Transformer capacity calculation
First, select the rated voltage of the transformer. The voltage on the high-voltage side is equal to the voltage of the connected power grid, and the voltage on the low-voltage side is 10% or 5% higher than the voltage of the low-voltage side power grid (depending on the transformer voltage level and impedance voltage); rated capacity selection.
Calculate the load carried by the transformer (requires statistics of the maximum comprehensive load, and converts the active load kW value into apparent power kVA). If there are two transformers, the capacity of each transformer can be selected according to 70% of the maximum comprehensive load. One transformer should be considered according to the total load, and an appropriate margin should be left. Other brand parameters can be appropriately considered in combination with the transformer product.
For example: select a 35/10kV transformer. Assuming the maximum load is 3500kW and the power factor is 0.8, select two transformers, capacity S=0.7×3500/0.8=3062kVA, you can choose a 3150kVA transformer, the voltage ratio is 35kV/10.5kV. Then select the model from the product catalog.
1. Transformer capacity calculation formula
1.1. Calculate the maximum power of each phase of the load
Add the load power of each phase of phase A, phase B, and phase C independently, such as the total load power of phase A is 10KW, the total load power of phase B is 9KW, and the total load power of phase C is 11KW, and take the maximum value of 11KW. (Note: The power of each single-phase device is calculated according to the maximum value on the nameplate, and the power of the three-phase device is divided by 3, which is equal to the power of each phase of this device.) For example: Total load power of phase C = 300W × 10 computers) + (2KW × 4 air conditioners) = 11KW
1.2. Calculate the total power of three phases
11KW × 3 phases = 33KW (total power of three phases of transformer)
Total power of three phases / 0.8, this 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 a power factor of 0.8.
33KW/0.8=41.25KW (total transformer power) total transformer power/0.85. According to the "Electric Power Engineering Design Manual", the transformer capacity should be selected according to the calculated load. For a single transformer with stable load power supply, the load rate is generally about 85%.
41.25KW/0.85=48.529KW (the power of the transformer to be purchased), so you can choose a 50KVA transformer when purchasing.
2. Some issues about transformer capacity calculation
2.1. The rated capacity of the transformer should be the maximum load apparent power that can ensure the normal operation of the transformer under the specified conditions of use;
2.2. This apparent power is the output power of the transformer, and it is also the apparent power that the transformer can carry the maximum load;
2.3. When the transformer is in rated operation, the output apparent power of the transformer is equal to the rated capacity;
2.4. When the transformer is in rated operation, the input apparent power of the transformer is greater than the rated capacity;
2.5. Since the efficiency of the transformer is very high, it is generally believed that when the transformer is in rated operation, the input apparent power of the transformer is equal to the rated capacity, and the calculation and results are basically accurate;
2.6. So when using the transformer, you only need to observe the current, voltage, power factor and apparent power of the transformer output equal to or less than the rated capacity to be safe (when the conditions of use are met);
2.7. Some people think that the transformer has losses and must be operated below 90% of the rated capacity, which is wrong!
2.8. When designing and selecting the capacity of the transformer, it is correct to multiply the safety factor according to the calculated load.
It can be seen that the transformer capacity calculation is not difficult. The main thing is to pay attention to some issues in the transformer capacity calculation.
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