored
• protected
• controlled

Substations contain equipment such as:

• transformers
• circuit breakers
• switches
• protective relays
• monitoring systems

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Stepping Voltage Down

High transmission voltage must eventually be reduced.

Large substations use step-down transformers to lower the voltage.

Electricity may pass through several voltage levels as it moves closer to consumers.

The general pattern is:

very high transmission voltage

↓

regional substation

↓

lower distribution voltage

↓

local transformer

↓

consumer voltage

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Local Distribution

After leaving a major substation, electricity enters the distribution network.

Distribution networks carry electricity through:

• cities
• towns
• industrial areas
• residential neighborhoods

Distribution lines may be:

• overhead
• underground

Further transformers reduce the voltage to levels appropriate for local use.

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Distribution Transformers

A distribution transformer is often one of the final transformer stages before electricity reaches consumers.

Depending on the electrical system, these may be mounted:

• on utility poles
• at ground level
• inside buildings
• in underground systems

Their role is to reduce distribution voltage to the voltage required by nearby consumers.

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Electricity Reaching a Home

A simplified pathway to a home is:

generator

↓

step-up transformer

↓

high-voltage transmission line

↓

transmission substation

↓

distribution network

↓

distribution transformer

↓

home electrical service

↓

electrical panel

↓

individual circuits and appliances

Each stage performs a specific role in delivering electricity safely and efficiently.

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Electricity Reaching Businesses and Industry

Not all consumers require the same electrical supply.

Small businesses may receive electricity in a manner similar to residential consumers.

Larger commercial or industrial facilities may require:

• higher voltages
• three-phase power
• larger transformers
• dedicated substations

Large industrial facilities can consume enormous amounts of electrical power, so their connection to the grid may be significantly different from that of a home.

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Transmission Efficiency

Efficiency compares useful output power with input power.

efficiency = P_output/P_input × 100%

For transmission:

P_input = P_output + P_loss

Therefore:

P_output = P_input − P_loss

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Worked Example: Transmission Efficiency

A transmission system receives:

P_input = 500 MW

and loses:

P_loss = 20 MW

Output power:

P_output = 500 − 20

P_output = 480 MW

Efficiency:

efficiency = 480/500 × 100%

efficiency = 96%

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Worked Example: Finding Power Loss

A transmission system receives:

P_input = 250 MW

and operates at:

98% efficiency

Use:

P_output = 0.98(250 MW)

P_output = 245 MW

Therefore:

P_loss = 250 − 245

P_loss = 5 MW

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Worked Example: Current and Transmission Loss

A system transmits:

P = 2.0 MW

at:

V = 20 kV

First convert:

2.0 MW = 2,000,000 W

20 kV = 20,000 V

Current:

I = P/V

I = 2,000,000/20,000

I = 100 A

Suppose the transmission line has:

R = 2.0 Ω

Power loss:

P_loss = I²R

P_loss = (100)²(2.0)

P_loss = 20,000 W

or:

20 kW

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Increasing the Transmission Voltage

Now transmit the same:

2.0 MW

at:

200 kV

Current:

I = 2,000,000/200,000

I = 10 A

Power loss:

P_loss = (10)²(2.0)

P_loss = 200 W

The transmission voltage increased by a factor of 10.

The current decreased by a factor of 10.

The resistive loss decreased from:

20,000 W → 200 W

a factor of:

100

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Voltage Drop

Transmission lines also experience a voltage drop because of their resistance.

For a simplified resistive line:

V_drop = IR

Reducing current therefore also reduces voltage drop.

This helps the grid deliver electrical energy more effectively over long distances.

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Why Not Use Infinite Voltage?

If high voltage reduces losses, it might seem that voltage should simply be made as large as possible.

However, increasing voltage creates engineering challenges.

Higher voltages require:

• larger insulation distances
• taller towers
• stronger insulators
• more expensive equipment
• greater protection against arcing
• more complex switching equipment
• greater safety precautions

Engineers therefore choose voltages that balance efficiency, cost, reliability, and safety.

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Transmission Line Resistance

The resistance of a conductor is approximately:

R = ρL/A

where:

• R = resistance
• ρ = resistivity of the material
• L = conductor length
• A = cross-sectional area

Resistance increases with length and decreases with conductor area.

Therefore, transmission losses can also be reduced by using conductors with:

• low resistivity
• larger cross-sectional area

However, larger conductors cost more and are heavier.

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Why Aluminium Is Commonly Used

Many overhead power lines use aluminium-based conductors.

Aluminium has several useful properties:

• relatively low electrical resistivity
• low density
• comparatively low cost
• good corrosion resistance

Although copper is more conductive, aluminium's lower mass and cost can make it advantageous for long overhead transmission lines.

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AC Transmission

Traditional electrical grids make extensive use of high-voltage alternating current (HVAC).

AC is particularly useful because transformers can efficiently:

• increase voltage
• decrease voltage

This makes it practical to use different voltage levels for:

• generation
• transmission
• regional distribution
• local distribution
• consumer use

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High-Voltage Direct Current

Not all long-distance transmission uses AC.

High-voltage direct current (HVDC) is used for some transmission systems.

HVDC can be advantageous for applications such as:

• very long transmission routes
• submarine cables
• connections between certain electrical grids
• transferring large amounts of power between distant regions

Electronic converter stations change between AC and DC.

This shows that modern grids use both AC and DC technologies where each is most suitable.

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Power Grid Balance

Electricity generation and electricity use must be carefully balanced.

At any moment, consumers may switch:

• appliances on and off
• industrial equipment on and off
• heating and cooling systems
• lighting
• vehicle chargers

Grid operators must respond to these changing demands.

Generation, storage, transmission, and control systems work together to maintain a stable supply.

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Peak Demand

Electricity demand is not constant.

Demand can vary with:

• time of day
• season
• weather
• industrial activity
• major events

Periods of particularly high electricity use are called peak demand periods.

The grid must have enough generation and transmission capacity to meet these peaks.

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Renewable Energy and the Grid

Renewable energy sources introduce new opportunities and challenges.

Solar generation varies with:

• sunlight
• clouds
• time of day

Wind generation varies with:

• wind speed
• weather patterns

An efficient grid can move electrical energy from regions where it is available to regions where it is needed.

This can help integrate renewable generation.

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Energy Storage

Energy-storage technologies can also support electrical grids.

Examples include:

• batteries
• pumped-storage hydroelectricity
• other large-scale storage systems

Storage can absorb electrical energy when supply exceeds demand and release it when demand is higher.

This can improve:

• reliability
• stability
• renewable-energy integration

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Smart Grids

A smart grid uses monitoring, communication, automation, and control technology to manage electricity more effectively.

Smart-grid systems can help:

• identify faults
• monitor power flow
• respond to changing demand
• integrate renewable energy
• coordinate energy storage
• improve reliability
• reduce unnecessary losses

Modern grids increasingly combine electrical infrastructure with digital control systems.

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Why Grid Efficiency Matters

Even a small percentage improvement in efficiency can become significant when enormous amounts of electrical energy are being transmitted.

Improved efficiency means:

• less energy wasted as heat
• less fuel required for some generating systems
• lower operating costs
• reduced strain on infrastructure
• more useful energy delivered to consumers
• potentially lower environmental impacts

Efficiency therefore has both economic and environmental importance.

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Grid Reliability

An efficient grid must also be reliable.

Consumers expect electricity to be available when needed.

Grid reliability depends on:

• adequate generation
• functioning transmission lines
• transformers
• substations
• protection systems
• maintenance
• backup connections
• accurate monitoring

Failure in one part of the grid can sometimes affect other connected regions.

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Redundancy

Electrical grids are often designed with multiple possible pathways for power.

If one line or generator becomes unavailable, electricity may sometimes be rerouted through other parts of the network.

This is called redundancy.

Redundancy can improve reliability, although it increases infrastructure complexity and cost.

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Transformers Throughout the Grid

Transformers are used repeatedly because different stages require different voltages.

A simplified grid may use:

generator voltage

↓

step-up transformer

↓

very high transmission voltage

↓

step-down transformer

↓

regional distribution voltage

↓

another step-down transformer

↓

local distribution voltage

↓

distribution transformer

↓

consumer voltage

The exact number of stages depends on the grid.

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Complete Transmission Problem

A power station needs to transmit:

P = 5.0 MW

through a transmission line with total resistance:

R = 4.0 Ω

The transmission voltage is:

V = 100 kV

Convert units:

5.0 MW = 5,000,000 W

100 kV = 100,000 V

Calculate current:

I = P/V

I = 5,000,000/100,000

I = 50 A

Calculate line loss:

P_loss = I²R

P_loss = (50)²(4.0)

P_loss = 10,000 W

P_loss = 10 kW

Useful output:

P_output = 5000 kW − 10 kW

P_output = 4990 kW

Efficiency:

efficiency = 4990/5000 × 100%

efficiency = 99.8%

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Comparing Two Transmission Voltages

Suppose the same 5.0 MW is transmitted through the same 4.0 Ω line at only:

20 kV

Current:

I = 5,000,000/20,000

I = 250 A

Power loss:

P_loss = (250)²(4.0)

P_loss = 250,000 W

or:

250 kW

Compare:

At 100 kV:

P_loss = 10 kW

At 20 kV:

P_loss = 250 kW

The higher-voltage system loses much less power.

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An Important Mathematical Relationship

Since:

I = P/V

and:

P_loss = I²R

we can substitute:

P_loss = (P/V)²R

Therefore:

P_loss = P²R/V²

for this simplified model.

If transmitted power and line resistance remain constant:

P_loss ∝ 1/V²

This means doubling transmission voltage reduces resistive loss to:

1/4

of its previous value.

Increasing voltage by a factor of 10 reduces loss to:

1/100

of its previous value.

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Evaluating Grid Design

An effective electrical grid must balance several factors:

• efficiency
• safety
• cost
• reliability
• environmental impact
• transmission distance
• expected demand
• renewable-energy integration
• maintenance requirements

The technically highest voltage or largest conductor is not automatically the best solution.

Engineering involves choosing a system that provides reliable electricity at acceptable cost and risk.

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Common Mistakes

Thinking high voltage itself reduces resistance

The resistance of the transmission wire does not automatically decrease because voltage increases. Higher voltage reduces the current required for the same transmitted power.

Using P = VI without considering losses

In real transmission systems, some input power is lost before reaching the consumer.

Forgetting that losses depend on I²

Transmission heating is described by:

P_loss = I²R

not simply IR.

Thinking transformers eliminate transmission losses

Transformers help reduce losses by allowing higher voltage and lower current. They do not make the transmission lines resistance-free.

Thinking high voltage means high current

For a fixed transmitted power, increasing voltage decreases current.

Confusing transmission with distribution

Transmission generally carries large amounts of power over long distances. Distribution delivers it more locally to consumers.

Assuming 100% efficiency

Real transformers, wires, substations, and other equipment all have some losses.

Ignoring unit conversions

Remember:

1 kV = 1000 V

1 MW = 1,000,000 W

1 kW = 1000 W

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Key Terms

Electrical grid — interconnected system used to generate, transmit, distribute, and control electrical power.

Transmission — movement of large amounts of electrical power, usually over long distances and at high voltage.

Distribution — delivery of electrical power from the transmission system toward local consumers.

Transmission line — conductor system used to carry large amounts of electrical power between locations.

Step-up transformer — transformer that increases AC voltage and decreases current in the ideal case.

Step-down transformer — transformer that decreases AC voltage and increases current in the ideal case.

Substation — grid facility used for voltage transformation, switching, protection, monitoring, and control.

Distribution transformer — transformer that reduces distribution voltage toward a level suitable for local consumers.

Power loss — electrical power converted into unwanted forms, particularly heat.

Transmission efficiency — percentage of input power successfully delivered as useful output power.

Resistive heating — conversion of electrical energy into thermal energy because current passes through resistance.

Voltage drop — reduction in electrical potential difference along a conductor due to its electrical properties.

Peak demand — period during which electricity use is particularly high.

Smart grid — electrical grid incorporating digital monitoring, communication, and automated control.

Redundancy — use of alternative components or pathways to improve system reliability.

HVAC — high-voltage alternating-current transmission.

HVDC — high-voltage direct-current transmission.

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Key Takeaways

• Electrical grids transport electricity from generating stations to consumers.
• Long-distance electricity transmission uses high voltage to reduce energy losses.
• Electrical power is related by P = VI.
• For a fixed transmitted power, increasing voltage decreases current.
• Transmission-line heating is described approximately by P_loss = I²R.
• Because loss depends on current squared, reducing current can dramatically improve efficiency.
• Step-up transformers increase AC voltage before long-distance transmission.
• Step-down transformers reduce voltage as electricity approaches consumers.
• Electricity usually passes through several voltage levels between a generator and a home or business.
• Substations transform, switch, protect, monitor, and control electrical power.
• Transmission moves large quantities of power over long distances, while distribution delivers power locally.
• Transmission efficiency can be calculated using efficiency = P_output/P_input × 100%.
• For fixed transmitted power and resistance, P_loss is proportional to 1/V².
• High voltage also creates engineering challenges involving insulation, clearance, safety, and cost.
• Grid design therefore balances efficiency with safety, reliability, and economics.
• Both AC and DC high-voltage transmission technologies are used in modern grids.
• Efficient grids reduce wasted energy and help reduce the amount of generation required.
• Modern grids increasingly integrate renewable generation, energy storage, monitoring, and automated control.
• Reliable electrical grids are essential for homes, businesses, transportation, communication, healthcare, and industry.

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Check Your Understanding

1. Explain why electricity is usually transmitted over long distances at high voltage.
2. A power line transmits 2.0 MW at 20 kV. Calculate the current.
3. The same 2.0 MW is transmitted at 200 kV. Calculate the new current and compare it with your answer to Question 2.
4. A transmission line has a resistance of 3.0 Ω and carries 100 A. Calculate the power lost as heat.
5. The current in Question 4 is reduced from 100 A to 20 A. Calculate the new power loss.
6. Explain why reducing current by a factor of 5 reduces resistive power loss by a factor of 25.
7. Describe the role of a step-up transformer near an electrical generating station.
8. Describe the path electrical energy might take from a generator to a home.
9. Explain the difference between electrical transmission and distribution.
10. A transmission system receives 600 MW and delivers 570 MW. Calculate its efficiency.
11. A system operates at 97% efficiency and receives 400 MW. Calculate the useful output power and the power lost.
12. Explain why electrical grids do not simply use the highest voltage technologically possible everywhere.
13. Explain how transformers contribute to reducing transmission losses without eliminating the resistance of transmission wires.
14. Describe three reasons why improving electrical-grid efficiency is important to society.
15. A city needs to receive a fixed amount of electrical power from a distant generating station. Explain how engineers could use voltage, conductor resistance, transformers, and grid design to reduce energy losses while maintaining a safe and reliable electricity supply.