A conceptual view of grid weakness in transmission and distribution systems

• Presents a systemic assessment of power grid and network weaknesses across various voltage levels. • Clarifies the interdependence of short-circuit ratio, stiffness ratio and impedance-based indicators in weak grids. • Summarizes different control and mitigation strategies for strengthening weak networks dominated by distributed energy resources. • Provide guidelines and a source of updated knowledge for power system planners and researchers. The increasing proliferation of renewable energy-based generation and inverter-based resources in the existing power system infrastructure has led to a decline in power system inertia, due to the reduction of conventional synchronous generators (SGs) connected to the grid. The SG reduction brings significant challenges concerning system stability, short-circuit current levels, and protection system coordination. This paper presents a unified approach to discuss these aspects, comprehensively exploring the technical and operational challenges involved in the integration of various resources in transmission and distribution systems. The aspects addressed include short circuit capacity, low inertia, voltage and frequency instability, and their systematic impacts on the power system. This contribution critically investigates network weaknesses in transmission and distribution systems, considering the different structural and operational characteristics of these systems, from the current literature and practical scenarios. Moreover, the paper discusses control and mitigation strategies aimed at better operation and efficient grid integration of renewable energy sources and other mixes of resources. The discussion on specific aspects emphasises the need to promote coordination between the transmission and distribution system operators, extended to the whole aspects of system control, modelling, and analysis, by considering both traditional and innovative approaches to address new challenges.