How to Choose the Rated Voltage and Rated Capacity of a Transformer
Transformer rated voltage is an important factor in ensuring proper operation. The high-voltage side should match the grid voltage, while the low-voltage side is typically 10% or 5% higher than the grid voltage, depending on the transformer's voltage level and impedance.
When selecting the rated capacity of a transformer, it’s essential to calculate the load. This involves determining the maximum combined load and converting active power (kW) into apparent power (kVA). For two transformers, each can be sized at around 70% of the total maximum load. It’s also important to consider the total load with some margin for future expansion or unexpected increases. Additionally, other parameters such as brand specifications and product availability should be taken into account during the selection process.
Example: Choosing a 35/10kV Transformer
Suppose the maximum load is 3500 kW with a power factor of 0.8. In this case, the required capacity per transformer would be calculated as follows:
S = 0.7 × 3500 / 0.8 = 3062 kVA. A 3150 kVA transformer would be suitable, with a voltage ratio of 35 kV / 10.5 kV. Once the size is determined, you can refer to the manufacturer’s catalog to select the appropriate model.
Transformer Capacity Calculation Formula
To calculate the maximum load per phase, add up the power for each individual phase (A, B, and C). For example, if phase A has 10 kW, phase B has 9 kW, and phase C has 11 kW, the maximum load is 11 kW. When calculating single-phase loads, use the maximum value from the nameplate. For three-phase equipment, divide the total power by 3 to determine the per-phase load.
Example: Total Load for Phase C
Phase C load = (Computer: 300 W × 10 units) + (Air Conditioning: 2 kW × 4 units) = 11 kW
Total Three-Phase Power = 11 kW × 3 = 33 kW
Next, divide the total power by the power factor (usually 0.8 for most transformers), which gives the apparent power.
33 kW / 0.8 = 41.25 kVA
According to the "Electric Engineering Design Manual," the transformer capacity should be selected based on the calculated load. If a single transformer is used and the load is relatively smooth, the load rate is generally around 85%. So, the required transformer capacity would be:
41.25 kW / 0.85 = 48.53 kW
This means a 50 kVA transformer would be a suitable choice.
Transformer Capacity Calculation Considerations
1. The rated capacity of a transformer should be the maximum apparent power it can handle under normal operating conditions.
2. This apparent power represents the output power when the transformer is fully loaded.
3. When operating at rated capacity, the transformer’s output power equals its rated capacity.
4. The input apparent power is usually slightly higher due to losses, but for practical purposes, it is often assumed equal to the rated capacity.
5. Transformers are highly efficient, so calculations using rated capacity are generally accurate.
6. To ensure safe operation, monitor the transformer’s current, voltage, and power factor. The apparent power should not exceed the rated capacity.
7. Some believe that transformers should never operate below 90% of their rated capacity, but this is a common misconception.
8. When designing a transformer system, it is correct to include a safety factor when sizing based on calculated load.
In reality, calculating transformer capacity is straightforward once you understand the key factors involved. Paying attention to these details ensures that the transformer operates efficiently and safely over its lifetime.
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