As the power system continues to evolve with an increase in inverter-based resources (IBRs), there could be a need to adopt future inverter technology and capability that could allow for operating a 100% IBR network in a safe, secure, and reliable manner. Such inverter technology has been labeled as grid-forming (GFM) in certain industrial discussions. However, we should take care to not operate on the assumption that the responsibility for the stability and reliability of a future, high-IBR power system falls exclusively on what are considered GFM IBRs. Rather, this responsibility is dispersed. So-called grid-following (GFL) IBRs can play a significant role—and they must, if we are to most efficiently transition to a high-renewables system.
Reliability is a Team Sport
Power system operation can be considered a team sport, where the responsibility and the burden of reliability can be expected to be spread around multiple participants. An improvement in stability, security, and reliability can manifest only when each player—in this case, each IBR or other resource—contributes a little in a beneficial manner. While some discussions seem to consider GFM IBRs a “most valuable player” (MVP), the entire burden cannot (and should not) fall on an MVP. Only under certain conditions, such as blackstart, is there a need for an MVP (Figure 1).
An example of reliability as a team sport is the frequency response obligation of each balancing area within an interconnection. Although each area may have an obligation of hundreds of MW, each generator that provides frequency response only needs to provide a few MW. As a team, frequency response is provided over the entire network, and no single resource is required to bear a large burden. However, this paradigm only works well when many resources participate.
Figure 1. Power System Reliability as a Team Sport
Getting the Most from the IBR Capabilities That We Have
Now, there are hundreds of GW of IBRs in the interconnection queue for which utilization/delivery of full capability either is not required or is optional (a market product). This can result in underutilization of IBRs’ potential contribution to reliability, and underutilization of this capability today can lead to an increased burden of capability provision on future IBRs. Thus, the burden of maintaining stability, security, and reliability of the power supply may fall only on a few resources, which can require these few sources to be of higher rating and exceptionally robust. An increased burden of response on such GFM resources could delay manufacturers’ ability to offer consistent and robust products in the marketplace.
Figure 2. Different Levels of IBR Capabilities
IBRs can potentially have a large spectrum of control objectives, and “grid following” (GFL) and “grid forming” terminology may be too general. The GFL/GFM distinction may fail to reflect the relevant variation in spectrum in IBRs’ capabilities, and cause the importance of existing IBRs’ capabilities to be underestimated.
For the purpose of this article, IBRs are categorized under four buckets:
- Legacy IBRs inject active power at unity power factor and provide no grid services. (This is what is usually meant by GFL IBR.)
- Conventional IBRs provide frequency and voltage response over multiple seconds.
- Enhanced IBRs may provide fast voltage/frequency response within 1s of an event in addition to providing a slower response similar to that of conventional IBRs.
- Future IBRs provide the above services, are capable of surviving/riding through severe load/generation mismatch, and are potentially able to provide blackstart services. (This is what is usually meant by GFM IBR.)
This categorization does not correspond to a particular IBR control structure, and IBRs using different technologies may be categorized in the same bucket if they provide similar services.
Why it’s Important to Recognize the Reliability Contributions of Existing IBRs
One might wonder why this is important. The reason is because GFL and GFM terminology could inadvertently create a perception of limitations of an IBR. When looking at a power network, if one expects a single resource to be able to “save” the network, all other resources that would not, on their own, be able to save the network can seem limited or ineffectual. However, it is possible that when all capability of today’s IBRs is utilized, as a group they can help each other and survive the loss of last synchronous machine in a large interconnected network.
An example of this behavior is shown in Figure 3, which shows results from a study that tested and compared the behavior of four devices following the trip of all synchronous machines in a network containing synchronous machines, different types of IBRs, and load.
Figure 3. Performance of various devices under test to a loss of all synchronous machines in a network
When the device under test is a conventional IBR (the orange line in each panel), the system is unable to survive the loss of synchronous machines, as both of the conventional IBRs in the system are too slow to provide a stable response. However, when the device under test is the enhanced IBR (the green line in each panel), while it cannot take the entire burden of maintaining system stability, it can help the remaining IBRs in the network ride through the initial disturbance and, subsequently, both IBRs jointly were able to bring the system back to stability (voltage and frequency return to within nominal levels in a reasonably damped manner).
Stability and Reliability will be Gained from the Collective Behavior of All Types of IBRs on a System
Now, this is not to imply that we don’t need new forms of IBR control in the network. Newer forms of control are most definitely required. However, as the power network transitions to one with high levels of IBRs, it is imperative that we recognize the vast capability that today’s IBRs can bring into the network and work towards effective utilization of the capability. As noted above, if the burden of maintaining the system’s stability, security, and reliability of power supply falls on only a few resources, and thus requires them to be exceptionally robust, this could delay manufacturers’ ability to offer the products the system truly needs.
Not every power network is going to operate at 100% IBR at all times of the day; therefore, understanding the capability that remains locked in today’s IBRs, and developing a framework that can help unlock such capability, can be crucial to enable the transition of the power system underway.
Senior, Technical Leader, EPRI