Methods of Transformer Drying Treatment

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Induction heating method

The induction heating method is to place the transformer body in the original tank, and the winding coil outside the tank is dried by electric current, using the eddy current heating of the tank skin. At this time, the temperature of box wall should not exceed 115℃～120℃, and the temperature of transformer body should not exceed 90℃～95℃. For the convenience of winding the coil, make the number of turns of the coil less or the current smaller as far as possible. Generally, the current is 150A, and the conductor can be used 35-50mm2. The oil tank wall can be padded with asbestos strips, and the wires are wound around the asbestos strips. The power required for induction heating is determined by the type of transformer and the drying conditions.

Hot air drying

The transformer is placed in the drying chamber and dried by hot air. The drying chamber can be overlapped with wall panels according to the size of transformer body. The wall panels are covered with asbestos board or other canvas or asbestos linen impregnated with fire-proof solution. The drying chamber should be as small as possible, and the distance between the panel and the transformer should not be greater than 200 mm. It can be heated by electric furnace and steam snake tube.

The electric power consumed in electric furnace is calculated by the following formula: hot air quantity Q in drying chamber and volume Q in drying chamber are selected for each minute, and Q = 15qm3 is generally used for calculation.

P≈0.07γQ(t2－t1)

P – Electric Furnace Power Required, kW

γ – Specific heat of air at constant pressure (both 0.31)

T2, T1 – Import Hot Air Temperature and Ambient Air Temperature,℃

When drying, the inlet hot air temperature should gradually rise, and the maximum temperature should not exceed 95 C. Filters or metal grids should be installed at the inlet of hot air to eliminate fire extinguishing stars and dust. Hot air should not be blown directly to the transformer body. It should be blown evenly from the bottom of the transformer body to all parts so that the moisture can be released through the ventilation holes in the box.

Vacuum drying method

Vacuum drying method is based on air as heat carrier. Under atmospheric pressure, transformer body or winding is gradually preheated to about 105 C before vacuum treatment begins. Because of the slow heat transfer and uneven internal and external heating (internal cooling and external heat), the transformer with high voltage and large capacity often needs more than 100 hours to preheat because of its thick insulating layer. The production cycle is very long and the drying is not thorough, so it is difficult to meet the transformer’s insulation requirements. But the equipment is simple and the operation is simple.

Gas-phase vacuum drying

The vapor phase vacuum drying method uses a special kerosene vapor as the heat carrier. The kerosene vapor introduced into the vacuum tank condenses on the transformer body and releases a large amount of heat energy, thus heating the dried body. Because of the high thermal energy of kerosene vapor (kerosene vaporization heat is 306 *103j/kg), the transformer body is dried and heated more thoroughly, more uniformly, with high efficiency and little damage to insulation materials. However, due to its complex structure and high cost, it is currently limited to the application of large transformer body drying treatment at 110 kV and above.

Transformer Drying Method-Requirements for Transformer Drying Treatment

(1) No matter which method is used to heat and dry the transformer, the temperature of the transformer body should not exceed 95 C in the absence of oil, and the oil temperature should not exceed 80 C in the drying with oil, so as to avoid oil aging. If drying with oil can not improve insulation resistance, all oil should be released, no oil drying.

(2) Insulation resistance and oil breakdown voltage should be measured every 4 hours by oil-dried method. When the oil breakdown voltage is stable and the insulation resistance is stable for 6 hours, the drying can be stopped.