Cooling method of oil immersed power transformer
Cooling method of oil immersed Power Transformer
The internal heating of the transformer mainly comes from loss, which consists of no-load loss (also known as iron loss) and load loss (also known as copper loss). The no-load loss includes the hysteresis loss and eddy current loss existing in the iron core, which is caused by the transformer iron. The main factor for the increase in core temperature. The load loss is composed of DC loss in the winding wire, eddy current loss, and stray loss in other iron structures such as transformer case and yoke. It is the most important factor affecting the rise in winding temperature.
The temperature rise of the transformer depends not only on the size of the loss (or source of heat), but also on the cooling effect. Therefore, in order to stabilize the temperature rise of the transformer within the required numerical range, the transformer must be required to have a certain heat dissipation capability in order to effectively dissipate the heat generated by the transformer heat source to the surrounding cooling medium. Since the heat source (ie, loss) of the transformer increases with its own capacity, the loss is almost proportional to the weight of the transformer or the cube of its linear dimension, and the heat dissipation area of the transformer is the square of the weight or its linear dimension. In proportion, therefore, when the capacity of the transformer increases, the increase of the loss is greater than the increase of the heat dissipation area. The oil-immersed transformer is divided into three types: ON, OF and OD according to the circulation mode of the oil.
1. ON means that the oil is cooled by natural convection to cool the windings;
2. OF represents oil cooling the winding in a forced convection manner;
3. OD represents the oil to cool the windings in a transformer with a guiding structure by forced convection cooling.
According to the cooling method of the radiator, it can be divided into three types: AN, AF and WF.
1. AN indicates that the radiator uses the natural convection of air to dissipate heat;
2. AF indicates that the radiator uses air to force convection to dissipate heat;
3. WF means that water is used as a cooling medium to dissipate heat from the heat sink.
By combining the heat dissipation methods in the transformer and in the heat sink, different forms of cooling can be obtained. Commonly used oil-immersed transformer cooling methods are as follows:
1 Oil-immersed self-cooling (ONAN): Transformers with a capacity of 6300kVA or less generally use this natural oil circulation and natural air cooling. It is a fuel pipe or a heat sink for heat dissipation or a heat sink composed of a flat tube (or a heat sink) welded to the tank wall or on the tank wall to increase the heat dissipation area to improve the cooling capacity.
2 Oil-immersed air-cooled (ONAF): medium and large transformers with a capacity of 8000~40000kVA or a voltage of 110~220kV generally adopt this natural oil circulation and blown cooling method. It is equipped with a blower on the self-cooling radiator to further improve the heat dissipation and cooling capacity.
3 Forced oil circulation type: The coolers used in the forced oil circulation cooling mode can be divided into two types: strong oil air cooling (OFAF) and strong oil water cooling (OFWF). Large transformers with a capacity greater than 31,500 kVA are often used. They connect the air cooler or water cooler with high heat dissipation efficiency directly to the transformer tank and perform circulating cooling. In order to give full play to the cooling effect of the forced oil circulation cooler, for large transformers with large radial size and complicated insulation structure, forced-oriented oil circulating air cooling (ODAF) or forced-oriented oil circulating water cooling (ODWF) may be used. It is characterized by forcing the oil flow to be guided back and forth in the winding, further improving the cooling efficiency.
4 Evaporative cooling method: This is a way to cool the transformer by using a mixture of fluorocarbon liquid and SF6 non-combustible gas. At present, this type of cooling has been adopted in foreign countries with large-capacity SF6 gas insulated transformers. And promotion stage.