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Vacuum Pressure Impregnation Process For Dry Type Transformer

1. Impregnated resin

Vacuum pressure impregnation technology is the best insulation treatment technology in the world. Generally, the modified heat-resistant unsaturated polyester resin used in impregnating resin belongs to thermosetting resin.For grade F impregnated resins, there are two main types of epoxide anhydride styrene and epoxide anhydride.Epoxy anhydride styrene resin has higher shrinkage, lower vitrification temperature and better toughness.Epoxy anhydride resin has low shrinkage, high glass transition temperature and strong rigidity.For grade C impregnation resin, there are mainly organic silicone resin, polyester resin including heat resistant unsaturated polyester resin, epoxy modified unsaturated polyester resin (without acid anhydride) imide modified unsaturated polyester resin.

This paper mainly introduces the technology of heat-resistant unsaturated polyester resin used in transformer industry.Commonly used impregnation resin: (1) Modified heat – resistant unsaturated polyester resin.It is mainly composed of modified heat-resistant unsaturated polyester, flame retardant resin, initiator and active diluent. (2) Unsaturated polyester imine resin.The reaction principle of unsaturated polyester resin is the esterification of diacids and diols, and then the reaction results in the cross-linking of unsaturated double bonds in the polyester chain with the Alkyl monomer.

2. Vacuum pressure impregnation (VPI) process
The dry transformer coils were processed by the Hangzhou Ruiqin Mechanical Equipment Co.,Ltd RQ-VPI-2220 vacuum pressure macerating equipment. First, the coils that needed to be impregnated were pre-dried, dehumidified and dehumidified. The pre-baked coils were set to constant temperature and time and naturally cooled to 30-40℃. Test the upper and lower 8 points of the coil with an infrared thermometer and remove dust and other metal particles and sundries from the coil with an air gun.The purpose of pre-drying is to remove the moisture in the winding and improve the temperature of the work piece when impregnation, improve the quality of the paint and the penetration of the paint.Note that the pre-drying heating should gradually increase the temperature, and the heating rate should not be greater than 20~30 °C/h. The temperature of pre-drying depends on the insulation level. Grade E 120~125℃, Grade B 125~ 130℃.After pre-drying, we need to wait for the winding temperature to be cooled to 60~ 80℃ before we start to soak.Because, if the paint temperature is too high, the solvent in the paint quickly volatilize, premature formation of paint film, it is not easy to make the paint impregnated to the coil;If the temperature is too low, it will lose the effect of pre-drying, so that the viscosity of the paint increased, fluidity and permeability become poor.

In the process of vacuum pressure immersion paint, the key process control indexes are vacuum degree and pressure.The purpose of vacuum extraction is to achieve a certain vacuum degree in the impregnated tank, so as to remove moisture, small molecule volatiles and air as far as possible from the coil insulation layer and prepare for the filling of the insulating paint.The purpose of pressurization is to press the insulating paint into the insulating layer in order to improve the permeability of the insulating paint and to fill all the gaps in the insulating layer.In the past, people often mistake vacuum degree is higher, pressure is bigger, the effect after macerate is better, but practice proves not to be such.In the past, vacuum degree control in 200 ~ 300 Pa, pressure control in 0.3~ 0.5 MPa is more appropriate, but the number of specific value depends on the actual process of impregnating paint and equipment to determine the value of vacuum degree and pressure.

In addition, the number of dip lacquer also can be increased appropriately, the number of dip lacquer increase, the electrical performance of the impregnated workpiece will be significantly improved.When the vacuum pressure impregnation is complete and the coil workpiece is out of the tank, it must be cured.Without curing process, polyester resin can not change from the viscous state to the glass state, visible curing process of importance.The reason for the great color difference of some products after impregnation is that the distribution thickness of polyester resin is not uniform, some areas have changed the color of complete curing, and some areas have not changed the color of complete curing.For example, the initiators in polyester resins change from reddish brown to orange yellow after curing.

3, Unsaturated polyester resin paint process index analysis
3.1 Viscosity (No. 4 viscometer)
Viscosity is a reliable method to determine the relative molecular weight of the mixture. Currently, the commonly used test method is to use viscometer, the specific requirement is 35 ~50s at 23℃.

3.2 Aging test
Samples should be taken once a week for aging tests.

3.3 Curing time
According to the difference of diluent, macerate lacquer can cent has solvent lacquer and have no solvent lacquer.The curing time of the two is different.Solvent paint is commonly added toluene and other chemical reactions can not participate in the emotional diluent.In the curing process, the heating rate should not be too fast, and – must start from room temperature;Solventless paint is generally added to styrene active dilute: release agent, curing process, the faster the faster the better, in order to reduce the loss of insulation paint, fully filled insulation layer.

3.4 Impregnation resin thickness
General dip film thickness of 60-80 PUm.Multiple dip curing is beneficial to increase the film thickness, thus improving the insulation breakdown field strength.

Characteristics of vacuum pressure impregnation (VPI) process
As is known to all, VPI vacuum pressure impregnation process is characterized by strong overload capacity, good moisture resistance, thermal shock resistance, impregnation coil repair is relatively easy to impregnate the quality of paint and impregnation process for the non-encapsulated dry transformer performance played a decisive role.

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