The National Grid

The National Grid is the network that distributes all the electricity in a city, usually the power comes from power stations, which normally produce lots of heat that then becomes thermal energy which then is turned into electricity.

The thing about power stations is that the amount of electricity they make is based on the amount of demand, the demand increases in the afternoon and evening, and since we use so much electricity in our daily lives, the power stations need to have extra capacity just in case, so often the stations run at much lower rate so that if there's a spike in electrical consumption they can output more power to satisfy the demand, which they can't do if they are running at high rates.

As we know the equation P=VT is power = voltage multiplied by time, but when high amounts of current flow through a wire, it causes the wire to heat up, this heat is then lost power, but if we need to transport this power all around the city or even the whole country this loss of power is extremely decremental.
But if we run a very low current, which is more efficient that would mean our voltage is going to be very high, meaning when we want to move the electricity around the country via the power lines and pylons we would need a way of increasing the voltage.

This is where Step-up transformers come in, they take the voltage from the power station and then turn up the voltage up to 400,000 volts then the cables transport it around the country.
Then after that, we would need a step-down transformer since 400,000 volts flowing into your house is too much if we were to pump that amount of voltage into a civilian house, firstly it would be very dangerous as if our devices or appliances they would explode having a high chance of losing lives.
In order to keep us safe from the high voltages is that we use a step-down transformer, that turns the 400,000 volts to roughly 230 volts.