Solar, wind and storage can provide seven key grid services

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From pv magazine USA.

Solar, wind and storage can provide all seven of a set of “essential” grid reliability services at a level ranging from good to excellent, according to grid consultant and researcher Michael Milligan, due to the capabilities of their inverters.

The table above, which synthesizes previous research, features grid services considered essential to restoring grid frequency after a generating unit outage or other disruption. Milligan, who examines renewables’ capability in a paper in The Electricity Journal, presented his findings at the policy conference of the National Association of Regulatory Utility Commissioners.

Grid operators can call on distributed as well as utility-scale solar, if small generators can respond to signals from a utility or aggregator – a practice that could be facilitated by a recently issued standard communications profile.

Milligan defined his seven essential grid services primarily using a concept paper prepared by the Electric Reliability Services Task Force of the North American Electric Reliability Corporation (NERC).

Milligan’s study builds on prior research into the grid services solar can provide. That body of work included a study by First Solar, the National Renewable Energy Laboratory (NREL) and the California Independent System Operator (CAISO) that demonstrated solar’s capabilities in frequency control, voltage regulation and ramping capabilities; a study sponsored by First Solar and conducted by E3 that concluded operating solar in full flexibility mode enabled a higher level of cost-saving solar on the grid; and a NERC report showing frequency response improved with more solar and wind on the grid.

Inverters hold the key

Milligan explains in his article, inverters form the basis of the grid capabilities of solar, wind and storage, and writes: “Because the power electronics devices that couple DC to AC power offer very fast response, it is now possible to use software to control how the resource interacts with the power system, subject to physical constraints.”

To provide grid services, the article notes solar, wind and storage must also “be in an appropriate operating state to provide services when needed” – for instance, solar must be “pre-curtailed” to provide ramping support – and must “have an economic incentive, and/or no economic disincentive, to provide the service”.

In other words, economic incentives are essential at this stage – regulators must provide appropriate incentives to enable participation by all technology types. That process has already begun. Milligan notes CAISO has adopted ramping as a market-based product while its counterpart Midcontinent ISO is developing a ramp capability product. He adds: “Any resource that is capable of providing a grid service should not be prevented by reliability rules or market rules from doing so.”

The author, who launched his consulting firm after retiring as a principal researcher at the NREL, said he wrote the article because: “There is no widespread understanding of the grid services that can be provided by alternative resource types. In some cases, there were unfounded claims that traditional resources could provide all of the required ERS [essential reliability services] and that new resources cannot.”

Milligan provided a graph showing how four of the grid services related to frequency work in sequence to restore grid frequency after network disruption. As the graph shows, “fast frequency response” kicks in during the initial seconds, slowing the rate of frequency decline and establishing a low point. Within around 20 seconds, “primary response” stabilizes the frequency. Additional power then comes online over the following minutes to restore normal frequency.

Image: Milligan Grid Solutions, based on a graph by Joseph Eto at Lawrence Berkeley National Laboratory, with input from Mark Ahlstrom at NextEra Energy