Depending on the voltage class, the preventive maintenance of a power transformer may be performed from once a year to once every four years. This process is most often performed by a three man team, with the transformer taken offline.

The maintenance includes a clear sequence of certain operations. First, general transformer condition is checked. This is done by inspecting all accessible parts, locations and components, as well as checking protective grounding. If the inspection reveals that one of the radiators heats more than the others, the cause must be determined and removed. Before taking the unit offline, contact connections must be inspected. Contacts prone to overheating must be disassembled and cleaned.

Then welding seams are checked and grounding bolts tightened.

If the tank, lid, radiator or expansion tank are dirty, the dirt must be removed. Drain valve and lower plugs of the radiator must be checked. If necessary, seals are replaced, lid and radiator bolts tightened.

The process of inspection and removal of dirt from under the expansion tank includes cleaning of the copper mesh. Oil level meter must be cleaned, dirty oil and water sediment must be drained through a special valve, providing connection to the expansion tank.

The need for silica gel change is indicated by chemical analysis. To redice transformer oil acidity, automatic transformer oil recycling process should be performed in the thermosyphon filter. To limit the content of moisture entering the oil with the air, special air filters must be installed on the filter.

In the course of operation, the condition of the silica gel is determined by color observation. An indication of the need for more attention is when some crystals turn blue. If blue color begins to prevail, the desiccant must be changed.

An important criterion is the oil level. Lowering of the oil level in transformers with film protection may cause vacuum, the destruction of the exhaust diaphragm and gas protection tripping. If an oil leak is detected the cause must be removed and oil must be added to the transformer.

The UVD type unit by GlobeCore allows to add degassed oil to high voltage power transformers though the air tight bushings during corrective and preventive maintenance, repairs and operation of transformers.

One of the advantages of the unit is the ability to select the required mode of operation. Depending on the current needs, the UVD unit can perform one of the following:

• preparation (degassing) of oil;
• oil storage;
• transportation of oil;
• adding oil.

The unit is quite mobile, making it easy to service several transformers on a location.