Minimizing Transformer Inrush Current

I have used three different Methods depending on the details of a specific project plus I do not use the Method 4.

Method 1. Secondary pre-charge
This is normally the most economical Method. It uses a small pre-charge transformer to apply a voltage to the transformer's secondary and I normally use 90% of the rated transformer's secondary voltage. The required current is approximately the transformer's magnetizing current. A typical value is 0.5% of the rated winding current for a transformer with a single secondary. A resistor is normally used between the pre-charge transformer and the main transformer to limit the initial current transient.

For this Method 1 to work a suitable AC voltage is required to feed the pre-charge transformer that is in phase with the operating voltage of the main transformer's secondary winding. When the pre-charge current is applied the transformer's primary voltage will eventually settle at to approximately 90% of the rated primary voltage. Most importantly the voltage across the transformer's primary breaker will be very low so that it can be closed with virtually no inrush current.

I normally use a check-synch relay for the transformer's primary breaker as a safety check. Provided the transformer's primary voltage on pre-charge is above 80% of rated the transformer's inrush current will be low. This gives the basis of the Method 1 and extra parts are needed for protection switching, isolation and optimal operation.

Method 2. Primary pre-charge
This Method 2 can be used when a suitable AC power supply for Method 1 is not available, but needs an extra AC supply energizing switch in addition to the transformer's primary breaker. The extra AC power supply energizing switch is connected via 3 pre-charge resistors to the transformer's primary connections.

When the extra AC power supply energizing switch is operated a lower inrush current can be achieved. After a short delay the transformer's primary breaker is energized which shorts out the 3 pre-charge resistors, that can be short time rated. This gives the basis of the Method 2 and extra parts are needed for protection switching, isolation and optimal operation.

Method 3. Transformer design
For Method 2 the cost of the extra parts can be significant. Method 3 is used when either Methods 1 & 2 are not viable or Method 3 is acceptable versus the extra cost. Data from the internet gives the following data shown in ""

"Transformers can be designed for reduced inrush current characteristics by tweaking some of the variables in the design process but this requires certain compromises and inevitably a higher cost. Some factors that contribute to reducing the inrush current are:

  1. Lower operating flux density reduces inrush. Typically larger core and/or more turns.
  2. Leakage Impedance. Leakage of primary winding to core not just impedance defined by the primary/secondary leakage.
  3. Large primary winding area. Position in winding, layers and air ducts.
  4. Relative number of conductor turns and relative core size
  5. Core magnetic properties and its geometry."

I have used high impedance transformers, with impedances of 15%, to minimize inrush current.

Method 4. Differential plus timed closing of transformer's primary breaker
I have not used this Method 4 for transformers due to the unknown residual flux level. When the transformer's primary breaker is opened its currents will continue to a near zero value. However some breakers will arc chop at a low current level leaving residual flux in the transformer's core.

Differential plus timed closing of transformer's primary breaker works very well for avoiding over voltage transients in harmonic / power factor correction capacitor circuits.

For good results it is essential to discharge the capacitors before closing and a simple circuit is used to do this.

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10/8/2016 6:26 AM
Is inrush current seen by the protective relay at the primary side or secondary side of transformer?
10/8/2016 6:39 AM
Inrush current is dependent on where in the voltage cycle the transformer is switched on. In steady state operation the applied voltage is opposed by the rate of change of flux, thus at peak voltage the rate of change of flux is at its peak and is passing through zero. So in simplistic terms if switching on of a transformer is done at peak voltage when the flux is zero there will be no inrush current, but if switching is done at zero voltage then there will be the largest inrush current as the transformer saturates in the next half cycle of the applied voltage. This will continue for every other half cycle to a lesser extent each time until the flux attains the steady state sinusoidal form. During inrush the flux will be a distorted sinusoid with a decaying dc component. Hence the inrush current will be distorted. I have seen inrush currents of 10 times normal full load current, but it depends very much on the B-H curve and how conservative the designer has been.
2/4/2017 11:09 AM
relation between permeability of core and inrush current