1How to choose the rated voltage and rated capacity of the transformer
Transformer rated voltage selection
The high-voltage side voltage is equal to the connected grid voltage, and the low-voltage side voltage is 10% or 5% higher than the low-voltage side grid voltage (depending on the transformer voltage level and impedance voltage)
Rated capacity selection
Calculate the load of the transformer (requires statistical maximum combined load, convert the active load kW value into apparent power kVA). If it is two transformers, the capacity of each transformer can be selected according to 70% of the maximum combined load. The transformer should be considered for the total load and with appropriate margin. Other brand name parameters can be properly considered in conjunction with transformer products
Case
How to choose a 35/10kV transformer?
Assume that the maximum load is 3500kW and the power factor is 0.8.
Then choose two transformers, the capacity S = 0.7 × 3500 / 0.8 = 3062kVA, choose 3150kVA transformer, the voltage ratio is 35kV / 10.5kV.
Then select the model from the catalog.
2 transformer capacity calculation formula
Calculation of maximum load power per phase
The load power of each phase of phase A, phase B and phase C is added separately. For example, the total power of phase A load is 10KW, the total power of phase B load is 9KW, the total power of phase C load is 11KW, and the maximum value is 11KW. (Note: The power of each unit of single phase is calculated according to the maximum value on the nameplate. The power of the three-phase equipment is divided by 3, which is equal to the power of each phase of this equipment.)
For example: C phase load total power = computer 300WX10 units) + (air conditioning 2KWX4 units) = 11KW
Calculation of three-phase total power
11KWX3 phase = 33KW (transformer three-phase total power)
The three-phase total power / 0.8, this is the most important step, the current market sales of transformers more than 90% of the power factor is only 0.8, so you need to divide by 0.8 power factor.
33KW/0.8=41.25KW (total power of transformer)
The total power of the transformer is 0.85. According to the "Electric Engineering Design Manual", the transformer capacity should be selected according to the calculation load. For a single transformer that supplies power to a smooth load, the load rate is generally about 85%.
41.25KW/0.85=48.529KW (the transformer power that needs to be purchased), then you can choose a 50KVA transformer at the time of purchase.
Transformer capacity calculation is confusing
1. The rated capacity of the transformer shall be the maximum load apparent power of the transformer under the specified conditions of use to ensure normal operation of the transformer;
2. This apparent power is the output power of the transformer and the apparent power of the transformer with the maximum load;
3. When the transformer is rated for operation, the apparent output of the transformer is equal to the rated capacity;
4. When the transformer is rated for operation, the input apparent power of the transformer is greater than the rated capacity;
5, due to the high efficiency of the transformer, it is generally believed that when the transformer is rated for operation, the input apparent power of the transformer is equal to the rated capacity, and the calculations and results are basically accurate;
6, so when using the transformer, you only need to observe the transformer output current, voltage, power factor and its apparent power is equal to or less than the rated capacity is safe (when the conditions of use are met);
7. Some people think that transformers have losses, and it is wrong to operate below 90% of rated capacity!
8. When the transformer is designed for capacity, it is correct to multiply the safety factor by the calculated load.
In fact, the calculation of transformer capacity is not difficult, mainly to pay attention to the problem that transformer capacity calculation is often overlooked.
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