Successful basketball teams include players with a variety of talents—three-point shooting, rebounding, defending, passing. However, assembling a team where every player excels in all these areas is impractical (and likely prohibitively expensive and difficult to coach). Instead, prudent coaches identify specific skills gaps and recruit players to fill the gaps.
In the energy world, a stable and reliable grid is like a winning basketball team with players filling in all relevant gaps. Grid-forming (GFM) inverter-based resource (IBR) technology is like a very solid player … sort of a star NBA player.
While certain grids and projects require GFM technology to operate stably, many others can operate stably with non-GFM technology. Identifying where GFM is essential (and not) is important to control costs, as GFM technologies other than batteries may require either additional non-standard hardware or a reduction in energy production to fully realize GFM performance. Further, state-of-the art grid-following (GFL) technology can perform as well as GFM in some applications mistakenly considered beyond GFL scope. Therefore, it’s important to avoid excluding these GFL “players” that can contribute to stability to keep costs low.
A few existing or emerging applications of IBR technologies are described below, together with an assessment of when our star GFM players are needed and when we can rely on our core GFL lineup. Short-circuit ratios (SCR) are provided with the application descriptions as a rough reference and can be considered at the medium-voltage connection point of the plant.
Application Space of Grid Forming
Meshed Region of the Bulk Power System (SCR > 2)
In strong grid connections like this, voltage and frequency tend to be stable independent of the IBR technology in a given plant. No clear advantage of GFM exists in this scenario unless the area is expected to become significantly weaker over time. This risk can likely be mitigated by deploying state-of-the art GFL technology with enhanced stability features, such as IBR-unit-level voltage control (e.g., voltage control at the low-voltage terminals of the IBR, in addition to conventional point-of-interconnection voltage control, to keep voltage stable during fast transients).
Remote Pocket of the Bulk Power System with Power Transfer Constraints (1 < SCR < 2)
In these types of weak grid applications, the transfer of active power from the remote pocket of the grid may approach the physical steady-state stability limits of the network. Operating near the steady-state stability limit also creates transient voltage stability issues. For example, one key challenge is maintaining voltage stability when transferring active power near the transfer limits, particularly after ramping power back up following a grid fault. In these cases, there’s no clear advantage of GFM over state-of-the art GFL technology, as maintaining stability involves carefully ramping active power while maintaining voltage magnitude, which can be achieved with either GFM or stability-enhanced GFL.
The advantages of GFM in weak systems, as discussed in industry forums, are primarily attributed to the fact that GFM commonly features IBR-unit-level voltage control (which can also be done in GFL, but is less common). Therefore, the benefits are not necessarily tied to GFM itself but the introduction of IBRs that have unit-level voltage control.
Similarly, GFM is commonly reported to reduce the risk of subsynchronous oscillations (e.g., due to series-compensated transmission). However, the stability risks are (at least in part) less associated with the GFM or GFL classification and more related to the specifics of how the control algorithms are designed and tuned. Therefore, risks with subsynchronous oscillations may be mitigated via proper design/tuning of GFL controls without necessarily requiring GFM.
Hybrid Applications with SCR << 1
In some hybrid applications, two or more IBR types (e.g., wind, solar, battery energy storage system) may be connected together within the medium-voltage circuit of the plant. The combined nameplate ratings of the IBR may be significantly higher than both the plant nameplate rating and the medium-voltage to high-voltage transformer rating. Design of plants this way may be desirable to improve the capacity factor of the overall plant and increase energy availability at times of high system load. For these applications, the SCR may be well below 1 from the perspective of the IBR assets within the plant. These plants are operated to inject power into the grid well below the combined rating of the IBRs, reducing the risk of approaching power transfer limits that would be expected in more traditional IBR plants with similar SCR. However, stability challenges may arise with the extreme weak connection of IBRs due to high coupling between active power and the voltage angle of the medium-voltage circuit. In these applications, having some portion of the IBR with GFM technology may be beneficial to stabilize this angle. However, stability can likely be achieved without requiring all the IBRs within the plant to be GFM.
Off-Grid Applications
In a permanent off-grid application with only IBR generation, a fraction of GFM-controlled IBRs is required to maintain voltage angle/frequency stability for the loads and other GFL IBRs. In this case the GFM IBRs maintain the generation/load balance in the system together with providing voltage support (potentially together with other voltage-supporting GFL technologies).
Another example of current interest involves large, highly variable data center loads operating off-grid, where thermal or gas synchronous generation might be used to supply these loads. In these cases, the synchronous generation acts as the primary GFM resource. Grid strength may be relatively high within applications like this, though GFM IBR technology may still be relevant—not as a means for establishing voltage/frequency for the system, but instead as a mechanism for “softening” the large load transients on the synchronous generation induced by the load. GFL IBR technology may also be applied in these systems as a supplemental energy source.
Other applications may involve temporarily operating off-grid. For example, portions of a larger grid may be temporarily islanded for certain events to maintain security. GFM IBRs within the islanded portion of the system are likely not needed while operating grid connected. However, some fraction of GFM IBRs is needed upon onset of the islanding scenario to maintain voltage/frequency within the island (similar to a permanent off-grid application).
Blackstart
Blackstart capability with IBRs requires GFM capability since the IBR is required to establish voltage and frequency for the unenergized system. In an IBR plant, it may be possible to have blackstart capability with relatively few GFM IBRs while other IBRs are non-GFM so long as the voltage and frequency may be maintained within reasonable bounds while energizing equipment and loads. Much like conventional thermal or gas plants, not all GFM IBRs have blackstart capability as this requires specific design considerations.
Conclusions
While a team full of stars may be desirable, the descriptions above indicate that we can have a winning team in many important applications by utilizing both existing technology as well as new/emerging GFM technology. Disaggregating applications where each technology is needed for stability purposes helps keep costs low while still maintaining grid reliability. Just as a successful basketball team wins by leveraging diverse talents, the grid can sustain the level of stability and reliability we’ve become accustomed to by strategically utilizing the right mix of technologies.
Dustin Howard
Technical Director – Energy Consulting
GE Vernova Consulting Services
Sebastian Achilles
GM, Grid Integration and Stability
GE Vernova Consulting Services
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