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 gridforming converter and the slow synchronous machine dynamics can contribute to the system instability.

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As renewable sources increasingly replace existing conventional generation, the dynamics of the grid drastically changes, posing new challenges for transmission system operations, but also arising new opportunities as converter-based generation is highly controllable in faster timescales. This paper investigates grid stability under the massive integration of grid-forming converters. We utilize detailed converter and synchronous machine models and describe frequency behavior under different penetration levels. First, we show that the transition from 0% to 100% can be achieved from a frequency stability point of view. This is achieved by retuning power system stabilizers at high penetration values. Second, we explore the evolution of the nadir and RoCoF for each generator as a function of the amount of inverter-based generation in the grid. This work sheds some light on two major challenges in low and noinertia systems: defining novel performance metrics that better characterize grid behaviour, and adapting present paradigms in PSS design.

This review covers the current state of "smart" grid research and demonstration projects. At present, smart elements are making their way into traditional electricity grid systems at every level, from transmission down to distribution. The vast size of the power grid makes the extension of digitally enabled electric infrastructure a question of cost. Drivers for this development are the growing security requirements and sustainability of supply in the face of rising demand and aging infrastructure. Information technology (IT) is one of the key elements of smart grids because it enables cooperation of distributed energy resources, local control, and globalized energy markets. Smart grids are expected to make our power system more resilient, "green," and efficient; a challenge that the automotive industry could only manage by introducing digital controls in engines. We now witness the same development in electric energy systems. This article provides an introduction to the topic, a snapshot of current activities, and a general outlook on what still is needed.