AC grid-forming (GFM) resources rely on power synchronization, i.e., transient transfer of real power in response to changes in frequency, to reach frequency equilibrium with other GFM resources in the associated grid. While operating at the limit of hardware constraints, such as converter current or power limits, and DC source constraints such as real power limit, inverter based resources (IBRs) are forced to suspend power-synchronization as no transient power exchange is feasible beyond the capacity limit. Despite the large class of research efforts for augmenting synchronization stability under currentlimiting operation, an overall ride-through strategy is missing under a comprehensive set of hardware and source constraints. In this work, we contemplate a broader fundamental question: What constitutes GFM response when an IBR reaches one or more such capacity limits? We propose a general framework for defining and optimizing GFM response under capacity constraints. The proposed control framework leverages constrained optimization to minimize the deviation in IBR response from that of an ideal GFM source. The resulting estimated deviation is utilized to dynamically adapt a droop model which enables continued power-synchronization while operating at capacity limit. An exemplary architecture of the proposed framework is presented with experimental validation utilizing a laboratory inverter prototype.

Grid-forming (GFM) control, considered a key enabler for heavy penetration of inverter based resources (IBRs) into the power grid, is subject to stricter current constraints compared to that experienced by legacy generation resources such as synchronous machines. Transient stability analysis plays a major role in the development and evaluation of ride-through strategies for such constrained GFM IBRs. Majority of literature leverage single-converter-infinite-bus (SCIB) configurations for transient stability assessment of GFM IBRs, which invariably leads to mutually exclusive study of contingency events such as voltage, phase, or frequency transients. However, loss of physical rotational inertia in IBR dominated grids, otherwise present in centralized generation architectures consisting of synchronous generators, attributes faster frequency changes as well as simultaneous voltage and frequency dynamics in response to contingencies. In this work, we revisit the adequacy of SCIB configurations for transient stability assessment of current-constrained GFM IBRs to be deployed in low-inertia networks. Furthermore, a two stage transient stability enhancement scheme is introduced to improve GFM control response under current constraints. Finally, effectiveness of the proposed ride-through scheme is verified through EMT simulations of full IBR penetration scenario in a modified IEEE 9-bus system.

Fast and accurate detection of symmetrical components is critical for ride-through of asymmetrical faults in grid-forming (GFM) inverter based resources (IBRs). Sequence-decomposed GFM control enables to emulate the behavior of a synchronous machine by an IBR in both positive- and negative-sequences, where current references are generated separately in each sequence from the extracted symmetrical components of the terminal voltage. Cross-coupled dynamics between the stationary frame components attributed by the symmetrical component extraction (SCE) complicates the analysis and design process. In this work, a small-signal model is developed for the analysis and design of such sequence-decomposed GFM control.  It is demonstrated that by virtue of its overall structure, sequence-decomposed GFM control enables simplified analysis eliminating the cross-coupled dynamics characteristic to SCE. Subsequently, comparative analysis is presented between delay based and filter based SCE methods focusing on their impact on small-signal stability. Design guidelines are provided along with supporting experimental evidence using a laboratory inverter prototype.