Transformer Capacity Calculation and Rated Current Calculation

Transformer Capacity Calculation

First, select the rated voltage of the transformer. The voltage of the high-voltage side is equal to the voltage of the connected grid, and the voltage of the low-voltage side is 10% or 5% higher than the voltage of the grid on the low-voltage side (depending on the voltage level of the transformer and the impedance voltage.

To calculate the size of the load carried by the transformer, it is required to count the maximum comprehensive load and convert the kW value of the active load into the apparent power kVA. If there are two transformers, then the capacity of each transformer can be selected according to 70% of the maximum comprehensive load, and one transformer should be considered according to the total load, with an appropriate margin. Other famous brand parameters can be properly considered in combination with transformer products.

For example: choose a 35/10kV transformer. Assuming that the maximum load is 3500kW, the power factor is 0.8, two transformers are selected, the capacity S=0.7×3500/0.8=3062kVA and the transformer of 3150kVA can be selected, and the voltage ratio is 35kV/10.5kV. Then select the model from the product catalog.

Transformer capacity calculation formula：

1、Calculate the maximum power per phase of the load

Add the load power of each phase of the A-phase, B-phase, and C-phase independently. For example, the total power of the A-phase load is 10KW, the total power of the B-phase load is 9KW, the total power of the C-phase load is 11KW, and the maximum value is 11KW. (Note: The power of each single-phase device is calculated according to the maximum value on the nameplate, and the three-phase device power is divided by 3, which is equal to the power of each phase of this device.)

For example: C-phase load total power = computer 300W x 10 units) + (air conditioner 2KW x 4 units) = 11KW

2、Calculate the three-phase total power

11KW×3-phase=33KW (transformer three-phase total power)

3、Calculate the total power of the transformer

Three-phase total power / 0.8 is the most important step. At present, more than 90% of the transformers sold on the market have a power factor of only 0.8, so it needs to be divided by the power factor of 0.8.

33KW/0.8=41.25KW (total transformer power)

4、Calculate the total transformer capacity

The total power of the transformer/0.85, according to the "Power Engineering Design Manual", the transformer capacity should be selected according to the calculated load. For a single transformer with a stable load power supply, the load rate is generally about 85%.

41.25KW/0.85=48.529KW (transformer power to be purchased), then you can choose a 50KVA transformer when purchasing.