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16. How to extend work time of transformer oil?

When operating the equipment, it is recommended to counteract the ageing and contamination of electroinsulation oil with one of the following methods:

• by limiting or eliminating the contact of oil with atmosphere, by using dehumidifiers, inert gasses, flexible membranes, air-tight sealing,
• periodical oil cleaning with suitable devices,
• application of equipment for continuous removal of ageing products and contaminants.

Air dehumidifiers used mainly on transformers should be operated in consistency with the Transformer Operating Instructions. The absorbent applied in the dehumidifier changes its colour depending on the degree of its humidity. When colour is changed to pink, the dehumidifier should be replaced. Guidelines for selecting the amount of silicagel to the amount of oil protected are chosen acc. to the equipment manufacturer's requirements, and if not recommended otherwise, according to the table below:

Amount of silicagel, kg	0,5	1	3	6	15
Amount of protected oil, kg	1 000	3 000	10 000	20 000	50 000

Protection with inert gasses is recommended for transformers below 110 kV, whereas it is prohibited to use it for transformers with voltage over 220 kV. Gas should be dried before use. The pressure of gas from the bottle must not be higher than 0,02 MPa. Gas pressure over oil should be fixed, close to atmospheric pressure. Inert gas concentration over oil should be periodically inspected. When concentration falls below 90 %, identify leaking places and seal them, and purge the space over oil with inert gas.