Because of difference internal structure of test transformer, there are 3 kinds of them. Same function but the internal insulation material is different essentially. Therefore, each of the three test transformers has advantages and disadvantages.
Dry test transformer is manufactured with internal iron core and epoxy resin casting technology. Integrated molding, no gas, no insulating oil, the cost is low, the price is cheap, but the volume is big, the weight is heavy. It's almost unmaintainable, and users need to be extra careful when using it, so it's preferred for small projects. If it is damaged carelessly during use, it can be disassembled. Replace the copper core or the insulating oil.
Oil immersion test transformer, as the name implies, internal use of insulating oil for insulation and arc extinguishing. It has the advantages of low cost, fast boost, strong pressure resistance, convenient and cheap maintenance. Procurement and use costs are relatively low. However, due to the internal insulation oil, the equipment is heavy, which is not conducive to moving to the outdoor use, but also has oil pollution and other shortcomings.
The inflatable test transformer uses SF6 gas for insulation and arc extinguishing. Because it is aerated, it has the advantages of small size, light weight, clean and oil-free, but if there is gas escape inside, it is difficult to repair and can only be returned to the factory. It is troublesome to handle and use, and the cost of equipment is also relatively high, which leads to the natural rise of the price.
In general, oil-type, inflatable and dry test transformers have their own advantages and disadvantages, which can meet the different needs of different electrical workers.