Electrical Energy and Power
5. Electrical Power Systems
Learning outcomes
- I can describe electrical power generation.
- I can explain electrical transmission.
- I can describe transformers qualitatively.
- I can explain why high-voltage transmission is used.
- I can relate circuits to national power grids.
Electrical Power Systems
From Power Generation to Our Homes
An electrical power system is the large network used to generate, transmit, distribute, and control electrical energy.
A national or regional electricity system connects:
energy sources → generators → transformers → transmission lines → substations → distribution networks → consumers
Consumers include:
- homes
- schools
- hospitals
- offices
- factories
- transportation systems
- data centres
Unlike a simple classroom circuit, a national power grid can contain thousands of generators, substations, transmission lines, and millions of loads operating together.
Electrical Power Generation
Electrical energy must first be produced by a generator or another electrical energy source.
Many large power stations use generators that convert mechanical energy into electrical energy.
The mechanical energy can come from:
- steam turbines
- water turbines
- wind turbines
- gas turbines
- engines
The generator then converts the rotational mechanical energy into electrical energy using electromagnetic induction.
Electromagnetic Induction
A changing magnetic field can produce an emf in a conductor.
This principle is called:
electromagnetic induction
Generators use relative motion between:
- magnetic fields
- electrical conductors
to produce electrical energy.
In many generators, turbines rotate part of the generator, creating the changing magnetic conditions needed to generate electricity.
Energy Transformations in Power Stations
Power stations do not create energy.
They transform energy from one form into another.
For example:
Hydroelectric
gravitational potential → kinetic → mechanical → electrical
Wind
kinetic energy of wind → mechanical → electrical
Fossil fuel
chemical → thermal → mechanical → electrical
Nuclear
nuclear → thermal → mechanical → electrical
Solar photovoltaic
radiation → electrical
Energy is conserved throughout these transformations.
Thermal Power Stations
Many conventional power stations use thermal energy to produce steam.
A simplified sequence is:
energy source → thermal energy → steam → turbine → generator → electrical energy
The thermal energy may originate from:
- coal
- natural gas
- nuclear reactions
- biomass
- geothermal sources
The steam turns a turbine connected to a generator.
Hydroelectric Generation
Hydroelectric power stations use moving water.
Water stored at a height has gravitational potential energy.
As it moves downward:
gravitational potential energy → kinetic energy
The moving water turns turbines:
kinetic → mechanical energy
Generators then convert this into:
mechanical → electrical energy
Wind Generation
Wind turbines extract some of the kinetic energy of moving air.
The blades rotate, producing mechanical energy.
A generator converts this mechanical energy into electrical energy.
The transformation is approximately:
wind kinetic energy → rotational mechanical energy → electrical energy
Wind generation varies with wind conditions, so power systems must manage changes in output.
Solar Electricity
Solar photovoltaic cells operate differently from turbine-based generators.
They convert energy from solar radiation directly into electrical energy.
Solar output depends on factors including:
- sunlight intensity
- time of day
- weather
- season
- panel orientation
- shading
Large power systems can combine many different generation technologies.
Electrical Power
Electrical power is the rate at which electrical energy is transferred.
P = E/t
Electrical power can also be calculated using:
P = VI
where:
- P = power (W)
- V = voltage (V)
- I = current (A)
Large generators may produce power measured in:
- kilowatts (kW)
- megawatts (MW)
- gigawatts (GW)
Remember:
1 kW = 1000 W
1 MW = 1,000,000 W
1 GW = 1,000,000,000 W
Why Electricity Must Be Transmitted
Power stations are often located far from the places where electricity is needed.
For example:
- hydroelectric stations may be near large rivers
- wind farms may be in windy regions
- solar farms may require large open areas
- large thermal stations may be outside cities
Electrical energy therefore needs to be transported over large distances.
This is called electrical transmission.
The Transmission Network
The transmission network uses high-voltage power lines to transport electrical power over long distances.
Large transmission lines are often supported by towers or pylons.
The transmission network connects:
- generators
- substations
- major population centres
- industrial regions
- different parts of the grid
The Problem of Resistance
Transmission wires are good conductors, but they are not perfect conductors.
They have resistance.
When current flows through resistance, electrical energy is transferred into thermal energy.
The power transferred as heat is:
P_loss = I²R
where:
- P_loss = power lost as heating
- I = current
- R = resistance of transmission line
This heating represents energy that is not delivered to consumers.
Why Current Matters So Much
Notice that current is squared:
P_loss = I²R
Suppose a transmission line has:
R = 2 Ω
and carries:
I = 100 A
Then:
P_loss = 100² × 2
P_loss = 10,000 × 2
P_loss = 20,000 W
or:
20 kW
If the current doubles to 200 A:
P_loss = 200² × 2
P_loss = 80,000 W
or:
80 kW
Doubling current produces four times the heating loss.
Why High Voltage Is Used
The power being transmitted is approximately:
P = VI
Rearrange:
I = P/V
For a given transmitted power:
higher voltage → lower current
Lower current dramatically reduces:
I²R losses
Therefore, electrical power is transmitted over long distances using very high voltages.
This is one of the most important ideas in electrical power systems.
High-Voltage Example
Suppose:
P = 1,000,000 W
or:
1 MW
is being transmitted.
At 10,000 V
I = P/V
I = 1,000,000/10,000
I = 100 A
If the line resistance is:
R = 5 Ω
then:
P_loss = I²R
P_loss = 100² × 5
P_loss = 50,000 W
or:
50 kW
Transmitting the Same Power at Higher Voltage
Now transmit the same:
1 MW
at:
100,000 V
Current:
I = 1,000,000/100,000
I = 10 A
Heating loss:
P_loss = 10² × 5
P_loss = 500 W
Compare:
at 10 kV → 50,000 W lost
at 100 kV → 500 W lost
Increasing the voltage by a factor of 10 reduced the current by a factor of 10.
Because loss depends on I², heating loss decreased by a factor of:
100
High Voltage Improves Transmission Efficiency
Therefore:
high voltage → low current → lower I²R losses → greater transmission efficiency
This allows large amounts of electrical energy to be transported over hundreds or even thousands of kilometres.
However, extremely high voltages require:
- strong electrical insulation
- large clearances
- specialized equipment
- appropriate transmission towers
- careful safety systems
So transmission voltage is chosen by balancing efficiency, engineering requirements, safety, and cost.
Transformers
A transformer is a device that changes AC voltage.
Transformers are essential to electrical power systems.
They allow voltage to be:
- increased for transmission
- decreased for distribution and use
Transformers operate using electromagnetic induction.
Basic Transformer Structure
A simple transformer contains:
- primary coil
- secondary coil
- magnetic core
Alternating current in the primary coil produces a changing magnetic field.
The changing magnetic field passes through the core and induces an alternating emf in the secondary coil.
The coils are not normally connected directly by a conducting wire.
Energy is transferred through the changing magnetic field.
Why Transformers Require Changing Current
A transformer requires a changing magnetic field.
AC naturally produces a continuously changing magnetic field.
Steady DC does not continuously produce the changing magnetic flux required for normal transformer operation.
Therefore, conventional transformers are designed for AC.
Step-Up Transformers
A step-up transformer increases voltage.
For an ideal transformer:
V_secondary > V_primary
Step-up transformers are used near generating stations to increase voltage before long-distance transmission.
Higher voltage means lower current for the same power.
That reduces transmission losses.
Step-Down Transformers
A step-down transformer decreases voltage.
For an ideal transformer:
V_secondary < V_primary
Step-down transformers are used throughout the distribution network to reduce very high transmission voltages to levels suitable for local distribution and eventually consumers.
Transformer Turns Ratio
The voltage produced by a transformer depends on the number of turns in the coils.
For an ideal transformer:
V_s/V_p = N_s/N_p
where:
- V_s = secondary voltage
- V_p = primary voltage
- N_s = secondary turns
- N_p = primary turns
If the secondary has more turns:
N_s > N_p
then:
V_s > V_p
The transformer steps the voltage up.
Transformer Example
Suppose:
N_p = 500 turns
N_s = 5000 turns
V_p = 10,000 V
Then:
V_s/V_p = N_s/N_p
V_s/10,000 = 5000/500
V_s/10,000 = 10
Therefore:
V_s = 100,000 V
The transformer increases the voltage by a factor of 10.
Power in an Ideal Transformer
An ideal transformer does not create energy.
Therefore:
P_input = P_output
Since:
P = VI
we can write approximately:
V_pI_p = V_sI_s
This explains another important relationship.
If voltage increases:
current decreases
If voltage decreases:
current increases
for approximately the same transferred power.
Transformer Example with Current
Suppose an ideal transformer receives:
V_p = 10,000 V
I_p = 100 A
Input power:
P = VI
P = 10,000 × 100
P = 1,000,000 W
Suppose the output voltage is:
100,000 V
Then:
I_s = P/V_s
I_s = 1,000,000/100,000
I_s = 10 A
Voltage increased by a factor of 10 while current decreased by a factor of 10.
Real Transformers
Real transformers are not perfectly efficient.
Some energy is transferred into unwanted forms because of:
- resistance in the coils
- heating in the core
- magnetic effects
- vibration and sound
However, large power transformers can be highly efficient.
Energy conservation still applies:
input energy = useful output energy + unwanted energy transfers
From Generator to Consumer
A simplified electrical power system might operate like this:
Generator
↓
Step-up transformer
↓
High-voltage transmission lines
↓
Transmission substation
↓
Step-down transformer
↓
Distribution network
↓
Local transformer
↓
Homes and businesses
Voltage is changed several times as electricity moves through the system.
Transmission vs Distribution
These terms describe different parts of the grid.
Transmission
Moves large amounts of electrical power over long distances.
Typically uses very high voltages.
Distribution
Carries electrical power from substations to local consumers.
Uses lower voltages than major transmission networks.
The exact voltage levels depend on the electrical system.
Substations
A substation is a facility that helps control and distribute electrical power.
Substations may contain:
- transformers
- circuit breakers
- switches
- protection systems
- measurement equipment
- control systems
Substations connect different parts of the grid and allow operators to control power flow.
The National Power Grid
A power grid is an interconnected network of:
- generators
- transmission lines
- substations
- transformers
- distribution lines
- consumers
Rather than every city having a completely independent power supply, large interconnected networks allow electricity to be transferred between different areas.
The Grid as a Large Circuit
The national grid can be thought of as an enormous and complex electrical circuit.
The same fundamental principles still apply:
P = VI
V = IR
P_loss = I²R
Kirchhoff's laws and conservation principles also remain fundamental.
However, real power grids involve AC systems, multiple generators, changing loads, transformers, protection systems, and complex control.
Electricity Supply and Demand
Electrical generation must continuously respond to the amount of electrical power consumers require.
This demand changes throughout the day.
Demand may increase when:
- people wake up
- businesses begin operating
- air conditioning use increases
- cooking occurs
- industrial equipment operates
Demand may fall during periods of lower activity.
Peak Demand
Peak demand is the period when electricity consumption is highest.
During peak demand, the grid must provide enough power to meet the combined load.
If millions of devices are switched on simultaneously, the total power requirement can become extremely large.
Power systems therefore need enough:
- generation
- transmission capacity
- distribution capacity
to handle expected demand.
Power Balance
At any moment, the electrical system must maintain a balance between generation and consumption, with storage and other control systems increasingly helping manage that balance.
In simplified terms:
power generated ≈ power consumed + system losses
If supply and demand become seriously unbalanced, grid frequency and voltage can be affected.
Variable Renewable Energy
Wind and solar generation depend on environmental conditions.
Solar output changes with:
- daylight
- cloud cover
- season
Wind output changes with:
- wind speed
- weather systems
This means their electrical output can vary.
Power grids can manage this variability using combinations of:
- different generation sources
- transmission connections
- energy storage
- demand management
- forecasting
- flexible generation
Energy Storage
Energy storage can help balance electrical supply and demand.
Examples include:
- batteries
- pumped-storage hydroelectricity
- other mechanical or thermal storage systems
When electricity production exceeds immediate demand, some energy can be stored.
Later, the stored energy can help supply the grid when demand is higher.
Pumped-Storage Hydroelectricity
Pumped storage uses electricity to pump water to a higher reservoir when excess electrical energy is available.
Energy transformation during storage:
electrical → mechanical → gravitational potential
When electricity is needed:
gravitational potential → kinetic → mechanical → electrical
It effectively acts as a large-scale energy storage system.
Grid Interconnections
Power grids can be connected to neighboring regions.
Interconnections can allow electricity to flow between areas.
For example, one region may temporarily have:
- surplus generation
while another has:
- high demand
Electrical interconnections can improve flexibility and reliability.
Grid Reliability
A reliable electrical system should continue supplying electricity even when individual components fail.
Power networks can improve reliability through:
- multiple generators
- alternative transmission paths
- protection systems
- backup equipment
- automatic controls
- interconnected regions
This means electricity may sometimes reach a region through more than one possible transmission route.
Grid Protection
Electrical faults can produce extremely large currents.
Power systems therefore use protection equipment such as:
- circuit breakers
- protective relays
- monitoring systems
- grounding systems
When a serious fault occurs, protection systems can disconnect the affected section while allowing other parts of the grid to continue operating.
Transmission Line Resistance
Transmission losses can also be reduced by reducing resistance.
For a conductor:
R = ρL/A
where:
- ρ = resistivity
- L = length
- A = cross-sectional area
Resistance can be reduced by:
- using low-resistivity conductors
- increasing conductor cross-sectional area
- reducing unnecessary transmission distance where practical
However, thicker conductors require more material and cost more.
Engineering involves balancing performance and cost.
Why Not Use Infinite Voltage?
If higher voltage reduces transmission losses, it might seem that transmission voltage should simply be made as large as possible.
But higher voltages require:
- more insulation
- greater spacing
- larger towers
- specialized transformers
- more expensive equipment
- greater clearances
Very high electric fields can also create additional engineering problems.
Therefore, transmission systems use voltages that provide a practical balance between efficiency, safety, and cost.
Power Grid Efficiency
Suppose a generator produces:
1000 MW
and:
950 MW
reaches consumers.
Efficiency:
efficiency = useful output / input × 100%
efficiency = 950/1000 × 100%
efficiency = 95%
The remaining:
50 MW
has been transferred in other ways within the system, including electrical and transformer losses.
Household Circuits and the Grid
When you switch on an electrical appliance, it becomes one of many loads connected to the power system.
For example, a 2300 W kettle adds approximately:
2.3 kW
to electrical demand while operating.
One kettle is a tiny load compared with a national system.
Millions of appliances together create the total electrical demand that generators and the grid must supply.
From One Circuit to Millions
The principles studied in simple circuits scale into power systems.
Simple circuit
Battery → wires → resistor
Household circuit
Grid supply → distribution board → branch circuits → appliances
National grid
Generators → transformers → transmission → substations → distribution → millions of loads
The scale changes enormously, but electrical energy still obeys the same physical laws.
Evaluating Power Generation Methods
Different generation technologies have advantages and limitations.
Factors include:
- reliability
- cost
- environmental impact
- power output
- location
- fuel availability
- construction time
- operating lifetime
- ability to respond to changing demand
A complete power system usually involves a combination of technologies rather than a single type of generator.
Common Mistakes
Mistake: Saying power stations create energy.
They transform energy from other stores or sources into electrical energy.
Mistake: Saying transformers generate electricity.
Transformers change AC voltage; they do not provide the original energy source.
Mistake: Saying high voltage means more transmission loss.
For a fixed transmitted power, higher voltage allows lower current and therefore lower I²R losses.
Mistake: Forgetting that current is squared in P_loss = I²R.
Mistake: Assuming transmission wires have zero resistance.
Real conductors always have some resistance unless special conditions such as superconductivity apply.
Mistake: Saying a step-up transformer increases both voltage and current.
For approximately constant power, increasing voltage is accompanied by decreasing current.
Mistake: Assuming transformers work normally with steady DC.
Conventional transformers require changing magnetic flux and are therefore associated with AC.
Mistake: Confusing transmission with distribution.
Transmission generally moves large amounts of power over long distances; distribution delivers power locally.
Mistake: Thinking the national grid operates independently of household circuits.
Household loads contribute to the total demand on the grid.
Key Terms
Electrical power system — network used to generate, transmit, distribute, and control electrical energy.
Generator — device that converts mechanical energy into electrical energy through electromagnetic induction.
Electromagnetic induction — production of an emf due to changing magnetic flux.
Transmission — movement of large amounts of electrical power over long distances.
Distribution — delivery of electrical power from substations to local consumers.
Power grid — interconnected network of generators, transmission lines, substations, distribution systems, and consumers.
Transformer — device that changes AC voltage using electromagnetic induction.
Step-up transformer — transformer that increases voltage.
Step-down transformer — transformer that decreases voltage.
Substation — facility used to transform, switch, protect, measure, and control electrical power.
Transmission loss — energy transferred in unwanted ways during transmission, particularly resistive heating.
Peak demand — maximum electrical power demanded during a particular period.
Load — device or system consuming electrical power.
Grid interconnection — electrical connection allowing power exchange between regions.
Energy storage — system that stores energy for later use.
Pumped storage — storage method using gravitational potential energy of water.
Key Takeaways
- Electrical power systems connect generators to consumers.
- Many generators convert mechanical energy into electrical energy through electromagnetic induction.
- Power stations transform energy rather than create it.
- Electricity must often be transmitted long distances from generators to consumers.
- Transmission lines have resistance and therefore produce heating losses.
- Transmission heating losses are described by P_loss = I²R.
- Electrical power is P = VI.
- For the same transmitted power, increasing voltage decreases current.
- Lower current greatly reduces I²R transmission losses.
- This is why electrical power is transmitted at very high voltages.
- Transformers change AC voltage.
- Step-up transformers increase voltage for efficient transmission.
- Step-down transformers reduce voltage for distribution and use.
- Transformer operation depends on electromagnetic induction.
- An ideal transformer approximately conserves power: V_pI_p = V_sI_s.
- Transmission and distribution are different stages of the electrical system.
- Substations transform, control, switch, and protect electrical networks.
- National power grids connect many generators and millions of loads.
- Electricity supply must continually respond to changing demand.
- Renewable generation, storage, interconnections, and control systems can work together to maintain a reliable grid.
- The circuit principles studied in small circuits remain fundamental to large electrical power systems.
Check Your Understanding
- What is an electrical power system?
- Describe the main energy transformation in an electrical generator.
- What physical principle allows generators to produce an emf?
- Describe the energy transformations in a hydroelectric power station.
- Explain the difference between electrical transmission and distribution.
- Why do transmission wires cause energy losses?
- Write the equation for heating power loss in a transmission line.
- A transmission line carries 200 A and has a resistance of 3 Ω. Calculate the power lost as heating.
- If the current in Question 8 is reduced to 100 A, calculate the new power loss.
- Explain why reducing current has such a large effect on transmission losses.
- A transmission system transfers 10 MW at 10 kV. Calculate the current using P = VI.
- Calculate the current if the same 10 MW is transmitted at 100 kV.
- Explain why high-voltage transmission is more efficient.
- What is the purpose of a step-up transformer?
- What is the purpose of a step-down transformer?
- Explain qualitatively how a transformer transfers electrical energy between its primary and secondary coils.
- Why are conventional transformers associated with AC rather than steady DC?
- Describe the journey of electrical energy from a generator to a household appliance.
- Explain how switching on millions of household appliances affects demand on the national power grid.
- Explain how generators, transformers, transmission lines, substations, distribution networks, storage systems, and consumers work together as one electrical power system.