How to calculate short circuit current for transformer?

A 1600 kVA electrical transformer short-circuit fault current calculation is essential for understanding the potential electrical faults and ensuring the system’s safety. The fault current refers to the current that flows when a short circuit occurs, which can be much higher than the normal operating current. Calculating this value helps in the selection of protective devices such as circuit breakers and fuses.

Key Parameters for Calculation:

1. Transformer Rating: The transformer in question has a rating of 1600 kVA (kilo-volt-amps).

2. Primary and Secondary Voltages: Typically, the transformer's voltage rating will be provided, for example, 11kV/400V (primary/secondary). The actual voltage levels need to be considered in the calculation.

3. Impedance (Z): The impedance of the transformer, usually expressed as a percentage (e.g., 5% or 6%), is a critical factor. Impedance limits the fault current and is a measure of the opposition that the transformer offers to the flow of electrical current during a fault condition.

Short-Circuit Fault Current Calculation:

The short-circuit current can be calculated using the following formula:

$$I_{sc} = \frac{S_{transformer}}{(Z \times V_{secondary})}$$

Where:

• $I_{sc}$ = Short-circuit current (in amperes)
• $S_{transformer}$ = Transformer rating in VA (1600 kVA = 1,600,000 VA)
• $Z$ = Transformer impedance (in per unit or percentage)
• $V_{secondary}$ = Secondary voltage (in volts, typically 400V for low voltage transformers)

Example:

For a 1600 kVA transformer with a 5% impedance and a secondary voltage of 400V:

1. First, convert the transformer's impedance percentage to per unit: $Z = \frac{5}{100} = 0.05$

2. Apply the values to the formula:

$$I_{sc} = \frac{1,600,000}{(0.05 \times 400)} = \frac{1,600,000}{20} = 80,000A$$

Thus, the short-circuit fault current for this transformer would be approximately 80 kA (kilo-amperes).

Importance of Fault Current Calculation:

• Safety: Knowing the fault current ensures that the electrical system can be designed with adequate protection to handle potential faults, reducing the risk of damage to equipment and personnel.
• Selection of Protective Devices: Circuit breakers, fuses, and other protective devices must be rated for the maximum fault current to prevent system failure during a short circuit.
• Coordination of Protection Systems: Proper fault current calculation helps in designing a coordinated protection system, ensuring that faults are isolated quickly and effectively.

Conclusion:

The calculation of short-circuit fault current for a 1600 kVA transformer is a crucial step in electrical system design. It helps in the selection of appropriate protective devices and ensures the safety and reliability of the electrical network.