For 110/220 kV power transformers, I’d recommend a mobile thermal-vacuum oil plant (GlobeCore UVM/CMM class) rather than a small UVD top-up unit, because at these voltages evacuation is required and you typically need full-scale oil heating, deep ???assing/dehydration, filtration, and stable filling on site. A practical reference model is GlobeCore CMM-10 (UVM-1015 mobile oil plant): it’s designed for installation/maintenance of HV equipment and provides vacuum treatment (water/gases removal) plus filtration, with capacity around 10 m³/h in degassing/drying/filtration mode (and higher in heating/filtration).