  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. This
article presents a low-inertia case study that includes synchronous machines
and converters controlled under various grid-forming techniques. In this work
1) the positive impact of the grid-forming converters on the frequency
stability of synchronous machines is highlighted, 2) a qualitative analysis
which provides insights into the frequency stability of the system is
presented, 3) we explore the behavior of the grid-forming controls when
imposing the converter dc and ac current limitations, 4) the importance of the
dc dynamics in grid-forming control design as well as the critical need for an
effective ac current limitation scheme are reported, and lastly 5) we analyze
how and when the interaction between the fast grid-forming converter and the
slow synchronous machine dynamics can contribute to the system instability
