Balancing electricity supply and demand

We're used to using electricity without giving it much thought. Flip the switch and it's there. There's quite a lot going on behind the scenes though, and it's important to understand some of this because of what it entails for how we generate electricity, the effects that has on the grid, and the prices that we pay. The physics of the situation limits what we can and can't do, and also has significant effects on the electricity market.

We talked about the difference between energy, power and electricity in a previous post, where we learned that electricity is a way transferring energy, and power is the rate the energy is transferred. There are a few things to know about the electricity that comes out of your wall socket via the electricity grid, or the "poles and wires" that connect electricity generators and consumers. 

The first thing is the voltage, which is like the pressure of electricity. Voltage causes electric current to flow, with increases in voltage causing an increase in the flow. Voltage (in volts) multiplied by current (measured in amperes) is power in watts, more or less. Practically speaking, in Australia the electricity supply is 240 volts (V) and your normal power outlet is limited to 10 amperes (A). This means that any device that plugs into an electrical outlet is not going to have a power rating greater than 240V x 10A = 2400W. The electrical supply isn't exactly 240V, it has an allowable range of between 216V and 253V [1]. This range ensures that connected equipment won't be damaged by under or over-voltage, but the allowed variation is an important part of balancing supply and demand.

We talked earlier about the idea that energy can't be created or destroyed. This means that the rate that energy is being pushed into the grid by electricity generators has to be exactly the same as the rate that energy is being pulled out of it by consumers - there's nowhere else for the energy to go. There is a little bit of wriggle room because of the allowable voltage range, but not very much. If there's a bit more generation than needed, the voltage will start to rise, and this will cause the current consumption by all connected devices to rise a little bit. This increases their power consumption slightly, so that it matches power generation. The allowable voltage range means that nominal electrical power generation has to be within a few percent of nominal electrical power consumption, otherwise parts of the system will go outside of the safe voltage limits and automated safety mechanisms will start shutting down parts of the grid. Then you have a blackout.

To avoid this, supply and demand on the grid has to match at all times of the day. Not just in an average sense, but literally minute by minute, and to within only a few percent. Electricity demand depends on what people have switched on at any given moment in time. This looks somewhat random for a given household, but the averaged over tens or hundreds of thousands of households, predictable patterns emerge. The Australian Energy Market Operator (AEMO) and equivalent organisations around the world have the job of tracking and forecasting demand, and then scheduling electricity generators every 5 minutes, 24 hours a day to keep the whole thing balanced.

You can see this in Figure 1 below, which is a screen grab from the AEMO home page at the time of writing. The historical electricity demand for the state of NSW is the solid green line on the left hand side of the chart, and the dashed green line on the right hand side is the forecast. The maximum demand of 8,193MW (mega, or million, watts) was yesterday at about 7pm when a lot of people were cooking their evening meal, dropping to a minimum of 5,878MW at 3am in the morning when many of us are asleep and not using much electricity. There is an even bigger drop around midday but I'll leave that, and discussion of pricing, to another post.

Figure 1 - Historical and forecast electricity demand for NSW, 29th October 2022. Source: aemo.com.au 

The second thing to know about electricity from your wall socket is that it's alternating current, as opposed to the direct current that comes from a battery. This means that the electrical current in the wires reverses direction, and in Australia it reverses direction 100 times every second (for a cycle frequency of 50 Hertz - abbreviated as Hz). While there's a few percent of allowable drift for voltage up and down, there's almost no room for error with frequency. All the generators across the eastern half of Australia have to "push" and "pull" electricity through the grid in near perfect unison, just like the muscle cells in your heart have to contract and relax together to pump blood around your body. If your heart cells lose synchronisation you get fibrillation and cardiac arrest. Death follows quickly. Something very similar happens to the electricity grid if generators get too far out of step.

If the supply and demand balancing act is not maintained, things can go wrong very, very quickly - during the 2016 South Australian blackout for example, from the first inkling of things starting going wrong to the point when most of the state was blacked out, only took 90 seconds [2]. This sort of cascading failure is normally precipitated by equipment fault rather than by forecasting error, but it does illustrate how challenging it is to keep the lights on.

So this is the key thing to appreciate from this post. Electricity demand changes quite a lot over the day, at different times of the year, and depending on the weather. It's technically extremely challenging to keep electricity supply balanced with this ever changing demand - it's quite literally a balancing act to prevent automated, fast reacting safety systems from tipping the state into darkness. 

Next time, we'll find out more about how supply is scheduled, and how electricity prices are determined.

1. Solarlink, https://solarlinkaustralia.com.au/grid-over-voltage-and-overcoming-it/ accessed 29/10/2022
2. Wikipedia, https://en.wikipedia.org/wiki/2016_South_Australian_blackout accessed 29/10/2022