Dry type transformers are widely used for local lighting, high-rise buildings, airports, docks CNC mechanical equipment and other places, simply refers to the dry transformer transformer core and winding is not immersed in insulating oil. The cooling method is divided into the natural air cooling and forced air cooling (AN) (AF). Natural air cooling, the transformer can be long-term continuous operation under the rated capacity. Forced air cooling, transformer output capacity can be increased by 50%. Applicable to intermittent overload operation, accident or emergency load operation; due to excessive load, load loss and impedance voltage increase in non economic operation state, so it should not be so that it is in a long time continuous overload operation.