Managing frequency in a modern electricity system

The electricity grid is changing as we use more renewable energy like wind and solar power and less conventional generation such as coal and gas.

Infrastructure investors are embracing the new over the old and consumers are actively taking part in the clean energy revolution.

Everyone knows that wind, solar and storage do not work the same way as gas or coal. These technologies are not the same and advocates for the status quo love to point this out. Some claim that renewable energy doesn’t provide ‘inertia’ to the grid, although this is often not followed by an understanding of what ‘inertia’ actually means.

A new Clean Energy Council paper released this week investigates the issue of inertia, how it works with the power grid and how renewables and storage can help using currently available technology.

Inertia relates to the power system’s response to unexpected shocks, and its ability to provide time for other supporting controls to react and restore the system’s balance.

It has traditionally been provided by thermal gas, coal and hydro generators, and there are some other options available through conventional technology as well.

What is not so well understood is the interaction between mechanical inertia and the power grid. In short, the main job of inertia is to slow down the rate at which frequency changes after a grid shock, such as the failure of a large power plant or a transmission line – the larger the shock or the lower the inertia the faster the rate of change of frequency.

In the event shown below, South Australia’s Northern Power Station (this coal-fired plant closed in 2016) failed in 2005, the Heywood interconnector linking Victoria and SA tripped offline. And this happened before there was any wind in the state[1].

The red line shows the rate of change of frequency. The rapid frequency decline was stopped by turning off around a third of the state’s customers leading to major blackouts.

The example above is a stark reminder that the grid has never been perfect. In these extreme conditions all inertia is doing is buying time by slowing down the effects of the shock (i.e. the interconnector going offline), ensuring there is enough time for other emergency controls to kick in.

In the example, these controls included switching off lots of customers and the automatic powering up of other operating South Australian generators.

Those who advocate for the status quo because of the inertia provided by synchronous generators should be aware that these technologies are far from perfect. For example, they can become unstable at low power output. And there is simply no information available on how effectively these generators can respond to fast rates of change of frequency if they started operating before 2007.

Another concern is that, in comparison to other markets around the world, the National Electricity Market (NEM) is designed to have a very heavy reliance on the inertia provided by synchronous power generators such as coal and gas plants, instead of brining in a controlled automatic generator response more rapidly. There are relatively easy fixes to this issue that will improve outcomes for customers and align the NEM to good industry practice.

Other technologies like wind turbines and batteries can also assist in arresting frequency changes, thus supplementing inertia.

Capability of modern renewable energy and energy storage

Frequency is controlled by the balance of supply and demand. This is analogous to a pool of water in a stream where the water level stays the same if the flow in is equal to the flow out.

Injecting power into the grid will push the frequency (water level) up while drawing power from the grid will pull it down. Arresting a sharp fall in frequency requires a fast-acting injection of the right amount of power to stop it falling.

Modern wind turbines can draw on the kinetic energy in their rotating blades to deliver a fast-acting power injection into the grid if trigged by an event. They can also be flexibly controlled to deliver the correct response to suit the local grid conditions and requirements.

Hydro Quebec has been utilising this technology since 2011 with hundreds of wind these turbines now installed and operating. Wind farms can also turn down their output very quickly if this is expected by grid operators.

Large- and small scale-batteries can also detect a rapidly-changing frequency and either inject into or draw power from the grid. This ‘Fast Frequency Response’ solution is already being implemented in other countries.

It is not hard to see a day where homes around Australia have battery systems that don’t just help with their bills and the use of their solar power they also help to secure the entire energy system from major shocks.

Designing a 21^st century grid

No one is pretending that these responses from wind turbines and storage are the same as the inertia from a synchronous generator. But the solution is to provide enough flexibility in the energy system to ensure new technologies are providing these services in a power system which is changing dramatically.

The grid has to move with the times – it has to embrace these differences and utilise every opportunity to deliver the right outcomes for consumers. The following actions will be needed to bring ours into the 21^st century:

• Establish appropriate standards for frequency conditions that apply to all technologies following major events, with a focus on the speed and accuracy of their contribution to arresting the change in frequency following a disturbance.
• Accelerate trials of fast frequency response from inverter-based technologies to further prove this solution in the context of the NEM.
• Undertake a review of the existing synchronous generator fleet to understand its performance in response to frequency changes during normal operation and in response to major frequency disturbances.

The paper Arresting Frequency Changes in a Modern Electricity System is available on the Clean Energy Council website.

[1] National Electricity Code Administrator, Report into power system incident on 14 March 2005 in South Australia, 2005.

Tom Butler is Clean Energy Council Director of Energy Transformation