Discussion of high penetration of solar and wind energy inevitably leads to the question: “but what about inertia?” To understand why many believe inertia is a critical function of conventional generation, one must first understand inertia’s role in the grid.
A consequence of Newton’s First Law of Motion, inertia is the resistance of an object to a change in its motion. In the electric grid, the motion in question is the rotating mass of a generator spinning at a rate synchronized with the system frequency. Inertia is stored rotating energy in the system.
During a system disturbance, generation and demand become unbalanced, resulting in a change of system frequency. Stored kinetic energy in those rotating generators is then released, slowing the drop in frequency to allow other generators time to restore balance.
What happens when rotating mass is replaced with solar?
Solar generation has no rotating mass; instead, photons collide with atoms and knock electrons loose, generating a flow of electricity that is converted to grid power via inverters. As there is no synchronous rotating mass, this is called asynchronous generation. Some believe that a predominance of asynchronous generators will lead to more pronounced system frequency drops and diminished system reliability due to the absence of inertia.
Enter “synthetic inertia”: inverter-based resources like solar and storage react so fast, they can mimic rotating mass inertia. Where inertia is similar to pumping the brakes on a moving freight train, synthetic inertia works by instantaneously increasing output to counter frequency drops via swift control of power electronics.
Recent testing by First Solar, CAISO, and NREL demonstrated that inverter-based solar resources have superior frequency response performance over fossil-fueled units, ironically due to the absence of rotating mass. In fact, First Solar projects are capable of ramping from 0MW to maximum output (at a given irradiance) in less than one second – significantly faster than any fossil plant.
How can solar projects provide this service? Potential solutions include:
- Headroom – Traditionally, solar projects have been designed and incentivised to maximize production at all times. But by maintaining headroom (withholding some generation capability), the resource can increase generation in case of a drop in frequency. This will require changes in how solar resources are compensated; however, there are numerous cost and efficiency benefits to this approach that have been expanded upon in other forums. During times where that inertial response is not necessary, the solar resource could be counted as spinning reserves or could provide traditional frequency regulation service – all with zero marginal cost and at significantly higher levels of accuracy than are capable from fossil plants.
- Solar + Storage – Another potential solution is to integrate storage with solar projects with the express purpose of providing reserve capacity and ancillary services. The grid operator could then dedicate a specific amount of generation to be stored and made available for immediate dispatch if frequency drops.
- “Running Hot” – All projects interconnected to the bulk power system have a maximum Point of Interconnection (POI) limit. Many solar projects, though, are slightly oversized to provide other services to the grid. For example, there is a requirement from FERC Order 827 that generators be able to provide reactive power support while not sacrificing active power production. First Solar modestly over-sizes each inverter at a project site, “clipping” their production in order to maintain POI. A plant controller could theoretically override that clipping algorithm for a short timeframe to arrest a frequency drop. This could slightly overload the interconnection transformers and other equipment – a.k.a., “running hot”, as it would exceed equipment temperature limits. However, this may be feasible for short periods without damaging equipment. This approach would require study before implementation is considered, but may be a cost-effective solution.
Where do we go from here?
PV power generation has the capability today to support dynamic frequency response. At higher levels of penetration, PV can be leaned upon to provide “synthetic inertia” to ensure system stability and reliability. Grid operators must begin testing the full suite of capabilities from inverter-based resources so that they gain comfort long before high penetration occurs. Additionally, regulators, utilities, and other key stakeholders should explore how to modify compensation to best incentivize solar projects to provide these types of services. The solar industry needs to lead by example, enabling all of the capabilities offered by the fossil fleet, rather than just being a must-take energy resource.
 See, for example, Nelson, J. et al. October 2018. Investigating the Economic Value of Flexible Solar Power Plant Operation. Energy & Environmental Economics. https://www.ethree.com/wp-content/uploads/2018/10/Investigating-the-Economic-Value-of-Flexible-Solar-Power-Plant-Operation.pdf.
 See Section 3.3.3, Goggin, M. et al. November 2018. Customer Focused and Clean: Power Markets for the Future. Wind Solar Alliance. http://q8j.1ac.myftpupload.com/wp-content/uploads/2018/11/WSA_Market_Reform_report_online.pdf.