Grid-forming (GFM) inverters are recognized as a viable solution to increase the penetration of renewable energy in bulk power systems. However, GFM inverters are physically different from synchronous generators (SGs) in terms of fault current level. To protect the GFM inverters and support the power grid under faults or severe disturbances, various current-limiting control methods are developed. In this paper, an overview of these methods is presented, which can be classified as direct, indirect, and hybrid ones. An overall control diagram of GFM inverters is developed to demonstrate the implementation of different current-limiting controls. The advantages and disadvantages of different methods are also illustrated. Finally, open issues of current-limiting control methods for GFM inverters, including control stability, fault recovery, and fault current injection, are summarized.

Impedance-based method has been increasingly adopted to assess the stability of inverter-based resources (IBRs). To get a better view of the state-of-art and challenges for implementing the impedance-based dynamic analysis, a survey with general/specific questions has been initiated by IEEE Task Force on Frequency-Domain Modeling and Dynamic Analysis of HVDC and FACTS. The feedbacks are collected from universities, national labs, transmission system operators (TSOs), power plant developers, as well as IBR vendors. It is interesting to note that while many common understandings have been established in practices, certain gaps still exist among different stakeholders. This article intends to bridge this gap by sharing a summary of the survey, including questionnaires, responses from different stakeholders, and the analysis of survey results. The challenges for different stakeholders using impedance-based method are identified, which hopefully shed a light on the future research work.