  An inevitable consequence of the global power system transition towards
nearly 100% renewable-based generation is the loss of conventional bulk
generation by synchronous machines, their inertia, and accompanying frequency
and voltage control mechanisms. This gradual transformation of the power system
to a low-inertia system leads to critical challenges in maintaining system
stability. Novel control techniques for converters, so-called grid-forming
strategies, are expected to address these challenges and replicate
functionalities that so far have been provided by synchronous machines. We
present a low-inertia high-fidelity case study that includes synchronous
machines and models of grid-forming converters. We study interactions between
synchronous machines and converters and analyze the response of various
grid-forming control approaches to contingencies, i.e., large changes in load
and the loss of a synchronous machine. Our case study highlights the positive
impact of the grid-forming converters on frequency stability and analyze the
potential limitations of each control technique when interacting with
synchronous machines. Our studies also analyze how and when the interaction
between the fast grid-forming converter, the dc source and ac current
limitations, and the slow synchronous machine dynamics contributes to system
instability. Lastly, we introduce an effective solution to address the
instability issues due to the GFCs ac and dc current limitation.
