Capability of IBR technologies to exhibit small-signal voltage source characteristics

Grid-forming (GFM) inverter-based resources (IBRs) are gaining attention as a viable mechanism for supporting significantly higher penetration of renewables as conventional synchronous generators are being retired or displaced in bulk power systems. To facilitate interconnection of these GFM resources, grid requirements and services are being drafted to establish baseline performance expectations for the technology. Certain desired GFM characteristics can be quantified by its small-signal response to changes in grid voltage. Various technologies have differing capability of meeting the desired frequency response characteristics. For example, some grid-following (GFL) resources with non-standard or stability-enhanced controls may be able to meet specific small-signal response aspects that are associated with GFM performance. Further, equipment hardware constraints may limit the response of GFM assets at certain frequencies but otherwise provide the desired response outside of this constrained region (for example, wind turbines with mechanical resonances). This paper focuses on the capabilities and limitations of GFL and GFM assets to achieve the expected frequency response. Guidelines for small-signal performance requirements and services are also provided that achieve the desired grid benefits and avoid excluding various technologies that can contribute to providing these grid stabilizing benefits.