1. Oil Natural Air Natural (ONAN)

   • The working principle of ONAN involves utilizing the natural convection of oil to transfer heat generated by the transformer to the tank walls and radiator surfaces, where it is then dissipated into the surrounding air through natural convection and radiation. This system does not require any additional cooling equipment. Suitable for transformers up to 31,500 kVA at 35 kV or below, and up to 50,000 kVA at 110 kV.

2. Oil Natural Air Forced (ONAF)

   • Building upon the ONAN principle, ONAF systems incorporate fans mounted on the tank walls or radiators to enhance cooling through forced air circulation. Adding fans can increase the transformer's capacity and load handling capability by approximately 35%. During operation, losses such as iron loss, copper loss, and other forms of energy loss convert into heat, raising the temperature of core and windings. Heat is first transferred from the core and windings to the oil via thermal conduction, then circulated naturally within the oil before being cooled by fan-assisted airflow over the tank and radiator surfaces. Suitable for transformers ranging from 12,500 kVA to 63,000 kVA at 35 kV to 110 kV, under 75,000 kVA at 110 kV, and up to 40,000 kVA at 220 kV.

3. Oil Forced Air Forced (OFAF)

   • OFAF systems use oil pumps to circulate oil through coolers, which are equipped with fans to enhance cooling efficiency. Suitable for transformers ranging from 50,000 kVA to 90,000 kVA at 220 kV.

4. Oil Forced Water Forced (OFWF)

   • Typically used in hydroelectric power plants, OFWF systems utilize water as a cooling medium to dissipate heat from the transformer oil through heat exchangers. Suitable for step-up transformers at 220 kV and above, with capacities of 60 MVA and above. In both OFAF and OFWF systems, the oil is cycled through the cooler using pumps, and the shape of the cooler is designed to facilitate efficient heat dissipation. Increasing the oil circulation speed threefold can enhance the transformer's capacity by about 30%.

5. Oil Directed Air Forced (ODAF)

   • ODAF systems direct oil flow through channels between the core and windings to maximize heat extraction. Afterward, the heated oil is pumped through coolers equipped with fans for further cooling. Suitable for transformers of 75,000 kVA and above at 110 kV, 120,000 kVA and above at 220 kV, and transformers at 330 kV and 500 kV levels.

6. Oil Directed Water Forced (ODWF)

   • Similar to ODAF but uses water as the cooling medium instead of air. Suitable for transformers of 75,000 kVA and above at 110 kV, 120,000 kVA and above at 220 kV, and transformers at 330 kV and 500 kV levels.
   • 

Transformer Cooling System Operation

Traditional power transformers are controlled manually, with each unit having six sets of air-cooled motors controlled by thermal relays. The control circuits for these fans are managed through contactors based on measurements of oil temperature and overload conditions. Traditional controls rely on manual mechanical contacts, which have significant drawbacks: all fans start and stop simultaneously, causing high inrush currents that can damage circuit components. When temperatures range from 45°C to 55°C, full operational engagement often leads to substantial energy waste and complicates maintenance.

Components of Oil-Pump Forced Air-Cooled Transformers

Cooling units consist of heat exchangers, fans, motors, ducts, oil pumps, and oil flow indicators. Fans remove heated air from the heat exchanger, while oil pumps installed at the bottom of the cooler ensure oil circulation from top to bottom. Oil flow indicators are positioned prominently to allow operators to monitor pump performance.

Functions of Transformer Tanks and Cooling Devices

Transformer tanks serve as enclosures housing the core, windings, and transformer oil, contributing to heat dissipation. Cooling devices create an oil circulation loop when there is a temperature differential in the upper oil layer, directing the oil through coolers back into the tank to reduce oil temperature. Enhanced cooling can be achieved through air cooling, forced oil-air cooling, or forced oil-water cooling methods.