How to Calculate Solid-State Transformer Capacity Correctly

The transformer capacity refers to the apparent power at the transformer's main tap position, and the capacity indicated on the transformer nameplate is the rated capacity. In the operation of power transformers, there are cases of under-loading due to excessive capacity, as well as instances of overloading or overcurrent operation leading to equipment overheating and even burnout. These improper capacity matching practices directly affect the reliability and economy of power supply in electrical systems. Therefore, determining the appropriate transformer capacity is crucial to ensuring reliable and economical power system operation.

The capacity calculation for solid-state transformers must consider the following factors:

• Input Voltage: The input voltage refers to the voltage value supplied to the transformer. Solid-state transformers typically have a specified input voltage range (e.g., 220V ~ 460V), and an appropriate transformer should be selected based on this range.

• Output Voltage: The output voltage refers to the voltage value delivered by the transformer. Solid-state transformers also have a defined output voltage range (e.g., 80VAC ~ 480VAC), which must be considered when selecting a suitable transformer.

• Rated Capacity: The rated capacity indicates the maximum load capacity the transformer can handle, usually expressed in kilovolt-amperes (kVA). The rated capacity is typically determined based on demand; if the load requires a large total current, a transformer with a larger capacity must be selected.

• Input Power: Input power equals the input voltage multiplied by the input current, generally expressed in kilowatts (kW).

Therefore, considering these factors, the capacity calculation formula for a solid-state transformer can be expressed as:
Capacity (kVA) = Input Voltage (V) × Input Current (A) / 1000.

Note: Solid-state transformers differ from traditional power transformers. A solid-state transformer is a combination of a converter and a transformer, making it highly suitable for static power conversion applications. However, its calculation methods differ from those of conventional transformers.

The capacity calculation methods for single-phase and three-phase transformers are similar. The following explanation uses three-phase transformer capacity calculation as an example. The first step in transformer capacity calculation is to determine the maximum power per phase of the load (for single-phase transformers, this is simply the maximum single-phase load power).

Sum the load power independently for each phase (A, B, and C). For example, if the total load power on phase A is 10 kW, phase B is 9 kW, and phase C is 11 kW, take the maximum value, which is 11 kW.

Note: For single-phase devices, the power per unit is taken as the maximum value listed on the device nameplate. For three-phase equipment, divide the total power by 3 to obtain the per-phase power. For instance:
Total load power on phase C = (300W × 10 computers) + (2kW × 4 air conditioners) = 11 kW.

The second step in transformer capacity calculation is to determine the total three-phase power. Use the maximum single-phase power to calculate the total three-phase power:
Maximum single-phase power × 3 = Total three-phase power.

Using the maximum phase C load power of 11 kW:
11 kW × 3 (phases) = 33 kW. Thus, the total three-phase power is 33 kW.

Currently, over 90% of transformers available on the market have a power factor of only 0.8. Therefore, the total power must be divided by 0.8:
33 kW / 0.8 = 41.25 kW (required transformer apparent power in kW).

According to the Electrical Engineering Design Manual, transformer capacity should be selected based on calculated load. For a single transformer supplying a steady load, the load factor β is generally taken as around 85%. This is expressed as:
β = S / Se
Where:
S — Calculated load capacity (kVA);
Se — Transformer capacity (kVA);
β — Load factor (typically 80% to 90%).

Thus:
41.25 kW (apparent power requirement) / 0.85 = 48.529 kVA (required transformer capacity).Therefore, a 50 kVA transformer would be suitable.