The evolution of load-dominated to active distribution networks with significant dispersed generation calls for accurate models of these networks in the context of system stability analysis. Embedding detailed distribution network models in a dynamic transmission system model is impracticable due to model complexity. Complexity-reduced equivalent dynamic models solve this obstacle. However, equivalent models that consider grid forming converters emulating inertia have not been addressed in research yet. To fill this gap, this work proposes a novel gray-box method for aggregating active distribution networks containing grid forming converters. The approach utilizes voltage sensitivities to represent the grid’s strength at the grid forming converter’s connection point. We compare the proposed method with an existing gray-box approach capable of creating equivalent models for networks dominated by conventional grid following converters. Both aggregation approaches are applied on an active distribution network model. Simulation results for different events of the detailed and both equivalent models are compared. The equivalent model aggregated with the proposed approach reproduces the detailed network’s dynamic behavior adequately, while the existing approach fails to meet validation criteria. Hence, the proposed method provides an equivalent model that is capable of substituting detailed distribution network models in stability studies of renewable power systems.

In this study, the integration of grid‐forming (GFM) converters in power systems is discussed in terms of both the fundamental aspects of system stability and the technical possibilities of converter‐based resources. The paper provides a survey and comparison of various GFM control concepts with respect to their transient and stationary behavior. A method for black box characterization and conformity testing of GFM converters is presented and relevant results of lab and field tests with GFM converters are discussed. Finally, important system aspects are addressed. This includes an approach to consider the behavior of GFM converters in distribution grid models and investigations on the minimum share of GFM units required to ensure system stability. The findings of this paper provide valuable insights for the design and implementation of GFM converters in modern converter‐based power systems.

A microgrid is an independent power system that can be connected to the grid or operated in an islanded mode. This single grid entity is widely used for furthering access to energy and ensuring reliable energy supply. However, if islanded, microgrids do not benefit from the high inertia of the main grid and can be subject to high variations in terms of voltage and frequency, which challenge their stability. In addition, operability and interoperability requirements, standards as well as directives have addressed main concerns regarding a microgrid’s reliability, use of distributed local resources and cybersecurity. Nevertheless, microgrid systems are quickly evolving through digitalization and have a large range of applications. Thus, a consensus over their testing must be further developed with the current technological development. Here, we describe existing technical requirements and assessment criteria for a microgrid’s main functionalities to foster harmonization of functionality-level testing and an international conception of system-level one. This framework is proposed as a reference document for assessment frame development serving both microgrid research and implementation for a comprehensive understanding of technical microgrid performance and its current assessment challenges, such as lack of standardization and evolving technology.

Detailed active distribution network modeling in the context of stability studies of future power systems can be avoided by applying complexity‐reduced equivalent dynamic models. Previous work developed an approach to derive such equivalent dynamic network models of systems dominated by grid following and grid forming converters. Nevertheless, this approach lacks an investigation of the aggregation of multiple grid forming converters to one equivalent component. Also, the approach needs to be evaluated for a closed ring topology to substantiate its validity independent of the detailed network topology. This work applies and validates the developed approach on active distribution networks in the context of three scenarios. The detailed and equivalent network models are simulated for three events and the results are used to compare the models with respect to accuracy and complexity. The derived equivalent dynamic network models with an individual representation of grid forming converters reproduce the dynamic behavior of the corresponding detailed networks very well. The aggregation of grid forming converters results in an adequate reproduction of the detailed network's dynamic behavior under the constraint of aggregating only neighboring grid forming converters of one branch. This allows for the consideration of the detailed network's topology in the corresponding equivalent dynamic model.