How to Choose the Rated Voltage and Rated Capacity of a Transformer
Choosing the correct rated voltage and capacity for a transformer is essential to ensure efficient and safe operation. The rated voltage on the high-voltage side should match the grid voltage it’s connected to, while the low-voltage side is usually 5% or 10% higher than the grid voltage, depending on the transformer’s voltage level and impedance.
When selecting the rated capacity, you need to calculate the total load that the transformer will handle. This involves determining the maximum combined load and converting the active power (in kW) into apparent power (in kVA). For two transformers, each one can be sized at around 70% of the maximum load. It’s important to consider the total load and include some extra margin to account for future growth or unexpected demand.
For example, if the maximum load is 3,500 kW with a power factor of 0.8, the required capacity would be: 0.7 × 3,500 / 0.8 = 3,062 kVA. You could then choose two 3,150 kVA transformers with a voltage ratio of 35 kV / 10.5 kV. After selecting the right model from the manufacturer's catalog, you can proceed with installation.
Transformer Capacity Calculation Formula
To calculate the maximum load per phase, add up the power for each individual phase (A, B, and C). For instance, if Phase A has 10 kW, Phase B has 9 kW, and Phase C has 11 kW, the maximum is 11 kW. Remember, single-phase equipment should use the maximum nameplate value, while three-phase equipment should divide its total power by three for each phase.
For example, the total power for Phase C might be calculated as: (300 W × 10 units) + (2 kW × 4 units) = 11 kW. Then multiply this by 3 to get the total three-phase power: 11 kW × 3 = 33 kW.
Next, divide this by the power factor (usually 0.8 in most cases) to get the apparent power: 33 kW / 0.8 = 41.25 kVA.
According to the "Electric Engineering Design Manual," the transformer’s capacity should be based on the calculated load. If a single transformer is supplying a steady load, it should operate at about 85% of its rated capacity. So, 41.25 kVA / 0.85 ≈ 48.53 kVA. Therefore, a 50 kVA transformer would be a suitable choice.
Understanding Transformer Capacity
1. The rated capacity of a transformer is the maximum apparent power it can deliver under normal operating conditions.
2. This is also known as the output power when the transformer is at its maximum load.
3. When operating at rated capacity, the transformer’s output is equal to its rated capacity.
4. The input power, however, is slightly higher due to losses.
5. Despite these losses, it's commonly assumed that the input power equals the rated capacity for practical calculations.
6. As long as the output current, voltage, and power factor stay within the rated capacity, the transformer is considered safe.
7. Some people mistakenly believe that transformers should not operate below 90% of their rated capacity.
8. It is recommended to include a safety factor when designing the transformer’s capacity.
In reality, calculating transformer capacity isn’t complicated, but it requires attention to detail. Always make sure to account for all factors such as load balance, power factor, and future expansion needs. Proper planning ensures reliable performance and avoids unnecessary costs.
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