Dry-type transformers (specifically referring to transformers insulated with epoxy resin) are primarily used in locations with high fire safety requirements, such as high-rise buildings, airports, and oil depots.

1. Types of Resin Insulation:

Dry resin-insulated transformers can be classified into three types based on manufacturing processes: epoxy-quartz sand mixture vacuum casting type, epoxy non-alkali glass fiber reinforced vacuum pressure difference casting type, and non-alkali glass fiber wrapped impregnation type.

1.1 Epoxy-Quartz Sand Mixture Vacuum Casting Insulation:

In this type of transformer, quartz sand is used as a filler for epoxy resin insulation. The coils pre-treated with insulating varnish are placed in the mold, and the epoxy-quartz sand mixture is poured into the mold under vacuum conditions. Due to difficulties in the casting process meeting quality requirements, such as residual bubbles, local unevenness in the mixture, and potential local thermal stress cracking, these insulated transformers are not suitable for humid and hot environments or areas with significant load variations.

1.2 Epoxy Non-alkali Glass Fiber Reinforced Vacuum Pressure Difference Casting Insulation:

Epoxy non-alkali glass fiber reinforcement involves using non-alkali glass short fibers as insulation between coil layers. The outermost insulation layer typically ranges from 1 to 3 millimeters thick. The epoxy resin casting material is mixed according to a specific ratio and poured under a high vacuum to remove bubbles. As the insulation layer is relatively thin, inadequate impregnation can lead to local discharge points. Therefore, thorough mixing of casting material, complete vacuum bubble removal, and control of low viscosity and casting speed are essential to ensure high-quality impregnation of the coil during casting.

1.3 Non-alkali Glass Fiber Wrapped Impregnation Insulation:

Transformers with non-alkali glass fiber-wrapped impregnation involve treating the coil insulation and impregnating the coil during winding. This method eliminates the need for the two aforementioned impregnation processes but requires resin with low viscosity and ensures no residual bubbles during coil winding and impregnation.

2. Insulation Characteristics and Maintenance of Resin Transformers:

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

2.1 The average temperature rise of resin transformers is higher than that of oil-immersed transformers. Therefore, higher heat-resistant grades are required for insulation materials. However, if insulation materials are selected solely based on average temperature rise if they are improperly selected, or if resin transformers operate under prolonged overload conditions, it may affect the transformer's service life. Since the measured temperature rise of transformers often cannot reflect the temperature of the hottest spot in the transformer, it is advisable to use an infrared thermometer to inspect the hottest spot of resin transformers under full load conditions and adjust the direction and angle of fan cooling equipment accordingly to control local temperature rise and ensure safe operation of the transformer.

2.2 The magnitude of partial discharge in resin transformers is related to factors such as the distribution of electric fields, uniformity of resin mixtures, and the presence of residual bubbles or resin cracks. The magnitude of partial discharge affects the performance, quality, and service life of resin transformers. Therefore, measuring and accepting the partial discharge of resin transformers is a comprehensive assessment of their processes and quality. Partial discharge measurements should be conducted during transformer handover acceptance and major maintenance, and changes in partial discharge should be evaluated to assess the stability of their quality and performance.

As dry-type transformers are increasingly widely used, when selecting transformers, it is essential to have a clear understanding of their process structure, insulation design, and insulation configuration from an electrical engineering perspective. Choosing products with reliable production processes, complete quality assurance systems, and strict production management and ensuring the product quality and thermal aging life of transformers can improve the safety and reliability of transformer operation and power supply.