Transformer Capacity Calculation and Rated Current Calculation(3)

Transformer Rated Current Calculation

1. Transformer rated current calculation

Rated current refers to the current of the electrical equipment when it is running at rated voltage and rated power. It can also be defined as the current that electrical equipment can continuously work for a long time under rated environmental conditions, such as ambient temperature, sunshine, altitude, installation conditions, etc. The current of the electrical appliance in normal operation should not exceed its rated current.

The rated current is the current flowing through the winding wire end calculated by dividing the rated capacity of the winding by the rated voltage of the winding and the corresponding coefficient (single-phase is 1, three-phase is √3). 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, the rated current of the winding 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 operating at the rated capacity, the current of the winding is the rated current.

The bushing should also have a corresponding overload capacity, and the temperature of the winding hot spot and the top oil temperature should not exceed 140°C and 105°C, respectively.

2. Transformer rated current calculation：

Transformer rated current I1N/I2N, the unit is A, kA. It is the current that the transformer can bear during normal operation, and it represents the line current in the three-phase transformer.

Transformer rated current calculation formula: For single-phase transformer: I1N = SN/U1N; I2N = SN/U2N

For the three-phase transformer: I1N=SN/[√3*U1N]; I2N=SN/[√3*U2N]

U1N is the voltage that should be applied to the primary side during normal operation.

U2N is the voltage when the rated voltage is applied to the primary side and the secondary side is at no load. The unit is V.

Note: Rated voltage refers to line voltage.

I1N is the rated current of the primary side transformer during normal operation; I2N is the rated current of the primary side transformer. The unit is A.

SN is the rated capacity of the transformer, the unit is VA, kVA, and MVA.

Empirical formula: transformer secondary side current (I2N) = transformer rated capacity * 1.44.