What is system strength?
A stable power system can operate normally under all reasonably expected conditions. Importantly, it is able to quickly return to normal if there is a disturbance on the network, like any generation or transmission unexpectedly cutting out.
If the power system isn’t stable, a disturbance can instead lead to wider problems and interrupt the supply of electricity to many consumers.
In Australia, we expect that stability from our power system. What gives the system the stability it needs?
Power system stability relies on the complex interaction of many electrical and mechanical elements. Helpfully, engineers summarise the whole package as “system strength”.
Why does system strength matter?
A strong power system can respond better to disturbances than a weak one.
There are two important things to know about system strength:
It is local, so some parts of a network can be strong and others weak. System strength can’t be imported from another part of the system, but must be supplied locally. It is important to identify weak parts of the system and take action to strengthen them. (Local can mean geographically close, but can also relate to how the system is configured.)
It is tied closely to the presence of ‘synchronous ’ machines in the local area.
What makes a power system strong?
Because of how the power system is designed, synchronous machines – like coal, gas and hydro generators – make the system stronger just by operating.
It’s important to say ‘synchronous machines’, not synchronous generation, because it doesn’t have to be synchronous generation keeping the system strong. Synchronous condensers are machines that don’t generate power but can do the same job to make the system stronger.
On the other hand, non-synchronous generators, like wind and solar, connect to the power system differently and don’t contribute to system strength as an automatic by-product of operating.
That means we never had to think about building system strength in the past. It just happened naturally as traditional generators operated. But now, as non-synchronous generation grows, we do have to plan, to make sure all parts of the power system are strong enough to be stable.
In any part of the power system where lots of non-synchronous generation starts up, that local area can become weaker and potentially unstable. (For example, AEMO has identified system strength issues in South Australia, and potential emerging issues in north-western Victoria.)
To strengthen a weaker part of the system, enough synchronous machines have to be operating alongside the non-synchronous generation. If more non-synchronous generation is operating, the contribution of synchronous machines also needs to increase, to keep the proportion right.
In operating the system, AEMO ensures there is always enough system strength in every local area. We do that by requiring a minimum number of synchronous machines to be online in weaker parts of the network when non-synchronous generation is operating.
That action helps maintain system strength, so the power system is stable and can keep operating normally and supplying customers with electricity, even if there are small or large disturbances.