Rated current refers to the current when the electrical equipment is operating at the rated power under the rated voltage. It can also be defined as the current that electrical equipment can work continuously for a long time under rated environmental conditions (ambient temperature, sunshine, altitude, installation conditions, etc.). The current of the electrical appliance during normal operation should not exceed its rated current.
The nature of the rated currentThe rated current is 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) and the current flowing through the winding ends. Therefore, the rated current of the transformer is the rated current of each winding, which refers to the line current. However, for a single-phase transformer that forms a three-phase group, if the windings are delta-connected, the rated current of the windings is represented by the line current as the numerator and √3 as the denominator. For example, if the line current is 500A, the rated current of the winding is (500 /√3) A.
When the transformer is running under the rated capacity, the current of the winding is the rated current. According to the national standard GB/T151641—1994 "Guidelines for the Load of Oil-immersed Power Transformers", the transformer can be overloaded and the three-phase rated capacity does not exceed 100 MVA (single-phase does not exceed 33.3 MVA), the load rate (load current) /Rated current) is not more than 1.5 for occasional overload, and when the capacity is larger, it can withstand the occasional overload with a load rate of not more than 1.3.
The bushing should also have corresponding overload capacity, and the hot spot temperature of the winding and the temperature of the top oil should not exceed 140°C and 105°C, respectively.
(1) Rated current selection of ordinary reactors
(1) The reactor has almost no overload capacity, so the reactor of the main transformer or the outgoing circuit should be selected according to the maximum working current of the circuit instead of the normal continuous working current.
(2) For the reactor of the busbar segmented loop of the power plant, the largest generator should be cut off according to the bus accident, which may be selected by the current of the reactor. Generally take 50% to 80% of the rated current of the generator.
(3) The reactance of the substation bus section loop should meet the requirements of the user's primary load and most secondary loads.
(2) Rated current selection of split reactor
(1) When the generator or main transformer circuit of a power plant is used, it is generally selected according to 70% of the rated current of the generator or main transformer.
(2) When used in the main transformer circuit of a substation, it should be selected according to the maximum load current passed in the arm with the larger load current. When there is no load data, it is generally selected according to 70% of the rated current of the main transformer.
Rated current and starting currentWhen the motor outputs the rated power, the current flowing through the power circuit is called the rated current, and the unit is A (ampere). When the motor is just switched on, the current passing through the stator winding is called the starting current. When the current is just turned on, the stator winding generates a rotating magnetic field. Since the rotor has not yet rotated, the relative movement speed between the stator magnetic field and the rotor is the largest. The rotor winding cuts the rotating magnetic field of the stator winding at the maximum speed, so the maximum induction is generated in the rotor. Electric current and induced current increase the current in the stator winding through the induction of the air gap magnetic field. Therefore, the starting current of the motor can reach 6 to 9 times of the rated current.
Larger starting current will cause the voltage drop of the circuit to increase, affect the use of the same circuit electrical appliances, and prolong the electrical starting time, causing the temperature of the motor to rise. Therefore, it is hoped that the starting current is as small as possible. In the test and detection, the starting current is usually replaced by the locked-rotor current. The locked-rotor current is the effective value of the steady-state current input from the power supply loop measured when the rotor is blocked at the rated voltage and rated frequency.
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