Question

How do you calculate the short-circuit impedance of a transformer?

Answer 1

Calculating the short-circuit impedance of a transformer involves determining the equivalent impedance that the transformer presents to the flow of short-circuit current. This impedance is useful in understanding the fault current and voltage drop during a short-circuit condition. The short-circuit impedance of a transformer is typically represented as a percentage of the transformer's rated voltage.

To calculate the short-circuit impedance of a transformer, you will need the following information:

Rated Voltage: The voltage at which the transformer is designed to operate under normal conditions.

Rated Power (kVA): The maximum apparent power that the transformer can deliver at its rated voltage and frequency.

Short-Circuit Test Data: This data is obtained from the short-circuit test performed on the transformer. It includes the short-circuit current and the corresponding applied voltage at a specific rated voltage and frequency.

Once you have this information, follow these steps to calculate the short-circuit impedance:

Step 1: Determine the Short-Circuit Power (kVA)
The short-circuit power is the apparent power that the transformer can deliver under short-circuit conditions. It can be calculated using the formula:

Short-Circuit Power (kVA) = (Short-Circuit Current)^2 * Impedance Voltage

where Impedance Voltage is the voltage applied during the short-circuit test (usually expressed as a percentage of the rated voltage).

Step 2: Calculate Short-Circuit Impedance (Zsc)
The short-circuit impedance (Zsc) is given by the formula:

Zsc = Rated Voltage / Short-Circuit Power

Step 3: Express Short-Circuit Impedance as a Percentage
To express the short-circuit impedance as a percentage, divide it by the transformer's rated voltage and multiply by 100:

Short-Circuit Impedance (%) = (Zsc / Rated Voltage) * 100

Keep in mind that transformer impedance is typically specified on the transformer nameplate, and the short-circuit impedance can be one of the values provided.

It's essential to note that short-circuit impedance can vary depending on the type of transformer (e.g., power transformers, distribution transformers), the construction, and the design specifications. Therefore, it's always best to refer to the transformer manufacturer's documentation or consult with a qualified electrical engineer to obtain accurate and up-to-date information for a specific transformer.