Transformers are important equipment in power systems, but their insulation is relatively weak. Oil-impregnated paper insulation is the most widely used insulating material in power transformers. The insulating oil plays a dual role of heat dissipation and insulation. In the transformer accident, insulation accidents account for a large proportion.
Insulation of air (including creepage) outside the transformer tank is often referred to as external insulation. It is directly affected by external conditions (pressure, humidity, dirt, etc.); insulation in the tank, including insulating oil, and paper soaked in oil And cardboard, etc. are called inner insulation. The internal insulation is divided into main insulation and vertical insulation. The main insulation refers to the insulation between the live conductors and the grounded components (cores, fuel tanks), including the insulation of high and low voltage windings and their insulation to the ground. Insulation of other windings, insulation of tap changer to ground and other windings. Longitudinal insulation refers to the insulation between the parts of the same winding, including insulation, insulation between layers, insulation between different layers.
After the transformer is put into operation, it must withstand the effects of working voltage, heat, and chemical factors for a long period of time, and may withstand the impact of lightning overvoltage and internal overvoltage in a short time. In addition, insulation is also affected by mechanical forces and environmental factors. Therefore, it must meet certain requirements in terms of electrical properties, heat resistance, mechanical strength, environmental resistance, and aging resistance.
(1) Electrical properties. In order to enable the transformer to operate at rated voltage in K phase and withstand various overvoltages that may occur, it is necessary to carry out tests such as 1rain power frequency voltage withstand test, lightning impulse full wave test and wave chopper test.
(2) Mechanical properties. When there is current flowing through the transformer windings, the interaction of this current with the leakage flux generates electromagnetic force. Especially when a short circuit occurs, the short-circuit current by r may reach 20 to 30 times of the rated current, and the short-circuit electromagnetic force on the winding may reach several hundred to nearly one thousand times normal. If the windings are not stable or the insulation material is aged and crisp, there is
It can lead to deformation, looseness, etc. of the winding and cause accidents.
(3) Thermal properties. During the operation of the transformer, both the winding and the core have energy loss and heat. The long-term allowable temperature of oil-impregnated paper, paperboard, linoleum, and phenolic tubes used in power transformers is 105°C. The increase in temperature will cause the cable to heat up rapidly. In order to ensure the necessary life, it is necessary to improve the cooling conditions, such as the use of forced oil circulating cooling for larger transformers.
(4) Other properties. Internal insulation can be affected by environmental factors during operation. For example, transformer oil is exposed to moisture, and contact with copper and aluminum conductors and iron cores accelerates the rate of aging. Therefore, it is required that the insulation has certain anti-aging ability and good chemical stability, and at the same time prevent moisture or other pollution of the insulation during design, manufacture and operation.