Short Answer:

Inrush current in transformers is the sudden surge of current that flows into the transformer when it is first energized. This current is much higher than the normal operating current and lasts for a few milliseconds to a few seconds.

It happens because the magnetic core of the transformer needs to be magnetized when power is first switched on. During this time, the core draws a large amount of current, especially if the switching is done near the zero-crossing of the voltage waveform. Although temporary, inrush current can cause mechanical stress and affect protection devices.

Detailed Explanation:

Inrush current in transformers

Inrush current is a common phenomenon that occurs in transformers during their initial energization. When a transformer is switched on, especially for the first time or after being off for a while, it draws a very large current from the supply. This current is called inrush current, and it can be 6 to 10 times higher than the normal full-load current of the transformer.

This sudden rush of current is not due to a fault or overload, but due to the behavior of the magnetic core and how it reacts to the applied voltage at the moment of switching.

Why Inrush Current Occurs:

When a transformer is energized, the magnetic core needs to build up a magnetic field. If the core is not magnetized or has some residual flux from a previous operation, it takes a large magnetizing current to align the magnetic domains in the core. This leads to a high current flow in the primary winding.

The magnitude of inrush current depends on several factors:

1. Point on the AC Waveform:
   If the transformer is switched on at the zero-voltage point, the inrush current is highest because the flux reaches a peak very quickly.
2. Residual Magnetism:
   If there is some remaining magnetism (residual flux) in the core from previous use, and it adds to the newly applied flux, the total flux can exceed the core’s saturation limit, causing more current draw.
3. Transformer Size:
   Larger transformers with bigger cores experience higher inrush currents.
4. Impedance and Supply Strength:
   The strength of the power source and the internal impedance of the transformer also affect the inrush current magnitude.

Effects of Inrush Current:

1. Mechanical Stress:
   The sudden high current can cause physical stress in the windings due to electromagnetic forces.
2. Nuisance Tripping:
   Protection devices like circuit breakers and relays might interpret inrush current as a short circuit and trip unnecessarily.
3. Voltage Dip:
   Inrush current can cause a temporary dip in voltage in the connected power system, affecting other nearby devices.
4. Transformer Aging:
   Repeated exposure to high inrush currents can reduce the lifespan of the transformer.

How to Reduce Inrush Current:

1. Point-on-Wave Switching:
   Switching at the voltage peak rather than at the zero-crossing helps reduce inrush current.
2. Pre-insertion Resistors:
   Temporarily inserted resistors during switching limit the current flow.
3. Controlled Switching Devices:
   Intelligent circuit breakers and contactors can control the exact switching time.
4. Magnetic Flux Control:
   Ensuring the transformer is de-energized in a way that reduces residual magnetism.

Conclusion:

Inrush current is a natural and temporary surge of current in transformers that occurs during initial energization. It is mainly due to the need to magnetize the core and can be influenced by switching conditions and residual flux. While it does not indicate a fault, managing inrush current is important to protect transformer components and avoid false tripping of protection systems. Proper design and switching techniques help in minimizing its effects.