Drytype transformers are classified by insulation levels, which are mainly divided according to the allowable maximum temperature rise of the insulation material (under the rated operating state), and the insulation level is named after the maximum allowable temperature of the insulation material (the temperature of the hottest point of the winding).

　　The common insulation levels of drytype transformers are as follows:

　　1. Class A insulation

　　The maximum allowable temperature of the insulation material is 105℃

　　The allowable temperature rise (based on the ambient temperature of 40℃) is 60K

　　Common insulation materials: cotton, silk, paper, etc., which need to be impregnated with insulating paint

　　It is rarely used in modern drytype transformers now

　　2. Class E insulation

　　The maximum allowable temperature of the insulation material is 120℃

　　The allowable temperature rise is 75K

　　This type of insulation is also less used in current drytype transformers

　　3. Class B insulation

　　The maximum allowable temperature of the insulation material is 130℃

　　The allowable temperature rise is 80K

　　Common insulation materials: mica, glass fiber, asbestos, etc., combined with organic adhesives

　　It was once a more commonly used insulation level in drytype transformers, and is now gradually replaced by higher insulation level types

　　4. Class F insulation

　　The maximum allowable temperature of the insulation material is 155℃

　　The allowable temperature rise is 100K

　　Common insulation materials: glass fiber, mica, etc., combined with hightemperature resistant organic adhesives

　　It is a widely used insulation level in drytype transformers at present, which can adapt to higher temperature working environments

　　5. Class H insulation

　　The maximum allowable temperature of the insulation material is 180℃

　　The allowable temperature rise is 125K

　　Common insulation materials: inorganic materials such as mica, glass fiber, and silicon rubber

　　It is suitable for drytype transformers in special hightemperature working environments, such as some industrial sites with high ambient temperature

　　6. Class C insulation

　　The maximum allowable temperature of the insulation material is above 180℃

　　Common insulation materials: polytetrafluoroethylene, quartz, etc.

　　It is only used in some special ultrahigh temperature working scenarios, and is rarely used in ordinary drytype transformers

　　The insulation level of the drytype transformer will directly affect its applicable environment, load capacity and service life. Generally, the higher the insulation level, the stronger the ability of the transformer to adapt to high temperature, and the longer the service life under the same working conditions.