What is the role of a transformer in the national grid?

Answer 1

A transformer changes electricity from one voltage to another - and in doing so, the current was alter in proportion (losses ignored)

There is more than one reason why we need these. For example, to start with, the voltage which comes from most of the three phase alternators at power stations is often as "little" as 15,500 volts - but at very high current. This requires VERY thick windings, capable of handling many thousands of amps - even for quite a small power station alternator.

Typically, this voltage will go through a "step-up" transformer which gives out around 400,000V which is then tied to the supergrid. This huge increase in voltage means that the cable can now be MUCH thinner.

In addition to the reduction in copper/aluminium conductor sizes required, losses are also reduced as they are a function of I squared R (Current * Current * Resistance). So if you halve the current (by doubling the voltage), you reduce the losses to a quarter. A ten fold increase in voltage = 100 fold decrease in transmission losses (in a perfect world).

Now, 400,000 is an efficient way to cart electricity around but is way to high for even heavy industry to use directly - so near cities you start to find substations which take in the power from the grid at 400,000 and transform it down to more manageable levels - first perhaps to 275,000 volts and 132,000v for some small pylons off to towns.

Further substations then also take down to much lower levels for local industry and power transmission out to villages - 33KV and 11KV

Finally when the power gets close to your home, it is transformed yet again right down to 415V which will also provide the regular 0-240 (230 soon) we all know and love.

Keeping the voltage as high as possible until the end keeps losses to a minimum. The reason we do not operate everything at a high voltage is partly for safety and partly because high voltages can arc-over if the conductors are not kept very far apart. Something which is not practical or safe in the home.

Some industry (eg: aluminium smelting) takes HUGE amounts of power and they DO take power from the grid at very high levels - even though they still end up using HUGE Transformers themselves as the actually process uses quite low voltages - but at around a hundred thousands amps. (As and aside, they also have to rectify this, as the smelting process requires DC - not AC). DC does not work with a transformer, so all power is delivered to consumers as AC (Alternating current).

*Final note* There are also some HVDC systems which use extremely high voltage DC to transmit power - and inverter systems to enable them to convert to AC and link to the grid. Advantages of HVDC links are that they can enable 50Hz and 60Hz systems to be linked - which using regular transformers is impossible. Using AC > DC ======= DC > AC however permits voltage AND frequency to be different at both ends of the system.