Insulated cylinder type: can be used in a single unit or cascaded, modular structure, easy to stack up. The insulated cylinder is made of epoxy reinforced glass fibers and filled with oil. Such a structure is suitable for indoor use, usually for short time operation (in accordance with IEC standard) at rated current. After running for the specified maximum time at certain current, the test transfromer needs to be cooled to ambient temperature, which means the usual pattern is intermittent operation.