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Grid synchronous inertia: Vital to stabilising the grid and preventing the next blackout

By Dr Javier Cavada, Gary Preece

Taking the UK's transmission system and the circumstances which led to a dramatic blackout as a direct example, Javier Cavada and Gary Preece of liquid air energy storage (LAES) systems company Highview Power look at how such events could be avoided, cost-effectively and with lesser environmental consequences.  

The stability of the energy grid in any country is vital to guard against outages. Delivering accessible electricity that is safe to use is a priority, and for the most part, the national grid in the UK is reliable. However, to guard against the kinds of blackouts that occurred last year and to safeguard the system, National Grid PLC is now researching energy storage technology.

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Outages occur for a wide variety of reasons, one of the most likely being a systemic failure in grid “frequency” due, in part, to an underabundance of “system inertia”. While critical to understanding the behavior of the grid—and key to understanding how we can make renewables a baseload power, inertia and frequency are terms that are often overlooked and misunderstood when reading about any outage. It is important to understand how the grid works in order to decide what is needed as the UK transitions to renewables.

To begin with, almost all traditional power sources operate on the same general principle: something is burned—coal, natural gas, oil—which boils water, creating steam, which is pressurised and used to spin a turbine. That spinning turbine is connected to a generator, which creates the power that is injected onto the grid. This is where inertia comes into play; inertia is the tendency of an object to resist changes in motion and direction. The spinning turbine, for example, is difficult to stop once it is going—it has high inertia. Electrical inertia works in much the same way, it resists disruptions in the grid caused by changes in demand. Because the spinning turbine is difficult to stop, the power it can generate is relatively consistent.

What happened when the UK grid’s frequency dropped and was restored on 9 August 2019. Image: National Grid.

Grid frequency: It’s complicated

Things become a little more complicated with grid frequency, which is the number of times per second alternating current changes direction. A consistent frequency is important to the grid because it prevents damage to its infrastructure. As it turns out, frequency is directly proportional to the speed of the spinning turbine coupled to the grid which is where the term “synchronous inertia” comes from. On a large scale, all turbines that are supplying power to the grid are synchronous or spinning at the same speed—nominally 3,000 RPM—thus, producing at the same frequency.

In the UK, the grid operates on a frequency of 50 Hertz (Hz) and deviations as small as 1% can result in systemic infrastructural damage. If two power plants simultaneously, or in quick succession, for example, experience transmission interruptions, this can cause a significant drop in grid frequency. To prevent damage, the energy demand on the grid will then have to be reduced, which means manually disconnecting parts of it, resulting in an inevitable outage.

This begs the obvious question: how can we prevent this from happening? There are two answers to the question. The first may be an infrastructural solution because, regardless of how reliable our power plants are, without reliable power infrastructure, the grid can blackout at any time. The second is to increase the inertia of the grid, therefore, allowing for more flexibility to maintain a consistent frequency. 

Making the grid easier to maintain

Twenty-first Century grids should be more reliable than those built decades ago, but this is often not the case because the increasing deployment of renewables has had a deleterious effect on grid inertia. In fact, in 2019, the supply of energy in the UK to have come from sources that are considered to have no inertia, in particular wind farms and solar, rose to 40% of the total electricity supply, according to reports. Renewables generate power in a way that Is “asynchronous,” meaning that there are no spinning turbines to synchronise with the existing turbines of traditional energy plants, so they have no inertia.

Pumped hydro is an exception because falling water is used to spin a turbine, which is coupled to the grid making it synchronous, but it is a rare exception in the panoply of renewable energy sources. For the sake of simplicity, we can say that wind and solar have no inertia. Therefore, as more and more solar arrays and wind farms come online and contribute to our energy needs, the overall inertia of the grid drops further and further, which makes a consistent frequency more difficult to maintain.

Going back to the solution of increasing the inertia of the grid—and thereby making the grid frequency easier to maintain—we need renewables to have synchronous inertia with the grid. Wind, solar and lithium-ion (inverter based systems) require a stable system to function correctly. This is understood by National Grid in the UK, which is why, over the past year, it has invested £180 million (US$225.44 million) in securing this kind of technology.

Our company, Highview Power, has developed a cryogenic energy storage system that will promote both grid resiliency and the deployment of renewable energy sources. The CRYOBatteryTM is a long-duration, cryogenic energy storage system that allows renewables to be grid synchronous, with each plant maintaining grid sync for up to 24 hours, promoting the overall health and stability of the power grid. With synchronous, grid-scale energy storage for renewable power, grid-threatening frequency oscillations can be less frequent, resulting in fewer blackouts.

Cover image: New York city suffering a weather event-induced blackout. Image: Flickr user Dan Nguyen.  

Close-up of part of a Highview Power CRYOBattery installation. Image: Highview Power.

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