The National Grid
Electricity is distributed from power stations to consumers through the National Grid, which allows distant power stations to be used. It also allows a mix of different energy resources to be used efficiently to supply the country’s electricity, whatever the local demand.
The National Grid ensures a reliable supply of electricity. If one power station breaks down, the grid will continue to supply electricity from other power stations in the grid. The National Grid also responds to the demand for electricity – supplying more at peak times.
Main features
This diagram shows the main features of the National Grid.
Power stations produce electricity at 25,000 volts (V). Step-up transformers change the voltage to the very high values needed to transmit electricity through the National Grid power lines. Electricity is sent through these at 400,000 V, 275,000 V or 132,000 V. The voltage of household electricity is about 230 V.
Electrical power can be calculated using this equation.
\(\text{power = }{\text{current}}\times{\text{voltage}}\)
Question
A power station produces 3 × 109 W of power with a current of 4 × 104 A. Calculate the voltage produced.
\(\text {voltage = }\frac{\text{power}}{\text{current}}\)
= (3 × 109) ÷ (4 × 104)
= 75,000 V
Question
The transformer increases the voltage by a factor of 6. What is the output current? Assume that the transformer is 100% efficient.
If the transformer is 100% efficient, the input power is equal to the output power.
Output power = 3 × 109 W
The output power is 6 times greater than the input voltage: 75,000 × 6 = 450,000 V
\(\text {voltage = }\frac{\text{power}}{\text{current}}\)
= (3 × 109) ÷ 450,000
= 6,667 A
Note, this current is 6 times smaller than the input current – we could have divided the input current by 6. It is useful to be able to calculate it fully in case the question involves a transformer that is less than 100% efficient.