I. Transformer Materials
1. Core material:
The core materials used in transformers mainly include iron sheets, low-silicon sheets, and high-silicon sheets. Adding silicon to the steel sheets reduces their conductivity and increases their resistivity, which in turn reduces eddy currents and thus losses. These silicon-added steel sheets are commonly referred to as silicon steel sheets. The quality of the silicon steel sheets used greatly affects the quality of the transformer. The quality of silicon steel sheets is usually expressed by magnetic flux density (B). Generally, the B value for iron sheets is 6000-8000, for low-silicon sheets it is 9000-11000, and for high-silicon sheets it is 12000-16000.
2. Commonly used materials for winding transformers include
Enameled wire, sandpaper wire, silk-covered wire—enameled wire is the most commonly used. The requirements for the conductor are good conductivity, sufficient heat resistance of the insulating varnish layer, and a certain degree of corrosion resistance. Generally, it is best to use high-strength polyester enameled wire of type Q2.
3. Insulation materials
In transformer winding, insulating materials are used for insulation between coil frame layers and between windings. Phenolic paperboard can be used for general transformer frame materials, polyester film or telephone paper can be used for insulation between layers, and yellow wax cloth can be used for insulation between windings.
4. Impregnation material:
After the transformer is wound, the final step is to impregnate it with insulating varnish. This process enhances the transformer's mechanical strength, improves its insulation performance, and extends its service life. Generally, cresol varnish can be used as the impregnation material.
II. Transformer Classification
(1) Power Transformer
Currently, representative products already in operation include: 1150kV, 1200MV·A; 735～765kV, 800MV·A; 400～500kV, 3-phase 750MV·A or single-phase 550MV·A; 220kV, 3-phase 1300MV·A power transformers; and ±500kV, 400MV·A DC transmission converter transformers. Power transformers are mainly oil-immersed, with two structural types: core-type and shell-type. Core-type transformers account for 95% of production, while shell-type transformers account for only 5%. There is no overwhelming advantage between core-type and shell-type transformers; core-type transformers are simply simpler in process and are therefore adopted by most companies, while shell-type transformers have a more complex structure and process, and are only used by traditional factories. Shell-type transformers are particularly suitable for high voltage and large capacity applications, offering advantages in insulation, mechanics, and heat dissipation, and are suitable for transportation in mountainous hydropower stations.
(2) Distribution transformer
Overseas distribution transformers can reach capacities of 2500 kVA, and come in both circular and elliptical core forms. Circular cores are the most common, while elliptical cores, due to their smaller M0 (pitch between core columns), require less material, and their corresponding coils are elliptical. Low-voltage coils are available in wire-wound and foil-type configurations, and oil tanks include those with cooling pipes (a minority) and corrugated types (the majority).
(3) Dry-type transformer
Recently, dry-type transformers have experienced rapid development in China. In major cities like Beijing, Shanghai, and Shenzhen, dry-type transformers account for 50% of all transformers, while in other large and medium-sized cities, they account for 20%. There are four types of dry-type transformer structures: epoxy resin casting, filler casting, wrapped, and impregnated. Currently, open-ventilated H-class dry-type transformers are widely used in Europe and America. These are new H-class dry-type transformers developed by incorporating features of the wrapped structure and using Nomex paper, based on the impregnated type. Due to their high price, they have not yet been widely adopted in my country. Currently, the dry-type distribution transformer with the largest short-circuit test capacity in China is 2500 kVA, 10/0.4 kV; the dry-type power transformer with the largest short-circuit test capacity is 16000 kVA, 35/10 kV.
(4) Amorphous alloy transformer
Although amorphous alloy transformers have poor short-circuit withstand performance and high noise levels, they are energy-efficient, thus their future development prospects are promising. Currently, China's largest amorphous alloy transformer core manufacturer has an annual core production capacity of 3,000 to 4,000 tons. However, the production technology of cores and transformers is not the key factor restricting the promotion of amorphous alloy core transformers; breakthroughs in amorphous alloy strip materials are needed to drive a qualitative leap in product quality.
(5) Winding core transformer
Currently, the production of wound-core transformers is mainly concentrated in the 10kV class, with capacities generally less than 800kVA. A 1600kVA capacity has also been trial-produced, but power departments primarily purchase transformers with capacities below 315kVA, suitable for rural power grids. There are over 200 wound-core transformer manufacturers in China, with about 20% being of a certain scale. China's production capacity for wound-core transformers is approximately 16 million kVA, but actual output is relatively low.
