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Question

What is the role of inertia in the future grid?

Paul Hobcraft's picture
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As the grid evolves with increasing penetrations of inverter-based resources—e.g., wind, solar photovoltaics, and battery storage—that do not inherently provide inertia, questions have emerged about the need for inertia and its role in the future grid. Understanding the role of inertia requires understanding the interplay of inertia and these other services, particularly primary frequency response, which is derived mainly from relatively slow-responding mechanical systems. How will this be transitioned?

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This is a great question; I think of the Hornsea use case. Although the power transferred through renewables to the system is through power electronics, power electronics typically do not provide an inertial response to the grid. An example of this was August 9, 2019, from the Hornsea Windfarm in the UK.

The Hornsea Project is an offshore wind farm located off the Yorkshire coast within the Hornsea Zone in the southern North Sea. During the evening rush hour, London and surrounding areas suffered a widespread power outage Friday, August 9, 2019. During the blackout, renewable generation was a significant portion of the overall contribution to the transmission system. Unfortunately, the lack of inertia coupled with improper settings on the wind farm did not correctly respond to the dip in frequency from the transient event. As a result, a cascading blackout occurred that dropped power to hundreds of thousands of people.

With renewables that have transient power output and power electronic sources, becoming the primary source of generation to the grid, grid flexibility is key to not simply having an after-action review after a blackout but to prevent outages in the first place. Dynamic controls coupled with a state estimator and real-time power flow tool will allow the grid to maintain a two-way power flow while assuring the generation matches the demand.

Paul, the relatively slow responding mechanical systems to which your refer are an asset, not a liability. The steady output of large power plants provides a stable basis for frequency and voltage that is unavailable from renewable sources of energy.

The much-anticipated "energy transition" to solar and wind is already accompanied by a corresponding transition to unreliability, and it will only get worse.

I believe some HVDC facilities can also provide "synthetic inertia". I seem to recall the Cross Sound Cable between Long Island and ISO-NE's grid have some ancillary service capabilities, using its inverter.

Building on the comments from Doug Houseman and Roger Arnold, there is definitely some consideration, and action, given to this issue, especially on smaller grids where inverter-based resources can make up a significant percentage of total generation at times.  I recalled hearing about a project in the UK, just in the last few years, that will specifically target the addition of inertia to the grid.  Here are a few links with more info: 

Giant flywheel project in Scotland could prevent UK blackouts

National Grid ESO - ’Cheaper and greener’ – the project that’s changing how we keep the grid stable

Additionally, there are also efforts to install equipment to monitor, and measure, grid inertia for decision making on dispatch/curtailment of inverter-based generation:

UK installs 5 MW ultracapacitor as ‘sonar’ to detect power grid inertia

Synchronous condensers (the generators at large retired power plants are a good example) can also help maintain far more than inertia. As the plants the transmission grid were designed around, the ability of the transmission grid to move power decreases, so leaving the generator in place as a synchronous condenser provides both ancillary services and increased transmission capacity (In the docket for DTE's IRP filling is a wonderful filing from ITC on this topic, please read that if you need more background).

I am not worried about how, I am worried about if people will pay enough attention to inertia. Pharma, computer chip making, some chemical processes, spot welding, and food manufacturing of some sorts are all sensitive to power quality, if we don't make it a priority in the transition, much of that manufacturing will be forced to go elsewhere, at a time where we are already struggling with supply chain issues and high quality jobs. 

Full sine-wave inverters can be designed to provide a degree of "synthetic inertia". In fact they can simulate infinite rotational inertia in their response to transients, if the transients are small. To do so, their DC inputs need to be buffered with enough capacitance to sustain bursts of power well above the steady-state rated power of the inverter. The inverter circuits must also be able to tolerate bursts of over-current without blowing out.

What an inverter can't do -- economically -- is to keep up its simulation of high rotational inertia for very long in the face of the kinds of transients that arise when re-making the connection to a black sub-grid. Synchronous condensers are probably a better option for that. A synchronous condenser is a free-spinning synchronous motor-generator. It provides true rotational inertia.

Bob Meinetz's picture
Bob Meinetz on Feb 17, 2022

Roger, questions:

1) Are these sine-wave inverters you're envisioning located at the output of solar / wind farms, or individual panels?

2) By "black sub-grid", are you referring to that of the solar / wind farm, or the local distribution grid?
3) Why would they need to break / re-make a connection?

Roger Arnold's picture
Roger Arnold on Feb 18, 2022

One of the first articles I posted to the old EnergyPulse website back in March 2005 was Distributed Energy Resources: Why IEEE 1547 won't be the Last Word. It was a deep dive into the version IEEE 1547 that had recently been released. It was focused on anti-islanding, and defined a standard for small inverters that allowed rooftop solar installations to connect to the grid. My conclusion was that it would be fine for the levels of penetration that were likely to be reached within that decade, but that new revisions would be needed for higher levels of penetration. The standard defined features that would enable an inverter that was providing power to the grid to detect that the connection to a central power generator had gone down. The inverter would then disconnect automatically, to avoid electrocuting line workers.

I was right that extensions would be needed; the standard has been revised and extended several times since that time to allow for higher levels of penetration by DERs. I haven't followed the revisions, and so I can't say whether synthetic inertia is now included in the standard. My guess would be that for small inverters, it's not. For the large inverters that you'd find for connecting a solar PV farm to the.grid, I'd think it would be. However I don't know. It's possible that the connection to a wind farm would be controlled directly by the TSO's SCADA system.

By a "black sub-grid" I'm referring to a local black-out. The grid has breakers that trip automatically for self-protection in the event of a short circuit or open circuit, or that can be commanded to trip to shed load in a power emergency. But eventually power will need to be restored, and that means re-making a connection to a section of the distribution grid that has a lot of unpowered loads attached to it. For a few tens to hundreds of milliseconds, there's a huge current rush. 

 

Bob Meinetz's picture
Bob Meinetz on Feb 18, 2022

Thanks.

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