Inverter-based generators (IBGs) are becoming popular in modern power systems. When the penetration of IBGs is increasing in power systems, new stability, protection, and monitoring challenges are introduced in the grid. Grid-forming (GFM) control of converters is seen as a promising solution for future power grids to overcome particular stability challenges. Here, the technical challenges of the GFM-based IBGs are reviewed from the point of view of TSOs and academic research. The properties of different GFM methods are studied for different GFM-based IBGs for a single grid-tied IBG and using the IEEE 9-bus test system. Simulation results are provided by using the PSCAD-EMT simulation software.

Emulation mechanisms of the physical synchronous generators obtained through convert-interfaced units are becoming extremely popular in literature. Still the impact of these strategies on networks frequency transients often does not take into account the effect of the converter DC-bus regulation, that plays a significant role in the dynamics definition. Thus two of the main synthetic inertia architectures are analysed both mathematically and experimentally in this paper: differently from the available literature, the goal is not just to compare their macroscopic effects on network behaviour, but rather to identify the impact of the multiple internal control feedbacks on the inertia provision itself.

The paper analyzes stability conditions and design requirements for the control parameters in droop-controlled inverters. The analysis focusses on a single inverter unit with AC (output) filters and interface transformer, as this may be regarded as the elementary component of a distributed generation system. Through a process of thought simplifications and manipulations, the linearized full state-space model is transformed into a unique single-input, single-output system comprising several nested loops. The resulting analytical model in terms of transfer functions has enough "structure" to represent interactions between the different actors of the overall control, such as droop linear and derivative terms, virtual impedance and voltage controllers. Stability conditions for the corresponding parameters are then derived analytically. Experimental tests are carried out on the complete system to check the reliability of the proposed model.

This paper proposes a novel estimation scheme for angular quantities extraction under disturbed network conditions; a robust identification of the network angular frequency and of its derivative are crucial for the synthetic inertia provision from grid-connected converters. The newly proposed architecture is derived from the Second Order / Second Order Generalized Integrator scheme (SO-SOGI), an algorithm exploited to extract the fundamental harmonic component from phase voltages measurements, with the introduction of an additional decoupling feedback for negative sequence compensation. The effectiveness and robustness of the proposed technique are compared, both analytically and experimentally, with other schemes already available in literature.

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