Transformer inrush current refers to the high peak current that flows into a transformer's primary winding when it is energized. This inrush current can cause voltage sags, disrupt the operation of other equipment on the same electrical system, and potentially damage the transformer and associated equipment. Here are several methods to reduce transformer inrush current:

1. Transformer Design:

   • Using transformers with lower leakage reactance and higher core saturation levels can help reduce inrush currents.
   • Employing transformers with special designs, such as zig-zag or flux-cancellation transformers, can also mitigate inrush currents.

2. Use of Voltage Regulators:

   • Voltage regulators can be installed on the primary side of the transformer to limit the initial voltage applied to the transformer, thereby reducing the inrush current.

3. Controlled Switching:

   • Gradually energizing the transformer by using controlled switching devices such as soft starters or phase-angle control devices can reduce the abruptness of the voltage change and limit the magnitude of the inrush current.
   • In systems with multiple transformers, staggered energization of transformers can also help reduce the overall inrush current.

4. External Resistors:

   • Connecting external resistors in series with the primary winding during energization can limit the rate of rise of the magnetizing current, thereby reducing the peak inrush current.
   • These resistors are typically shorted out once the transformer has reached full operating voltage.

5. Use of Pre-insertion Resistors or Reactors:

   • Pre-insertion resistors or reactors can be connected in series with the primary winding temporarily during energization to limit the rate of rise of the energizing current and reduce the inrush current.

6. Inrush Current Limiting Devices:

   • Inrush current limiting devices such as inrush current limiters (ICLs) or surge arresters can be installed in parallel with the transformer winding to absorb excess energy during energization and reduce the peak inrush current.

7. Tap Changers:

   • Tap changers can be used to adjust the transformer's turns ratio during energization, which can help reduce the inrush current by adjusting the magnetizing impedance.

8. Proper Sizing of Fuses and Circuit Breakers:

   • Ensuring that fuses and circuit breakers are properly sized to accommodate the inrush current without nuisance tripping can help prevent unnecessary disruptions to the system.

9. System Modeling and Analysis:

   • Conducting detailed modeling and analysis of the electrical system using simulation software can help identify potential issues with transformer inrush current and evaluate various mitigation strategies.

By implementing one or more of these methods, it's possible to effectively reduce transformer inrush current and mitigate its associated impacts on the electrical system. The most suitable approach will depend on factors such as the specific characteristics of the transformer, the requirements of the electrical system, and cost considerations.