Production Process of Dry-Type Transformers

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# Core Manufacturing
1. **Cutting of Silicon Steel Sheets**
- Select high-quality silicon steel sheets, such as cold-rolled silicon steel sheets with high magnetic permeability and low losses. According to the design requirements of the transformer, use a shearing machine to cut the silicon steel sheets into appropriate sizes and shapes. During the cutting process, it is necessary to ensure the dimensional accuracy of the silicon steel sheets. Generally, the cutting size error should be controlled within ±0.5mm to ensure the lamination quality of the core.

2. **Core Lamination**
- Laminated the cut silicon steel sheets. There are various lamination methods, such as direct lamination and mitered joint lamination. Mitered joint lamination can reduce the no-load loss of the core. Generally, the miter angle is between 30° and 60°. During the lamination process, it is necessary to ensure the alignment of the silicon steel sheets. The misalignment between adjacent silicon steel sheets should not exceed 0.2mm, and it is also necessary to ensure the tightness of the lamination. Appropriate pressure is applied through a pressure device to make the stacking factor of the core reach more than 95%.

3. **Core Clamping and Curing**
- Use channel steel or clamping parts to clamp the laminated core to prevent the core from loosening during operation. The clamping force should be uniform, and generally, the clamping pressure is between 2 - 3MPa. For some cores with high-performance requirements, curing treatment is also required. Use insulating materials such as epoxy resin to impregnate and cure the core to enhance its mechanical strength and insulating properties. The curing temperature is usually between 120°C and 150°C, and the curing time is about 2 - 4 hours.

### Winding Manufacturing
1. **Selection and Preparation of Conductors**
- Select appropriate conductors according to the capacity and voltage level of the transformer. For dry-type transformers, common conductors include copper conductors and aluminum conductors. Copper conductors have better electrical conductivity and mechanical properties. The specifications of the conductors are determined according to the current density. Generally, the current density is controlled between 2 - 4A/mm². Before winding, pre-treat the conductors, such as cleaning and straightening, to ensure that the surface of the conductors is smooth and free of burrs and oxide layers.

2. **Winding Winding**
- Use a winding machine to wind the windings. During the winding process, it is necessary to control the winding tension. Excessive tension may cause the conductors to stretch and deform, while too little tension will make the windings loose. Generally, the winding tension is controlled between 10% and 20% of the breaking force of the conductors. At the same time, it is necessary to ensure the accuracy of the number of turns of the windings. The error of the number of turns should be controlled within ±0.5%. For multi-layer windings, pay attention to the interlayer insulation. Use insulating materials such as polyester film and Nomex paper as interlayer insulation. The thickness of the insulation is generally between 0.1 - 0.3mm.

3. **Winding Drying and Impregnation**
- The wound windings need to be dried to remove the moisture in the windings. The drying temperature is generally between 100°C and 130°C, and the drying time is about 12 - 24 hours. After drying, the windings are impregnated with paint. Use solvent-free or low-solvent paints, such as epoxy resin paint. The purpose of impregnation is to improve the insulating performance and mechanical strength of the windings. After impregnation, put the windings into an oven for curing. The curing temperature is between 130°C and 150°C, and the curing time is about 4 - 8 hours.

### Body Assembly
1. **Assembly of Core and Windings**
- Put the dried and impregnated windings onto the core. It is necessary to ensure the concentricity of the windings and the core. The concentricity error should be controlled within ±1mm. Adjust the position of the windings and use positioning devices to make the windings firmly installed on the core. At the same time, check the insulation distance between the windings and the core to ensure that the insulation distance meets the design requirements. Generally, the insulation distance between phases should not be less than 10mm.

2. **Lead Connection**
- Connect the winding leads to the external terminals. The leads can be copper bars or flexible connections. The connection methods include welding, bolt connection, etc. For welded connections, it is necessary to ensure the welding quality. The welds should be full and free of defects such as incomplete welding and pores. For bolt connections, it is necessary to ensure a tight connection. The tightening torque of the bolts should comply with the regulations. Generally, the tightening torque is between 10 - 20N·m. The connected leads should be insulated. Use materials such as heat-shrinkable tubes and insulating tapes to wrap the leads. The thickness of the insulation should not be less than 2mm.

3. **Assembly Inspection**
- Conduct a comprehensive inspection of the assembled body. The inspection contents include the fixing conditions of the core and windings, whether the lead connections are correct, whether the insulation distance meets the requirements, etc. The insulation resistance of the body can be measured using an insulation resistance tester. The insulation resistance value is generally not less than 1000MΩ. At the same time, check the appearance of the body to ensure that there are no foreign objects entering and the appearance is clean.

### Final Assembly and Testing
1. **Installation of Enclosure**
- Select an appropriate enclosure according to the usage environment and requirements of the dry-type transformer. The enclosure materials include steel plates, aluminum alloys, etc. Install the enclosure on the outside of the body. It is necessary to ensure the tightness of the enclosure to prevent dust, moisture, etc. from entering the inside of the transformer. The grounding connection should be made between the enclosure and the body, and the grounding resistance should not be greater than 4Ω.

2. **Installation of Cooling Devices**
- For dry-type transformers with larger capacities, cooling devices such as fans need to be installed. The selection of fans should be determined according to the loss and heat dissipation requirements of the transformer. The installation position of the fans should be reasonable to ensure that the transformer can be effectively cooled. The connection between the fans and the transformer should be firm, and the electrical connection should be made to ensure the normal operation of the fans.

3. **Performance Testing**
- Conduct multiple performance tests, including no-load test, short-circuit test, withstand voltage test, etc. The no-load test mainly measures the no-load loss and no-load current of the transformer. The no-load loss generally does not exceed 10% of the value specified in the national standard, and the no-load current does not exceed 5% of the rated current. The short-circuit test measures the short-circuit impedance and short-circuit loss of the transformer. The deviation of the short-circuit impedance should be controlled within ±10%. The withstand voltage test includes power frequency withstand voltage test and induced withstand voltage test. The test voltage should comply with the national standard. For example, the power frequency withstand voltage test voltage is generally twice the rated voltage plus 1000V, and the duration is 1 minute. During the test, there should be no breakdown, flashover and other phenomena in the transformer.