Insulation level refers to the motor (or transformer) winding insulation material used in the heat resistance level. Motor and transformer insulation materials commonly used in the grade for A, E, B, F, H five. Each insulation level of insulation materials have corresponding limit allowable working temperature (motor or transformer winding the hottest point of temperature). Motor or transformer operation, the temperature of the hottest point of the winding shall not exceed the corresponding provisions, otherwise it will cause accelerated aging of the insulating material, shorten the life of the motor or transformer; if the temperature exceeds the permissible value of a lot, the insulation will be damaged, resulting in the motor or transformer burned out.

The insulation level of a transformer is not a concept of insulation strength.

The insulation level of a transformer, also referred to as the insulation strength, is the level that is compatible with the level of protection as well as with the other insulating parts, i.e. the withstand voltage value, which is determined by the highest voltage of the equipment. The highest voltage of the equipment Um for the transformer is the highest phase-to-phase voltage effective value of the windings, from the insulation aspect, Um is the highest voltage effective value of that system to which the windings can be connected, therefore, Um is the standard value which can be greater than or equal to the rated voltage of the windings.

All outgoing ends of the winding have the same frequency withstand voltage to ground winding insulation is called full insulation; winding the grounded end or neutral point of the insulation level is lower than the end of the winding insulation is called graded insulation.

Rated withstand voltage of the winding with the following letter code sign.

LI a lightning impulse withstand voltage SI a operation impulse withstand voltage AC a frequency withstand voltage.

Transformer insulation level is according to high-voltage, medium-voltage, low-voltage windings in the order of listed withstand voltage value to indicate (shock level in the front), separated by a slash. Neutral insulation level of graded insulation plus a horizontal line is listed in the end of the line after the insulation level.

For example: L1850 AC360 - -L1400 AC200/L1480 AC200 - LI250 AC95/LI75 AC35.

Meaning: 220KV three-side graded insulation of the main transformer, the first for the high-voltage side of the lead end, neutral point, medium-voltage side lead end, neutral point, low-voltage side.