Insulation and characteristics of dry type resin transformers

Dry-type transformers (here referred to as epoxy insulated transformers) are mainly used in places with high fire ...

Dry-type transformers (here referred to as epoxy insulated transformers) are mainly used in places with high fire protection requirements. Such as high-rise buildings, airports, oil depots and so on.

1. The type of resin insulation

Epoxy resin insulated transformers can be divided into three types according to the characteristics of the manufacturing process: vacuum casting type of epoxy quartz sand mixture, epoxy alkali-free glass fiber reinforced vacuum pressure differential casting type and non-alkali glass fiber wrapped impregnation type.

(1) Vacuum casting insulation of epoxy quartz sand mixture

This kind of transformer is filled with quartz sand as epoxy resin. The coil is wrapped after impregnation with insulating paint and put into the casting mold of the coil. The mixture of epoxy resin and quartz sand is then used for drip pouring under vacuum conditions. Because the casting process is difficult to meet the quality requirements, such as residual air bubbles, local non-uniformity of the mixture and may lead to local thermal stress cracking, such insulated transformers should not be used in hot and humid environments and areas with large load changes.

(2) Epoxy alkali-free glass fiber reinforced vacuum differential pressure casting insulation

Epoxy alkali-free glass fiber is reinforced by wrapping the outer insulation of the insulation between winding layers with short alkali-free glass mat. The outermost layer of insulation is generally thin insulation with wrapping thickness of 1 ~ 3mm. It is mixed with epoxy resin castable ratio, and the bubble casting is removed under high vacuum. Due to the thin insulation thickness around the wrap, it is easy to form partial discharge points when impregnation is not good, so it is required that the mixing of castables should be complete, the vacuum debubble should be thoroughly, and the low viscosity and pouring speed of castables should be well controlled to ensure the high quality of impregnation of wire package in the pouring process.

(3) Alkali free glass fiber wrapped impregnated insulation

Alkali-free glass fiber wrapped impregnation transformer completes the insulation treatment between the wire and coil impregnation while winding the transformer coil. It does not need the winding mold in the process of impregnation in the above two ways. But the resin viscosity is small, the resin should not remain small bubbles in the process of coil winding and impregnation.

2. Insulation characteristics and maintenance of dry-type resin transformer

The insulation level of the dry-type resin transformer is not significantly different from that of the oil-immersed transformer. The key lies in the temperature rise and partial discharge of the dry resin transformer.

(1) The average temperature rise of resin transformer is higher than oil-immersed transformer. Therefore, the corresponding requirements for insulating material heat-resisting level is higher, but the average temperature rise of the transformer does not reflect the temperature of the hottest part of the winding. When the insulation heat resistance grade is only selected according to the average temperature rise, or improper selection, or long-term overload operation of cast resin dry type transformer will affect the service life of the transformer.

The rise of temperature of the transformer measurement often can't reflect the temperature of the hottest part of the winding. Therefore, it is best to use the infrared thermometer to check the hot spot of the resin transformer under the maximum load of the transformer, and adjust the direction and angle of the fan cooling equipment, control the local temperature rise of the transformer, and ensure the safe operation of the cast resin dry type transformer.

(2) The local discharge of the cast resin dry type transformer is related to the electric field distribution of the transformer, the uniformity of the resin mixture and whether there are residual bubbles or resin cracking and other factors. The local discharge affects the performance, quality and service life of the resin transformer. Therefore, the measurement and acceptance of the local discharge of the resin transformer is a comprehensive assessment of its process and quality. The partial discharge measurement test should be carried out after the handover and acceptance of the resin transformer and the overhaul, and evaluate the stability of its quality and performance according to the change of partial discharge.

With more and more extensive application of dry type transformers, we should understand the process structure, insulation design and insulation configuration when choosing transformers. The safe operation and power supply reliability of transformers can only be improved by selecting the products with perfect production process and quality assurance system, strict production management and reliable technical performance to ensure the product quality and heat-resistant life of cast resin dry type transformers.