Abstract
In the last decade, different control concepts for the synchronisation of voltage-controlled power converters have been proposed in order to form converter-based power systems. The interoperability of these grid-forming power controls is often analysed based on reduced-order models covering only the slow controls or modes. In this article, the coupling of the outer, power-related and inner, inverter output-related control of multiple grid-forming power converter systems is analysed, based on a minimal working example. The elementary study cases each consist of a different grid-forming converter coupled with an external (and passive) grid. Here, the investigated stability problems are already manifested in the simplest possible setup. The analysis of these coupling effects is performed by modelling the system in impedance-based, state-space and phase portrait-based frameworks. In particular, small coupling impedances, like short transmission lines or small short circuit impedances, can be challenging for the controller stability of grid-forming converters while the inner controls can even enhance this issue. The impact of this phenomenon and the participating subsystems are identified in this work. Thus, recommendations concerning modelling techniques and their legitimate assumptions are given. Laboratory experiments validate the performed analysis by indicating a close correlation between analytical models and experimental results.
Highlights
Power electronic-based (PE) converters play a key role in future electrical grids
The main concepts can be divided in grid-feeding, grid-supporting and grid-forming controls. We define these concepts as follows: gridfeeding converters are internally operated as a current source that synchronises to an existing grid voltage; grid-supporting controls include additional functionalities in order to assist in frequency and/or voltage control; grid-forming converters are operated as a voltage source that is often accompanied
1) STATIC PHASOR-BASED MODELLING Some basic properties of the synchronisation-related transients are assumed in the conventional phase-portrait analysis, namely: (a) all relevant transients can be represented by phasors of the fundamental frequency; the electrical power system can be represented by static impedances and admittances; (c) the inner converter controls can be neglected due to their fast nature; (d) only the power-related control states are taken into account
Summary
Power electronic-based (PE) converters play a key role in future electrical grids. The increasing penetration of renewable distributed energy resources (DER), high-voltage direct current (HVDC) transmission systems and battery energy storage systems (BESS) are, among others, popular examples of the concentration of converters that occurs in the power system sector. A more precise model that includes the grid states and some low-frequency characteristics of the inner controls is derived by a systematic model-order reduction method Since this contribution focuses on legitimate modelorder reductions, a comprehensive and general analysis of mutual power-related couplings in grid-forming converters is missing. As a contribution of this article, the authors’ aim is to extensively elaborate the constraints of separating the powerrelated control loop from the fast inner control for two different grid-forming converters that can consist of three different inner control variations This analysis will contribute to the identification of the relevant electrical and control states to cover the main phenomena in converterdominated power-related stability studies that predominantly are based on grid-forming concepts, i.e. microgrids.
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