System strength and weak grids: fundamentals, challenges, and mitigation strategies

Converter-based renewable energy sources (RES) and battery energy storage (BES) devices that are asynchronously connected to the system are becoming more and more widespread. A number of relevant stability issues, usually in areas with little synchronous generation, are being experienced, particularly in terms of voltage stability. In this context, the scope of this work is to review the key aspects of the so-called 'system strength' that is associated with 'weak grids' where such instabilities arise, its definitions, assessment metrics, and adverse operational impacts that may emerge. The paper is then discussing two main mitigation strategies to improve short-term voltage stability in weak power systems. The first is to provide voltage stability services by converter-based technologies to support the system voltage in weak areas. The second proposition is to employ FACTS devices to increase system strength. Finally, limitations and challenges regarding the operation of converter-based technologies and FACTS devices in weak grids are presented and followed by key recommendations