Rated current refers to the current of an electrical device operating at rated voltage and rated power. It can also be defined as the current that electrical equipment can operate continuously for a long time under rated environmental conditions (such as ambient temperature, sunlight, altitude, installation conditions, etc.). The current of an electrical appliance during normal operation should not exceed its rated current. The rated current is the current flowing through the winding terminals calculated by dividing the rated capacity of the winding by the rated voltage of the winding and the corresponding coefficient (1 for single-phase and √ 3 for three-phase). Therefore, the rated current of a transformer is the rated current of each winding, which refers to the line current. However, for single-phase transformers composed of three-phase groups, if the winding is delta connected, the rated current of the winding is represented by the numerator of the line current and the denominator of √ 3. For example, if the line current is 500A, the rated current of the winding is (500/√ 3) A. When the transformer is operating at rated capacity, the current of the winding is the rated current. According to the national standard GB/T151641-1994 "Load Guidelines for Oil Immersed Power Transformers", transformers can operate under overload. When the rated capacity of three phases does not exceed 100 MVA (single-phase does not exceed 33.3 MVA), they can withstand occasional overloads with a load rate (load current/rated current) not exceeding 1.5, and when the capacity is larger, they can withstand occasional overloads with a load rate not exceeding 1.3. The casing should also have corresponding overload capacity, and the hot spot temperature of the winding and the top oil temperature should not exceed 140 ℃ and 105 ℃, respectively
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