How to Select the Cooling Method for Power Transformer

HIMALAYAL - SHANGHAI - CHINA



A transformer is a static machine used for transforming power from one circuit to another without changing frequency. You can click here for further information about power transformer. Here we present its cooling methods: Oil Natural Air Natural (ONAN), Oil Natural Air Forced (ONAF), Oil Forced Air Forced (OFAF), Oil Forced Water Forced (OFWF), Oil Directed Air Forced (ODAF) and Oil Directed Water Forced (ODWF).   

1. ONAN :  31500kVA and below, 35kV and below products       50000kVA and below, 110kV products 

2. ONAF: 12500kVA～63000kVA, 35kV～110kV products    Below 75000kVA and 110kV     40000kVA and below, 220kV

3. OFAF: 50000～90000kVA, 220kV     

4. OFWF :  220kV and above, 60MVA and above

5. ODAF or ODWF: 75000kVA and above, 110kV     120000kVA and above, 220kV      330kV and 500kV

If selecting the OFAF method, the power transformer is not allowed to operate for a long time when the oil pump and fan lose power supply. Two independent power supplies should be ready for use. If selecting the OFWF method, the power transformer must not be operated when the oil pump loses power supply. 

The ONAN method has no special cooling equipment. Relying on the natural convection of oil, the heat given out by windings and iron core is brought to the wall of oil tank or radiator; then the convection of air emits the heat. This kind of cooling method is often used for small-capacity transformer. In terms of ONAF method, the cooling fan is installed at the inside radiator and the fan is utilized to accelerate the cooling of radiator oil. It is a common practice to use ONAF method for medium-capacity transformer. For OFAF and OFWF methods, the hot oil in the oil tank of power transformer is into oil cooler by oil pump and then back to oil tank after being cooled. The cooling effect is strong so this method is sutiable for large-capacity transformer. 

There are two cooling methods for dry-type transformer: ① Air Natural (AN) ② Air Blast (AB). In most cases, the dry-type transformer is equipped with fans. Thus, strictly speaking, the dry-type transformer has only one cooling method-AN.