Complex sentence: The project was beset with technological challenges, most notable of these being the adoption of solid-state rectification devices in a large-scale commercial installation, whereas mercury arc valves had been the "de facto" standard for HVDC up to this time.
Split into simpler sentences: The project was beset with technological challenges, most notable of these being the adoption of solid-state rectification devices in a large-scale commercial installation. Mercury arc valves had been the "de facto" standard for HVDC up to this time.