I. General
Transformers generate heat during operation, primarily due to the electrical losses in the windings and the core. To maintain efficiency and prevent damage, various cooling methods are employed. These cooling methods can be broadly categorized into natural and forced types, depending on the size and power rating of the transformer.
II. Methods of Cooling
1. Oil-Immersed Transformer Cooling Methods
Oil-immersed transformers use oil for insulation and cooling. The transformer core and windings are submerged in oil, which dissipates heat through the following methods.
- ONAN (Oil Natural, Air Natural)
- The most basic method.Heat generated by the transformer is transferred to the oil, which rises and circulates naturally within the transformer tank. The oil transfers heat to the tank walls, where it is dissipated into the surrounding air through natural convection.
Used for smaller transformers. - ONAF (Oil Natural, Air Forced)
- In this method, natural oil circulation cools the core and windings, but the transformer is equipped with fans that force air over the radiators or external cooling fins to enhance heat dissipation. Typically used for medium to large transformers where natural air convection is insufficient.
- OFAN (Oil Forced, Air Natural)
- In this method, oil is forcibly circulated using pumps. The circulating oil transfers heat to the transformer tank, which then dissipates heat into the surrounding air naturally. Suitable for large transformers requiring more active cooling.
- OFAF (Oil Forced, Air Forced)
- Oil is forcibly circulated by pumps, and air is forced over the radiator fins by fans to improve cooling efficiency.
This method is used for large transformers with high power ratings. - ODAF (Oil Directed, Air Forced)
- Oil is directed to specific areas that require cooling, such as the windings, and forced circulation is used to increase the efficiency of heat transfer. Air cooling is also forced by fans for faster heat dissipation. Commonly used in very large power transformers.
- ODWF (Oil Directed, Water Forced)M
- Similar to ODAF, but instead of air, water is used to cool the oil. Water circulates in heat exchangers to absorb heat from the oil. Typically used in very large transformers installed in industrial environments where water is available for cooling.
2. Dry-Type Transformer Cooling Methods
Dry-type transformers do not use liquid for insulation or cooling, making them suitable for indoor or sensitive installations. The cooling methods are:
- AN (Air Natural)
- Used for smaller transformers, this method relies on natural air circulation around the transformer to dissipate heat.
There is no active cooling, making it suitable for low-power transformers. - AF (Air Forced)
- Fans blow air over the transformer windings to increase cooling capacity. Suitable for larger dry-type transformers or those operating in environments where higher load is expected.
- GN (Gas Natural)
- In this method, inert gas (such as SF₆) is used instead of air for insulation. The heat is dissipated through natural circulation of the gas within the transformer enclosure. This method is used for specialized installations, such as underground transformers.
- GF (Gas Forced)
- In this method, gas is circulated using fans or blowers to enhance the cooling of the transformer.
3. Water-Cooled Transformer
Water-cooled transformers use water as the primary cooling medium, typically in large industrial settings. Heat is transferred from the transformer oil or core directly to water through heat exchangers. Forced water circulation can handle very high heat loads, making it suitable for very large power transformers.
4. Hybrid Cooling Systems
In some cases, transformers use a combination of methods (oil, air, gas, or water) depending on the operational environment and power demand.
III. Standards for Transformer Cooling Methods
The cooling methods for transformers are governed by various national and international standards to ensure uniformity in design, safety, performance, and testing. These standards provide guidelines for manufacturers and operators to classify, describe, and test transformer cooling systems. Some key standards related to the cooling methods of transformers include:
1. IEC Standards (International Electrotechnical Commission)
- IEC 60076-2: Power Transformers – Part 2: Temperature Rise for Liquid-Immersed Transformers
- This standard defines the limits for temperature rise in the windings, core, and oil of liquid-immersed transformers, which directly influence cooling method design. It describes the measurement techniques for temperature rise and the permissible limits for each component.
- IEC 60076-7: Power Transformers – Part 7: Loading Guide for Oil-Immersed Power Transformers
- Provides recommendations for the loading of transformers, including how different cooling methods impact the permissible loading and heat dissipation capabilities. Explains how to calculate load capacity based on cooling performance and the ambient conditions.
- IEC 60076-11: Power Transformers – Part 11: Dry-Type Transformers
- Sets the specifications for dry-type transformers and the cooling methods such as Air Natural (AN), Air Forced (AF), and Gas Cooled systems. Includes testing procedures for temperature rise, insulation, and performance under various cooling conditions.
- IEC 60076-14: Power Transformers – Part 14: Liquid-Immersed Power Transformers Using High-Temperature Insulation Materials
- This standard specifies the design, construction, and performance characteristics of liquid-immersed transformers that use high-temperature insulation systems. It covers the cooling classifications and how the use of high-temperature materials can improve thermal performance.
2. IEEE Standards (Institute of Electrical and Electronics Engineers)
- IEEE C57.12.00: Standard for General Requirements for Liquid-Immersed Distribution, Power, and Regulating Transformers
- Provides general specifications for liquid-immersed transformers, including requirements for cooling methods like ONAN, ONAF, and OFAF. Specifies temperature limits for windings, insulation, and oil, which affect the transformer’s cooling design.
- IEEE C57.12.01: Standard for General Requirements for Dry-Type Distribution and Power Transformers
- Describes the classification and performance requirements for dry-type transformers, including cooling methods like Air Natural (AN) and Air Forced (AF). Sets limits for temperature rise and insulation, which directly influence cooling system performance.
- IEEE C57.91: Guide for Loading Mineral-Oil-Immersed Transformers
- Provides guidelines on transformer loading based on cooling method and ambient temperature. Discusses how cooling methods affect a transformer’s load capacity and the associated thermal characteristics.
- IEEE C57.92: Guide for Loading Dry-Type Distribution and Power Transformers
- Offers a guide for loading dry-type transformers based on their cooling method. Takes into account ambient conditions and the cooling performance of the transformer when determining its capacity.
- IEEE C57.119: Recommended Practice for Performing Temperature Rise Tests on Oil-Immersed Power Transformers
- Defines procedures to test the temperature rise in oil-immersed transformers under various cooling methods, ensuring that cooling systems are adequately designed and tested.
3. NEMA Standards (National Electrical Manufacturers Association)
- NEMA TR 1: Transformers, Regulators, and Reactors
- This standard specifies general requirements for distribution, power transformers, and regulators, including cooling classifications. Establishes temperature rise limits for windings, oil, and other components, influencing the design of cooling methods.
4. ANSI Standards (American National Standards Institute)
- ANSI C57.12.90: Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers
- Specifies testing procedures for liquid-immersed transformers, including tests for temperature rise and cooling performance.
Defines how transformers should be tested under different cooling methods like ONAN, ONAF, and OFAF. - ANSI C57.96: Guide for Loading Dry-Type Distribution and Power Transformers
- Provides guidance for the loading and testing of dry-type transformers, including the impact of cooling methods on their performance.
5. ISO Standards (International Organization for Standardization)
- ISO 12944: Paints and Varnishes – Corrosion Protection of Steel Structures
- This standard indirectly affects transformer cooling methods by specifying protective coatings for transformer tanks, especially those exposed to air (e.g., air-cooled transformers). Proper tank protection can improve heat dissipation performance in cooling systems like ONAN and ONAF.
IV. Transformer Cooling Classifications
The cooling classifications are commonly expressed using four-letter codes in these standards. Here's how they are typically defined:
First Letter: Internal fluid
O: Oil
A: Air
G: Gas
Second Letter: Circulation of internal fluid
N: Natural circulation
F: Forced circulation
Third Letter: External cooling medium
A: Air
W: Water
Fourth Letter: Circulation of external medium
N: Natural circulation
F: Forced circulation
Example of Cooling Types
ONAN – Oil Natural Air Natural
ONAF – Oil Natural Air Forced
OFAN – Oil Forced Air Natural
OFAF – Oil Forced Air Forced
ODAF – Oil Directed Air Forced
ODWF – Oil Directed Water Forced
AN – Air Natural (Dry-Type)
AF – Air Forced (Dry-Type)
Each method is chosen based on the transformer's rating, size, and cooling requirements, ensuring that the transformer operates efficiently and safely under varying load conditions.