Patent ID: 7833312

Claim:
In a method of drying an air dehumidifier filled with moisture-absorbing material for oil-insulated transformers, chokes, or tap changers having an expansion tank wherein the air dehumidifier communicates with the expansion tank via an intake, the moisture-absorbing material can be heated by an electric heater and can thereby be dried and can regenerated, and actuation of the heater is only possible when no air flows into the interior of the expansion tank, the improvement comprising the steps of: dividing a primary cycle into different time slots, determining at the beginning of a first time and at a second time of each time slot in each case the temperature (θ 2 −θ 1 ) at the intake and calculating a temperature gradient (dθ/dt) therefrom according to the relation θ 2 −θ 1 =dθ/dt, subsequently repeating these temperature measurements and calculations of temperature gradients for identical time slots in x−1 successive primary cycles, storing for each of the primary cycles those time slots in which the temperature gradient is greater than or equal to 0, thereafter determining during another cycle x whether during each of the previous cycles there are at least z consecutive identical stored time slots during which the temperature gradient is greater than or equal to 0, and if this is the case, during cycle x at the beginning of the corresponding time slots of the previous cycles starting the electric heater, provided that at this time the temperature gradient is again greater than or equal to 0, or, if this is not the case, including another cycle x+1 in the comparison.