Abstract
In the last decade, the concept of grid-forming (GFM) converters has been introduced for microgrids and islanded power systems. Recently, the concept has been proposed for use in wider interconnected transmission networks, and several control structures have thus been developed, giving rise to discussions about the expected behaviour of such converters. In this paper, an overview of control schemes for GFM converters is provided. By identifying the main subsystems in respect to their functionalities, a generalized control structure is derived and different solutions for each of the main subsystems composing the controller are analyzed and compared. Subsequently, several selected open issues and challenges regarding GFM converters, i. e. angle stability, fault ride-through (FRT) capabilities, and transition from islanded to grid connected mode are discussed. Perspectives on challenges and future trends are lastly shared.
Highlights
The concept of grid-forming (GFM) converters originally introduced for micro and islanded grid applications [1], [2], has been proposed as a viable solution for enhancing system stability and resiliency of wider interconnected power networks with high penetration of power electronics-based generation
The results presented in [51], demonstrate a better dynamic behaviour of the VIRTUAL OSCILLATOR CONTROL (VOC) compared to a droop control when the frequency regulation range is higher than a certain threshold, resulting in an opposite trend for a small frequency regulation range
The results reported in the paper, emphasize the advantages of the overdamped response of the dispatchable VOC (dVOC) which, in contrast to the droop control with inertial behaviour, is able to re-synchronize after fault clearance even if the clearing time is beyond the critical fault clearing time (CCT), leading to similar conclusions as in [62], for the case of a power synchronization control (PSC)-voltage source converters (VSCs)
Summary
The concept of grid-forming (GFM) converters originally introduced for micro and islanded grid applications [1], [2], has been proposed as a viable solution for enhancing system stability and resiliency of wider interconnected power networks with high penetration of power electronics-based generation. The wider use of GFM converters gives rise to the need for a classification of the control approaches used to implement these types of converters, which are conceptually different from state-of-the-art grid-following (GFL) units. First the conceptual differences between GFM converters and state-of-the-art GFL converters are discussed. An overview of the control structures proposed in the literature for implementing GFM converters is presented, and once the main characteristics of this type of converter are identified, a generalized structure is proposed by splitting the control into subsystems with respect to their functionalities.
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