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Practical guide to understanding transformer inrush current  

Transformer inrush current defined

This provides the energy needed to magnetize the core. It’s typically much bigger than the normal operating current that flows through a transformer. The actual inrush current depends on the core design and residual magnetism in the core. 

In the chart below, the flux density (B) of a transformer in operation follows the hysteresis loop. As the sinusoidal input voltage transitions from positive to negative, the magnetizing current will respond in kind, driving the flux density developed in the core. When power is disconnected, a residual magnetic flux (Br) remains in the transformer. The magnitude depends on the magnetic properties of the material.

The chart below shows two scenarios. If power is disconnected at the peak magnetizing current illustrated by the red curve, there will be a residual magnetism left in the core as indicated by the (Br) associated with the red curve. If power is disconnected at a value below the peak magnetizing current as shown by the blue curve, there will also be residual magnetism, but it will be lower as shown by the (Br) value associated with the blue curve. It should be noted that magnetizing current can be positive or negative, and the resulting magnetism having a positive or negative polarity.

   Properly protect step-up transformers

  Designing step-up transformer applications

  Six steps to properly protect step-up transformers

 

Residual flux at different flux densities

eaton-hpx-figure-en-us (1).png

The residual magnetism is dependent on where in the hysteresis loop power is removed. This residual magnetism, and the point in the sine wave power is reapplied also drives the inrush current required by the transformer. The figure below illustrates a scenario that can lead to maximum inrush. The blue line represents the nominal flux density (Bm) that occurs in the transformer. The red line illustrates nominal magnetizing current. It has a non-linear relationship to the sinusoidal input voltage.

When power is disconnected at the instance shown, there is a residual magnetism indicted by (Br+). If power is reconnected at the point in the cycle where magnetizing current would be in direct opposite polarity, the transformer coil becomes saturated and peak inrush current results.

Residual flux density and inrush

eaton-hpx-figure-en-us (2).png
The peak inrush current that occurs during transformer energization typically dissipates within a few cycles. The next figure shows a typical dissipation curve. The rate at which the inrush current dissipates is directly related to the coil impedance. Coil losses are also a consideration for transformer designers, so they are a factor in designs for energy efficiency. In other words, reducing coil impedance to improve efficiency can extend the time for the inrush to dissipate, and overcurrent protection must be sized to handle inrush while still providing proper protection.

Typical magnetizing inrush current on a single-phase transformer

eaton-hpx-figure-en-us (3).png

The maximum possible inrush on a transformer would require a perfect alignment of three factors. 

  1. Power would need to be removed when the transformer is at maximum flux density.
  2. The maximum residual flux would have to be driven to saturation prior to power being removed.
  3. Reapplication of power would need to occur at the exact point where flux density would have been the direct opposite polarity of the residual magnetism.

The reality is that the maximum possible inrush current almost never occurs. For this reason, Eaton provides maximum practical inrush values, which are recommended for proper sizing of overcurrent protective equipment.

Understand what it takes to protect step-up transformers

Our white paper provides technical recommendations to advance safety and understand the related electrical codes to adequately size overcurrent protective devices.

Download the whitepaper

  • Communicate with an expert

    Do you have additional questions or want to connect with someone well-versed in how to protect step-up transformers? We have experts ready to answer any questions you may have.
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