🔋Just Keep Spinning

What the death of grid inertia means for energy reliability and cost.

Jul 17, 2024

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There is a strong push for renewables around the globe as the energy transition has gained momentum. With it comes uncertainty about how the grid will fare, mostly due to the intermittent nature of wind and solar. Last week I discussed the growing battery storage market and how price arbitrage will be more lucrative the more renewables the grid has. You may not expect that the primary use for batteries on the grid currently is not the obvious price arbitrage, but categorized as frequency response.

This falls under the idea of grid inertia, another debated topic concerning its importance. Energy transition advocates tend to suggest that grid inertia is an outdated concept and unnecessary in the new energy paradigm. Critics on the other hand suggest that inertia is what maintains the reliability of the grid and stepping too much further will cause chaos in energy markets across the US. To see who is closer to the truth, we muddle into the gray area to learn more about this process. I do so with help from a report titled Inertia and the Power Grid: A Guide Without the Spin, from the National Renewable Energy Laboratory.

Inertia is the tendency of an object to either stay at rest or stay in motion. Imagine riding your bike at high speeds, where you would keep going absent friction and air resistance slowing you down. In the current grid, fossil fuel, hydro, and nuclear energy sources power large rotating generators. These generators are very big and the tendency for them to remain spinning is the concept of their inertia. When a part of the grid fails, the still-spinning generators allow some time delay for power operators to identify the problem and get new power to come online to keep things running smoothly. In short, renewables and batteries are called inverter-based resources (IBRs) that do not spin large rotating generators and thus have no intrinsic inertia which can lead to issues in a mixed energy source grid.

There are many generators all over North America which are all synchronized at the same frequency (60hz in the US) and split into four interconnections. For the health and durability of all the electrical equipment and infrastructure, the frequency of all the generators must remain within a tight range of around 60hz. When a generation source or transmission line fails, its power is removed from the system rapidly and without warning. This causes the frequency to decline steadily. If the frequency power is not restored and the frequency goes below a certain level (59.5 Hz in most of the NA, 59.3 Hz in ERCOT), some of the consumer load is disconnected. This is known as underfrequency load shedding (UFLS).

Primary frequency response (PFR) is the first line of defense against load shedding or disruption. Generators are equipped with devices to regulate the frequency and automatically adjust the rotation of the generator without the need for input from operators. The current system works like this figure, where the blue line indicates the system inertia allowing the frequency to slowly decrease after a contingency. PFR can often slow and reverse the issues and restore the declining frequency.

At a certain point new load is requested as the frequency gets too low. Hopefully, after a very short time, a new load will respond to the request and begin to balance the frequency back to 60hz.

Things like contingency size, generator inertia, and grid size all affect the time after the contingency before load shedding occurs. Additionally, energy demand ebbs and flows throughout the day and week, leading to varying grid inertia depending on when it is. Overall grid inertia depends on how many generators are running, so as more variable generation sources like wind and solar replace traditional sources, the less grid inertia there will be.

This should help you realize why some grid experts are concerned about adding wind, solar, and batteries to the grid and how it has the potential to be destabilizing. Take the example below, where with 30% wind on the grid, at different times there will be wildly different rates of frequency decline after a contingency which could mean the difference between load shedding and not.

In the US, ERCOT (Texas) has gone the furthest into the “danger zone” as they have added the highest share of renewables on their grid and are smaller than other interconnections. The largest contingency in both the Western Interconnect and ERCOT is a large nuclear power plant. If these went down, the contingency would represent 6.4% of the average load in ERCOT, but only 2.6% in the Western Interconnect leaving Texas with less time to respond to such an event. Frequency in ERCOT drops faster requiring the deployment of new load, while the Western Interconnect can get away with PFR due to its larger inertia.

There are several ways to address frequency stabilization, of which I will touch on a few.

System operation is where the operator keeps some generators on partially to keep inertia above critical levels but reduces the efficiency of the generators and can add costs.

Decreasing the contingency size can reduce the fall in frequency after a contingency. Grid operators may be more conscious of building large power plants and the tail risk it can have on the grid assuming more renewables come online as well.

Issuing a new load instead of using PFR can often respond quicker to a contingency, hence labeled Fast Frequency Response. ERCOT has employed this strategy more than any other to keep its grid functioning. Since wind and solar aren’t spinning massive generators, they can ramp up and down generation quickly. Using these IBRs below full output can allow them to ramp up to regulate frequency if need be.

Last week I discussed how the primary responsibility of grid battery storage systems was frequency response, not arbitrage as many expect. Batteries are the most exciting IBR for fast frequency response because they can increase output the quickest, can rapidly switch between charging and discharging, and offer the greatest flexibility. While batteries show promise in stabilizing frequency and microgrids have shown that zero-inertia conditions are possible, the NREL itself suggests, “The costs (or need) to develop a large system that can reliably operate under near zero-inertia conditions has yet to be analyzed in detail.”

In the end, both sides of the energy transition camp bring up good points. Renewables increase the frequency volatility, but at the same time allow unique ways to deal with frequency response challenges they might cause. Had we begun our grid with renewables, we wouldn’t likely be having the debate as the frequency response systems surrounding renewables would be built up around it. It is because our current grid operates with synchronous generators that rely on grid inertia that adding renewables causes complexity.

With Texas as an example and larger sizes, other interconnections should have less issue with frequency when adding renewables. NREL suggests that the grid allows for the reduction of inertia without negative consequences. While they may be right, the report does not speculate on economic consequences. It looks to be possible to solve the frequency regulation problems that arise, but I suspect that with many IBRs acting as frequency regulators instead of maximizing their use and low energy density, the cost to bring energy to consumers may not be as favorable.

-Grayson

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