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.