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Oil immersed transformer classification
Transformers can be categorized in various ways based on their design, application, and operational characteristics. One of the primary classifications is by the number of phases they handle. A three-phase transformer is typically used in three-phase power systems, which are common in industrial and large-scale electrical networks. However, when the required capacity is very high and transportation constraints make it difficult to move a single large unit, three individual single-phase transformers can be combined to form a three-phase transformer bank. This approach allows for more flexibility in installation and maintenance.
Another way to classify transformers is by the number of windings they have. The most common type is the two-winding transformer, which consists of a primary winding and a secondary winding, both wrapped around the same core. These are widely used in standard power distribution applications. For more complex systems that require connecting three different voltage levels, three-winding transformers are employed. These are typically used in high-capacity applications, usually above 5,600 kVA. In some specialized cases, transformers with more than three windings may also be used to meet specific electrical requirements.
In terms of construction, transformers can be either core-type or shell-type. In a core-type transformer, the windings are wrapped around the iron core, while in a shell-type transformer, the core surrounds the windings. Although these designs differ slightly in structure, their basic operating principles are similar. Most power transformers are of the core-type design due to its efficiency and ease of manufacturing.
Based on insulation and cooling methods, transformers are generally divided into oil-immersed and dry-type types. Oil-immersed transformers use transformer oil as both an insulating medium and a cooling agent. The heat generated by the windings and core is transferred to the oil, which then dissipates the heat through natural convection or forced circulation. Dry-type transformers, on the other hand, rely on air cooling and are often used in environments where flammable materials are present, such as street lighting systems or underground mines.
In addition to standard transformers, there are also special-purpose transformers designed for specific applications. Examples include test transformers used for high-voltage testing, furnace transformers for industrial heating, welding transformers for arc welding, and instrument transformers like potential and current transformers used for metering and protection purposes.
When it comes to cooling methods for oil-immersed power transformers, the options vary depending on the size and application. Common cooling methods include:
1. Natural oil circulation with natural cooling (oil-immersed self-cooling)
2. Natural oil circulation with air cooling (oil-immersed air-cooled)
3. Forced oil circulation with water cooling
4. Forced oil circulation with air cooling
These cooling techniques help maintain optimal operating temperatures and ensure the longevity and reliability of the transformer.
Under normal conditions, oil-immersed transformers are suitable for use at altitudes up to 1,000 meters, both indoors and outdoors. They can operate in ambient temperatures ranging from -25°C to +40°C, with a maximum daily average temperature of +30°C and an annual average of +20°C. Transformers designed for special environments, such as extreme climates or hazardous locations, can be custom-built upon request.