Introduction
Power systems around the world are rapidly transitioning to much higher shares of inverter-based resources (IBRs) with few synchronous generators remaining online. IBRs and synchronous generators have fundamentally different dynamic performance characteristics resulting in a difference in the overall power system dynamic performance. These differences will become more prevalent as IBR uptake increases in the power system. Reductions in system strength, inertia, damping of small-signal oscillations, fault levels, and other synchronous characteristics are the results of the transition from power systems with the dominance of synchronous generators to those with very few synchronous generators online.
IBRs are generally more flexible and controllable than synchronous generators, however at the same time exhibit significantly more complex control systems. Furthermore, new and emerging capabilities are being developed progressively and in particular the so-called grid-forming inverters. Grid-forming inverters (GFM) offer several new capabilities not previously possible with conventional grid-following inverters (GFL). However, they are not well understood currently when applied in a mega scale and moving forward when they will likely take over the role synchronous generators have been performing for several decades as the workhorse of system security support. It is also noted that similar to the GFL, GFM are current limited devices, and cannot be practically expected to address several grid needs at the same time as the solution for each problem will likely involve the provision of a portion of the total current available. Furthermore, like any other generation technologies including synchronous machines or conventional IBR, GFM have their own susceptibility and instability mechanisms. These limitations shall be accounted for along with their capabilities when designing the future power system and defining the technical requirements these inverters are sought to deliver.
Key questions currently in the technical community include the extent to which GFM shall be similar or different to each of the synchronous machines and conventional GFL, and how various control strategies, whether commercially available or under research and development, can assist in maximising the grid support capabilities sought and minimise or ideally eliminate any adverse impacts. The objective of this special issue is to provide insights into some of these unknowns.