Alternating current and the National Grid
The National Grid
The The network that connects all of the power stations in the country to make sure that everywhere has access to electricity. distributes electricity across the country. The National Grid connects power stations to homes, workplaces and public buildings all around the country. The electricity may be produced by a conventional power station turning a Device that converts kinetic energy into electrical energy. or by another method.
Learn more on the National Grid in this podcast
Listen to the full series on BBC Sounds.
Transformers
An electrical device that increases, or decreases, the potential difference (voltage) of an alternating current. are used to change voltages and Moving electric charges, eg electrons moving through a metal wire. in Thick cables used to carry electricity long distances through the National Grid.. A transformer is formed from two coils of wire around a magnetic core. The number of coils determines whether the transformers will step-up or step-down the voltage.
As the power transferred must stay the same:
- increasing voltage decreases current
- decreasing voltage increases current
In the National Grid, a A transformer that increases the voltage of the ac supply. is used to increase the voltage and reduce the current. The voltage is increased from about 25,000 Volts (V) to 400,000 V causing the current to decrease. Less current means less energy is lost through heating the wire.
To keep people safe from these high voltage wires, pylons are used to support transmission lines above the ground.
Before reaching the end user, a A transformer that decreases the voltage of the ac supply., reduces the voltage from the transmission voltage to the safer voltage of 230 V for home use.
Power in transformers
If the transformer is 100% efficient, the power in each coil will be the same - this means that:
potential difference across primary coil × current in primary coil = potential difference across secondary coil × current in secondary coil
\(V_{P} \times I_{P} = V_{S} \times I_{S}\)
Electricity is generated in a power station at 25,000 V, and to transmit the electricity across the country on the The network that connects all of the power stations in the country to make sure that everywhere has access to electricity., transformers increase the potential difference to 400,000 V.
This means that the transformer increases the potential difference by a factor of 16. So if there are no energy losses in the system, the current would also be reduced by a factor of 16 as:
power = potential difference × current
Reducing the current by a factor of 16 means that the heating effect of the current is reduced by a factor of 256 (162), as:
power = current2 × resistance
This means that the energy is transferred more efficiently, as less energy is dissipated as heat.
Transmission lines
As an electric current flows through the thick cables held up by the pylons, they will get hotter and dissipate energy to the surroundings. The electrical The energy transferred each second, measured in watts (W). Power = work done ÷ time taken. dissipated depends on current and resistance:
power = current2 × resistance
\(P = I^2 \times R\)
This is when:
- power (P) is measured in watts (W)
- current (I) is measured in amps (A)
- resistance (R) is measured in ohms (Ω)
To ensure that the minimum amount of power is lost from the cables:
- the cables are thick so that their resistance is low
- high voltages are used to reduce the current through the transmission lines
A low resistance and a low current mean that the transmission wires will not heat up as much. As a result, most of the power is delivered to the consumer, and not lost through the wires.
