What happens to "un-used" electricity?

Listener Andrew wants to know what happens to "un-used" electricity.He says:

"I live in Australia and am fairly used to periodic load-shedding when, for instance, the grid can't supply everyone's air-conditioning on very hot days. But what happens when there is very low demand? There is power when you switch a light on, but when it is switched off again, where does that electricity go. It's being generated and fed into the grid, but if everyone switched off their homes at the same time, what would happen to that electricity fed into the wires at the generator end?"

Energy industry specialist and now energy storage PhD researcher Rachel Lee from Sheffield University took this question on for us...

Rachel - In an electricity system like the Australian Grid, electricity generated must be matched on a second-by-second basis to the electricity required by consumers. So when you switch a light on in your house, somewhere in the country a generator puts more fuel in, or a hydro plant takes more water. When you turn it off the opposite happens.

Electricity is generated and transmitted as alternating current or AC, meaning that the it comes to your house as a wave that is constantly cycling from positive to negative and back again. How quickly that happens depends on the speed the generators are turning. In Australia and many parts of the world, we aim to have 50 of those cycles every second; we call that the frequency in Hertz, so that’s the 50Hz you may have heard of.

The operator of the network detects the change in load by a change in that frequency. If demand exceeds supply the generators slow down, just like your car going up a hill, and frequency falls and vice versa. Of course with just a light, the extra electricity required is so small compared to the total supply that it is lost in all the other things being turned on and off. But when the changes in load are large, for example when people get home at 5-6pm and all turn on their air-conditioners, the system frequency will start to fall. Some generators will automatically start increasing output to bring the frequency back to 50Hz, but in some cases the system operator may request additional plant to start.

If there isn’t enough plant available, then that is when load shedding could happen. However, load shedding is often a result of high demands locally in the system when the wires and transformers supplying your area get overloaded.

If everyone switched off at the same time, then the system frequency would rise very rapidly. Plants in automatic frequency control would quickly reduce their output, but if this wasn’t enough then eventually the frequency would go so high that ‘over-frequency’ protection on generators and other parts of the system would operate and disconnect generators completely.

通常系统频率紧密控制rolled; in Australia AEMO, The Australian Electricity Market Operator, aims to control the frequency between 49.85 and 50.15Hz. Within this normal range, generators are centrally set to their required output every 5 minutes and further signals are sent to raise or lower output on a second by second basis. If the frequency goes outside of those bounds, then ‘Contingency Frequency Control Services’ are called upon to operate and these aim to maintain the frequency between 49.5Hz and 50.5Hz. Depending on the service provided, these must respond between 6 seconds and 5 minutes.

Although the fine detail of frequency control varies around the world, the basic process remains the same everywhere; a system frequency below normal means more generation is required and a frequency above normal means less generation is required. There is never any spare electricity!