Analyzing the overload operation of transformer

　　Overload operation of a transformer means that the capacity transmitted by the transformer during operation exceeds the rated capacity of the transformer. The rated capacity of a three-phase transformer is SN=〖KF(〗3〖KF)〗, and the active output and reactive output of the transformer are recorded during operation, so conversion is required.

S=S2P+S2Q

　　When S>SN, the transformer is overloaded. Transformer overload must be when the operating current exceeds the rated current. At this time, the load loss of the transformer increases sharply, which is bound to cause the transformer temperature to rise. The increase in transformer temperature is extremely harmful to the operation and life of the transformer. Therefore, the overload operation of the transformer must be restricted. This restriction is actually the control of the hot spot temperature of the transformer winding. From this point of view, transformer overload can be divided into the following three situations.

　　1. Allowable overload

　　Although the transformer is overloaded, the degree of overload is not large. Before the overload, the transformer load is light, the oil temperature on the top of the transformer is not high, and the hot spot temperature of the transformer winding will not reach a harmful level. This overload is allowable for the transformer.

　　2. Limit overload

　　The transformer is overloaded to a large extent, causing the top oil temperature to rise, and the hot spot temperature of the transformer winding may reach a harmful level, but not a dangerous level. At this time, although the transformer can continue to operate, the insulation strength will decrease, threatening the safety of the transformer and affecting the life of the transformer. This overload must be limited.

　　3. Overload is prohibited

　　The transformer is overloaded to a large extent and for a long time, causing the oil temperature at the top of the transformer to exceed the allowable value, and the hot spot temperature of the transformer winding has reached a dangerous level. If the transformer continues to operate at this time, the insulating oil around the hot spot will decompose and produce bubbles, and the insulation strength will seriously decrease, which may cause a major failure of the transformer. This overload must be prohibited.

　　When the transformer is overloaded, special attention should be paid to the following points:

　　(1) Closely monitor the transformer winding temperature and top oil temperature.

　　(2) All cooling devices of the starting transformer should be prohibited from overloading the transformer when there are defects in the cooling device or the cooling efficiency does not meet the requirements.

　　(3) For transformers with on-load tap-changing devices, when the overload degree is large, use of the on-load tap-changing devices to adjust the tap should be avoided as much as possible.

　　(4) The main transformer can operate under normal overload and accident overload conditions. The allowable value of normal overload should be determined based on the load curve of the main transformer, the cooling medium and the load carried by the main transformer before overload. Accident overload and normal overload operations must be carried out when there are no abnormal phenomena in the main transformer. If the main transformer has a damaged cooler, serious oil leakage, or serious defects in the body protection, overload operation is not allowed.

　　GB/T 15164-94 "Guidelines for Loading of Oil-immersed Power Transformers" stipulates that for transformers designed according to GB1094, the reference value of the hot spot temperature is 98°C under rated load and normal ambient temperature (20°C), and the relative aging rate at this temperature is 1, that is, under this condition, the transformer consumes normal life. For every 6K increase in the hot spot temperature, the aging rate increases by 1. According to the factory temperature rise test of the transformer, the temperature converted to the hot spot at 20°C is much lower than 98°C (the difference is nearly 20°C), so in fact, the transformer has a large temperature rise margin. Because as the transformer is put into operation for an increasing period of time, the thermal efficiency of its cooling system will decrease compared to the factory due to reasons such as dirty radiators and reduced oil pump efficiency, and the actual temperature rise may be higher than the calculated one. Of course, if the manufacturer has provided a clear transformer overload capacity table, it should be implemented according to the data provided by the manufacturer.