. The high-voltage side voltage is equal to the voltage of the connected grid, and the low-voltage side voltage is 10% or 5% higher than the voltage of the low-voltage side grid (depending on the voltage level of the transformer and the impedance voltage); rated capacity selection.
(it is required to count the maximum comprehensive load and convert 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. The transformer should be considered according to the total load, and an appropriate margin should be left. Other famous brand parameters can be appropriately considered in combination with transformer products.
For example: select 35/10kV transformer. Assume that the maximum load is 3500kW and the power factor is 0.8. Choose two transformers with capacity S=0.7×3500/0.8=3062kVA. You can choose a 3150kVA transformer with a voltage ratio of 35kV/10.5kV. Then select the model from the product catalog.
1. Calculate the maximum power of each phase of the load
Add the load power of each phase A, B, and C independently. For example, the total power of phase A load is 10KW, the total power of phase B load is 9KW, and the total power of phase C load is 11KW. , take the maximum value of 11KW. (Note: The power of each single-phase device is calculated according to the maximum value on the nameplate. The power of three-phase devices divided by 3 is equal to the power of each phase of this device.)
2. Calculate the total power of the three phases
11KW × 3 phases=33KW (total power of three phases of the transformer)
Three-phase total power/0.8, is the most important step. More than 90% of the transformers currently sold on the market have a power factor of only 0.8, so the power factor needs to be divided by 0.8.
33KW/0.8=41.25KW (total power of transformer)
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 that supplies power with a stable load, the load factor is generally about 85%.
41.25KW/0.85=48.529KW (the power of the transformer that needs to be purchased), then just choose a 50KVA transformer when purchasing.
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. 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;
3. When the transformer is in rated operation, the output apparent power of the transformer is equal to the rated capacity;
4. When the transformer is in rated operation, the input apparent power of the transformer is greater than the rated capacity;
5. Because the efficiency of the transformer is very high, it is generally believed that when the transformer is operating at rated speed, the input apparent power of the transformer is equal to the rated capacity, and the calculations and results are basically accurate;
6. Therefore, when using a transformer, it is safe as long as you observe that the current, voltage, power factor, and apparent power output by the transformer are equal to or less than the rated capacity (when the usage conditions are met);
7. Some people think that the transformer has losses and must operate below 90% of the rated capacity. This is wrong!
8. When designing and selecting the capacity of the transformer, it is correct to multiply the safety factor based on the calculated load.
