What are the technical challenges in achieving 100% renewable electricity grids?

100% renewable challenges: resource variability (multi-day weather events, seasonal patterns), loss of synchronous inertia (requires grid-forming inverters, synthetic inertia), voltage control without synchronous generators, system strength (fault level reduction affects protection, stable inverter operation), transmission adequacy (geographic diversity requires strong interconnections), long-duration storage (bridging weeks of low generation), maintaining reliability standards, and market/regulatory frameworks for flexibility. Solutions include: hybrid plants, diverse renewable mix, demand response, sector coupling (heating, transport), green hydrogen for long-duration storage, and advanced grid-forming inverter control. Multiple pathways exist - no single technology solution.