Renewable energy sources – predominantly wind and solar – are no longer a novelty, but a necessity and a reality for the power grids of countless nations around the world. Every year, the number of renewable power plants, wind farms, and solar arrays increases, meaning that every year, the amount of energy generated from renewable sources and fed into the grid also increases. In short, we are receiving more and more of our domestic and industrial electricity from renewable energy, and this trend is only set to grow.
Electrical grids depend upon a regulated intake and output of energy to function faultlessly, but the increasing influx of ‘intermittent renewable power’ to the grid is upsetting this balance. Hence, there is a growing need for renewable energy frequency regulation. In this article, we discuss exactly how frequency regulation works.
Here, we shall discuss the current frequency regulation mechanics built into our electric grids: what they are, how they work, and why they’re necessary. We will then continue on to discuss the measures required in order to regulate the frequency of energies received to the power grid from renewable power plants and wind/solar farms.
How the Conventional Power Grid Is Regulated
The foundations of the electrical grid were designed before the introduction of renewable energy sources, and as such, its main regulatory mechanisms are unequipped to deal with the increasing influx of renewable power. But what are its regulatory mechanisms, and why are they important?
In order to deliver homes, businesses, and industries electricity, the power grid must be able to deliver a steady, consistent, and usable stream of electricity to every single connected ‘load’, regardless of any disturbances to the grid (which include unexpected or large load changes, partial generation, transmission, or distribution failures). In other words, no matter the fluctuations in supply and demand, the power grid has to be able to continuously connect consumers with energy sources in a safe, regulated manner.
This can only happen if three main variables are monitored and regulated. These three variables are:
- Voltage: Referring to the voltage level at every point on the grid. The operator has to monitor the voltage level from the biggest generation point, to the smallest load connected to the grid.
- Angles: As energy is transmitted in alternating current (AC), all voltages and currents come with an angle reference. The relation between angle references determine the flow of active power on the grid.
- Frequency: Last but not least, it is absolutely paramount that the grid’s AC transmission be made at the same frequency across the board. In the USA, this frequency must be 60Hz, whilst in other parts of the world it may be different (50Hz in Europe, for example).
Only by monitoring and controlling these variables is the grid operator able to diagnose potential problems, correct them, regulate the system, and establish preventative measures against similar future problems.
As it stands, grid operators rely primarily on the inertia afforded by rotating generators in order to regulate the grid’s power according to the above variables.
Rotating Generators as Frequency Regulators
Currently, the power grid’s frequency is determined by the speed at which the grid’s largest generators rotate. Deviations to this controlled frequency are the main triggers of the grid’s various in-built system protections, and these deviations are generally caused by an imbalance between the available supply of power and the end-user demand.
In order to mitigate these imbalances and the grid faults they can trigger, generators are used by grid operators to regulate the grid’s frequency. Up to a certain point, large generators can be sped up or slowed down, absorbing surplus energy fed into the grid, or working overtime to produce enough energy to meet a spike in demand. As such, rotating generators are used to create inertia (or physical stability) in the grid’s frequency.
Of course, rotating generators cannot regulate and stabilise the grid in this fashion indefinitely. What they do do, however, is provide grid operators the time they need to take corrective action against whatever sudden change has created the power imbalance.
Used in this way, the inertia granted by the rotating generators of conventional, non-renewable energy sources and power plants is called the ‘primary frequency response’ (PFR). To date, renewable energy sources and power plants do not have similar in-built frequency regulation measures.
Solar Generation and Frequency Regulation
Renewable Energy Generation
In recent years, the increasing presence of solar and wind energy on the national grid has presented some significant problems for grid operators. For one, renewable energy is what’s known as ‘intermittent’. In other words, it’s output fluctuates wildly throughout the day, and depending on the weather across the many thousands of renewable energy sites in any given country.
As an example, the US state of California has recorded a phenomenon commonly referred to as “The Solar Duck Curve”. The Solar Duck Curve refers to the shape made on a chart (which apparently resembles the silhouette of a flying duck), when peak energy demand is charted alongside the peak hours for renewable energy production. To put it simply, peak energy demand typically occurs in the early morning, and in the evening – times during which solar energy production is limited, if not altogether non-existent, given that solar power produces peak electricity in the hours around midday.
Problems with Renewable Energy Frequency Regulation
In relation to renewable energy’s problems of demand/supply imbalance, there is a major grid issue surrounding renewable energy plants’ capacity to regulate the imbalance. Because renewable energy sources such as solar do not create electricity with rotating generators, they do not have an innate ability to maintain inertia within the grid (which would help to regulate frequency during hours of demand/supply imbalance).
Currently, the procedure for a wind or solar plant in case of frequency or voltage deviation is simply to isolate itself from the grid. (IEEE 1547-2018 allows for renewable plants to operate from 56.5Hz to 62Hz with frequency droop included, outside of which they must disconnect from the grid.) Whilst useful as a means of minimising the risk of an under- or over-frequency grid complication, it does effectively render renewable power plants useless during periods in which they are producing too much, or too little power for the grid.
In the case of a total grid blackout, the IEEE 1547-2018 procedure is referred to as “anti-islanding” protection, as it prevents renewable plants from energizing the grid when the root cause of the blackout remains unsolved, or when there are workers at risk on the line.
Solutions to Renewable Energy Frequency Regulation
Of course, this doesn’t mean that the photovoltaic inverters used by renewable plants are not capable of frequency control. However, the frequency regulation provided by said inverters (at the current time) is innately problematic, as it still prevents renewable plants from working at maximum capacity (provided that maximum capacity would overload the grid).
As a solution, many renewable energy advocates propose large-scale battery storage as a solution. If we can store surplus energy produced by renewable energy sources in batteries, the theory goes, then we can effectively maintain the same inertia that rotating generators allow for. In other words, during peak production hours surplus energy could be stored in batteries, thus not overloading the grid, whilst the batteries could then be used to feed the grid a regulated voltage and frequency of electricity during peak demand hours.
Unfortunately, technology is yet to catch up with these aspirations, and mass energy battery storage is currently far too expensive for large-scale, industrial and national application. Thus, the best hope for renewable energy frequency regulation would appear to lie in changing the types of inverters we use in renewable power plants.
Grid-Following and Grid-Forming Inverters
Today, the photovoltaic inverters connected to the grid are all ‘grid-following’ inverters. What this means is that they regulate their power output not based on their energy sources, but based on the grid itself. The grid demands 60Hz of frequency at a specific voltage and angle, and so the grid-following PV inverter regulates its source’s output to match this demand.
There is, however, an alternative. Grid-forming inverters. A grid-forming inverter works by continuously controlling and regulating its output frequency and voltage, just like rotating generators do, thus dictating to the grid how much energy it will feed it, rather than regulating its power source to meet grid demands.
Grid-forming inverters are most commonly used in microgrids (such as off-grid homesteads and communities), because they actively regulate their output based on measured real and reactive power values. Grid-forming inverters are capable of operating as parallel voltage sources, with solid load-sharing capabilities, and an ability to maintain a stable output voltage and frequency, despite load variations (peaks and troughs).
More research needs to be conducted into grid-forming inverters, before they can be rolled out across the renewable energy grid. Nevertheless, studies would suggest that replacing grid-following inverters with grid-forming ones may be the key to negating our grid’s reliance on the inertia of rotating generators. Thus, it may also be key to the widescale insertion of renewable energy into the grid, which our world so desperately needs.
The power grid endures many fluctuations to the supply and demand of energy. Therefore, its voltage, angles, and frequency must be closely monitored and regulated to ensure that it can continuously deliver energy to consumers, in spite of these fluctuations (or any other issues). At present, renewable energy plants do not have the means to regulate frequency like non-renewable plants do (with the inertia afforded by rotating generators). As such, renewable energy plants are rendered greatly less efficient by an “anti-islanding” policy which automatically disconnects them from the grid when their energy production levels fall outside the 56.5Hz to 62Hz bracket. Until battery storage technology becomes a reality for renewable energy plants, the best frequency regulation alternative would appear to be the replacement of the current grid-following photovoltaic inverters with grid-forming inverters. These GFMs may present the most viable means of transitioning into a green, inverter-dominated, renewable energy grid.
Without the regulation of the grid’s frequency, we would experience many more disturbances (including blackouts) to our access to grid energy.
Grid operators regulate voltage, angles, and frequency by utilizing the inertia (or stabilising) capabilities of the rotating generators used in the production of non-renewable energy.
There is currently no reliably efficient means of regulating the frequency of renewable energy, without losing out on potential power, or reducing the operating capacity of the renewable power plants. As it stands, a policy of automatic disconnection from the power grid is the best means of ensuring that renewable energy production spikes and dips don’t disrupt the grid.
High-capacity, industrial-scale battery storage would arguably be the most efficient regulatory method for renewable energy, however the current technology available would not be able to affordably meet those demands. Instead, the future of renewable energy frequency regulation may lie in replacing old grid-following photovoltaic inverters with grid-forming inverters, instead.
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