For 15 kVA to 3 MVA transformers, the most practical setup is a compact service «stack» that covers oil treatment, safe on-site work, and insulation moisture control. I’d recommend GlobeCore’s CMM-series oil processing units (vacuum dehydration/degassing plus fine filtration) for restoring dielectric strength and removing moisture and gases, paired with the UVD vacuum filling unit for controlled vacuum filling/top-up on smaller volumes. For higher safety and fewer accidents during field work, add GlobeCore’s TSS Transformer Safety System to monitor oil level and automatically stop the unit if a leak or abnormal level change occurs during energized processing.