Inertia, Synchronous Generators and Frequency
What is inertia and how does it support the grid? Our electricity network was historically created for baseload power through thermal fossil fuel power plants and therefore works best with the conditions provided by those generators. That is constant output power, frequency, and signal inertia. More recently, the energy generation mix has been diversifying with the rapid uptake of renewable power generation and it brings with it a decreased level of inertia in the grid. Read on to learn more about inertia…
Synchronous generators: what are the moving parts?
Generating electricity from fossil fuels involves burning non-renewable fuel sources to produce heat. The heat is transferred to a gas, that is, steam in steam turbines and air in gas turbines. The high-pressure, high-temperature gas is used to spin a turbine, which in turn spins an alternator. It is the alternator that actually generates the AC electricity[i]. The term ‘synchronous generator’ comes from the fact that the speed that this generator spins (in revolutions per second) is synchronised with the frequency of the grid (which is necessary to maintain the appropriate AC signal).
What is Inertia?
Inertia in physics is defined by Newton’s first law of motion, also known as the law of inertia: a body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a net external force[ii]. Inertia, in the case of the electricity grid, is referring to the grid’s ability to ‘push back’ against any disturbances. The spinning turbine of synchronous generators (fossil-fuel fired power stations) provides constant grid inertia; the spinning turbine cannot stop quickly and only very slowly runs down if the pressurised gas slows or stops. Basically, it’s a big heavy piece of machinery that wants to keep spinning at the same speed.
What does low inertia result in? A grid that is more susceptible to minor disturbances disrupting the whole system. Another way to think of inertia is as a ‘buffer’. If a disturbance occurs, the turbine will keep spinning and allow some leeway which prevents the minor disturbance from affecting the whole grid.
Watt energy has an explanation about FACS here.
How is this related to frequency?
The frequency in the grid was set historically as the speed of the turbine and alternator of synchronous generators. You can see that the spinning inertia of a synchronous generator will keep the generated power at a consistent frequency. Globally, the frequency in each country is either set at 50 Hz or 60 Hz; the grid in Australia is kept at a constant frequency of 50 Hz, whereas the grid in the USA is set at 60 Hz.
What happens when the frequency drops?
Grid infrastructure is designed for a certain frequency and MW capacity, if an event occurs that exceeds the designed limits, the system shuts down to protect that infrastructure usually resulting in blackouts – read about the infamous 2016 South Australian Blackout[iii]. However, the Australia Energy Market Operator (AEMO) has processes in place to avoid total system failure; one of these is quick response energy storage.
Asynchronous generators
Many renewable power generators which are becoming very common on our grid are asynchronous. Wind turbines use rotation to generate electricity, the turbines spin at variable speeds set by the wind and not from constant high-pressure gas which limits the amount of inertia they hold. Solar photovoltaic (PV) cells only produce DC electricity when the sun is shining and have no moving turbines; solar PV has no spinning parts which would contribute to inertia. Both wind and solar are known as intermittent energy sources and have electronic frequency control to match the grid-required 50 Hz.
Energy storage & duration
To make up for the decreasing level of built-in inertia on the grid, other buffer systems have been installed – such as batteries. Quick response energy storage can be used to meet frequency variations, stabilise the grid and become that ‘buffer’ or ‘cushion’ to external events. Quick response energy storage is vital for the NEM’s shift to renewable power. However, a full combination of energy storage is needed to meet daily, weekly and seasonal demand while managing intermittent supply generation.
Medium and Long Duration Energy storage is also a rapidly expanding market. It refers to the effective length of storage, creating categories within the energy storage market. While batteries are very effective at quick response to electricity output, there are many other forms of energy storage that range from hours to months. Some medium-duration energy storage technologies such as Thermal Energy Storage (TES) can build in a level of inertia to the grid. As we transition to a net-zero energy network, a combination of energy storage technologies is a promising solution to create a stable, and robust grid.
MGA Thermal’s technology is medium to long-duration energy storage which is ideal for shifting the supply curve to meet demand on a daily to weekly basis. The TES can repurpose existing thermal power station infrastructure, to create cost-effective, sustainable, robust grid-scale energy storage. Learn more about the MGA Thermal technology here.
The next article in this series will discuss the supply curve, how it is expected to change with a full shift to renewables and how supply affects the price.
Published on 10/02/2023 - Arden Jarrett
A version of this blog was originally published on 18 January 2021.
References:
[i] https://adgefficiency.com/inertia-electricity-systems-energy-basics/#:~:text=Wind%20and%20solar%20are%20asynchronous,no%20moving%20parts%20at%20all
[ii] https://courses.lumenlearning.com/physics/chapter/4-2-newtons-first-law-of-motion-inertia/
[iii] https://www.abc.net.au/news/2016-10-06/uhlmann-on-power-blackout-in-south-australia/7906844
