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Importance o f Transf ormer Inrush Current (on pho to : Data Center Power Transf ormer bydigitalrealtytrust @ Flickr)

http://electrical-engineering-portal.com/importance-of- transformer- inrush-current February 8, 2013

Importance of Transformer Inrush Current

Edvard

Residual Flux

When a transformer is taken of f-line, there will be a certain amount ofresidual fluxthat canremain in the core due to t he properties of the magnetic core material.

The residual flux can be as much as 50 to 90% of the maximum operating flux, depending onthe type of core steel. When voltage is reapplied to t he transformer, the flux introduced by thissource voltage will build upon that which already exists in the core.

In order to maintain this level of f lux in the core, which can be well into the saturation rangeof the core steel, the t ransformer can draw current well in excess of the transformers rated fullload current.

Depending on the transformer design, the magnitude of this current inrush can be anywherefrom 3.5 to 40 times the rated full load current. The waveform of the inrush current will besimilar to a sine wave, but largely skewed towards the posit ive or negative direction. This inrush

current will experience a decay, part ially due to transformer losses, which will provide adampening ef fect; however, the current can remain well above rated current for many cycles.

This inrush current can have an ef fect on the operation of relays and fuses located in thesystem near the transformer.

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7/28/2019 Importance of Transformer Inrush Current

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Decent approximations of the inrush current require detailed information regarding thetransformer design which may be available f rom the manufacturer but is not typically availableto t he user.

Actual inrush currents will also depend upon where in the source voltage wave the switchingoperat ions occur, the moment of opening eff ecting the residual f lux magnitude, and themoment o f closing eff ecting the new flux.

Resource: James H. Harlow Harlow Engineering Associates