Question

Explain in brief the different types of cooling systems used for transformers.

Hint:

The oil used in oil-immersed transformers serves a dual purpose: it acts as a highly efficient heat-transfer fluid (coolant) and provides essential electrical insulation between the high-voltage windings and the grounded tank.

Answer 1

Step 1: The Necessity of Transformer Cooling:
Transformers generate substantial heat during operation due to copper losses ($I^2R$) in the windings and core losses (hysteresis and eddy currents) in the magnetic core. To prevent insulation breakdown and maintain transformer life, this heat must be continuously removed.

Step 2: Classifying Transformer Cooling Systems:
Cooling systems are categorized based on whether the transformer is dry-type or oil-immersed, and whether the cooling medium (air, oil, or water) circulates naturally or is forced mechanically.

• Air Natural (AN) Cooling: Used for small dry-type transformers (usually rated up to 1.5 MVA or 3 MVA). Heat is dissipated directly to the surrounding air via natural convection and radiation.
• Air Blast Forced (AB or AF) Cooling: For larger dry-type transformers, high-speed fans or blowers are used to force a continuous stream of air over the core and windings, increasing the rate of heat dissipation.
• Oil Natural Air Natural (ONAN) Cooling: This is the standard cooling method for medium distribution transformers. The core and windings are immersed in a tank filled with transformer oil.
  • Heat is transferred from the windings/core to the oil.
  • Heated oil rises naturally to the top of the tank, enters radiator tubes, cools down, and sinks back down, creating a natural thermal circulation loop.
  • The heat from the radiator tubes is carried away by the surrounding air via natural convection.
• Oil Natural Air Forced (ONAF) Cooling: Similar to ONAN, but fans are mounted below or beside the radiator tubes to force air over the cooling fins, significantly increasing heat transfer capacity. Used for transformers up to 30 MVA.
• Oil Forced Air Forced (OFAF) Cooling: Used for large power transformers (above 30 MVA). Oil pumps are installed in the cooling circuit to force the oil to circulate rapidly through the radiator banks, while high-velocity fans blow air across the radiators.
• Oil Forced Water Forced (OFWF) Cooling: Highly efficient system used in heavy-duty generator transformers at power plants. Hot oil is pumped through an external heat exchanger where flowing water is used to absorb and carry away the heat.