Transformer Inrush Current

In this post we will discuss the Magnetizing Inrush Current in a Power Transformer. Magnetizing Inrush urrent in Transformers results from any abrupt changes of the magnetizing voltage. This current in transformer may be caused by energizing an unloaded transformer, occurrence of an external fault, voltage recovery after clearing an external fault and out-of phase synchronizing of connected Generator.Because the amplitude of inrush current can be as high as a short circuit current, a detailed analysis of the magnetizing inrush current under various conditions is necessary for the making required setting of protective system for the Transformers.

First question which will come up in your smart mind, why a Power Transformer takes Inrush Current when energized in unloaded condition?

When a power transformer is energized while keeping its secondary circuit open, it acts as an inductance. In normal condition of a Power Transformer, the flux produced in the core is in quadrature with applied voltage i.e. Flux lags behind the applied voltage by 90° as shown in the figure below.

This means, Flux wave will reach its maximum value after 1/4 cycle or π/2 angle reaching maximum value of voltage wave. Hence as per the waves shown in the figure, at the instant when, the voltage is zero; the corresponding steady state value of flux should be negative maximum. But practically it is not possible to have flux at the instant of switching on the supply of Transformer. This is because, there will be no flux linked to the core prior to switching on the supply. The steady state value of flux will only reach after some finite time which in turn depends upon how fast the circuit can take energy. So the flux in the core also will start from its zero value at the time of switching on the transformer.

As we know that,
e = dφ/dt  where φ is the Flux in the core
Therefore assuming e = ESinωt,
Now suppose, Transformer is switched on when Voltage is zero. Therefore Flux will also start from zero. Therefore, total Flux at the end of first half cycle of voltage wave will be,

Where Øm = Maximum flux in the core in steady state or normal operating condition.

Therefore, the flux in the core of Transformer will be double the maximum value of flux in steady state condition. This phenomenon is also shown in figure below.

It is clear from the above graph that maximum flux in the core of Transformer will be 2Ømwhen the applied voltage is at its zero.

Now what will happen because of this higher value of flux in the core of Transformer?

Transformer core is saturated just above the maximum steady state value of flux Øm. But when we switch on power supply to the Transformer’s primary, the maximum value of flux will jump to double of its steady state maximum value Øm. As, after steady state maximum value of flux Øm, the Transformer core becomes saturated, the  current  required to produced rest (2Øm-Øm = Øm) of flux will be very high. So transformer primary will draw a very high current from the source which is called Magnetizing Inrush Current in Transformer or Inrush Current in Transformer. The nature of Transformer Inrush Current is shown in figure below.

It should be noted that waveform of Transformer Inrush Current is asymmetric which means in Transformer Inrush current mainly 2nd harmonic component will be present. It shall also be noted from the waveform that as time passes the Magnetizing Inrush Current of Transformer decays and becomes zero. Normally it takes few millisecond for Magnetizing Inrush Current to decay to zero.

The Magnetizing Inrush Current of Transformer may be up to 10 times higher than normal rated  current  of Transformer. Even though the magnitude of Magnetizing Inrush Currentis so high but it generally does not create any permanent fault in Transformer as it exists for few miliseconds. But still Magnetizing Inrush Current in Power Transformer is a problem, because during the time of Magnetizing Inrush Current the protection scheme of Transformer may operate and hence may trip the Primary side Circuit Breaker of Transformer which is not expected.

How do we prevent Tripping of Transformer due to Magnetizing Inrush Current?

In Transformer Differential Protection an intestinal time delay of 20 milisecond is provided to prevent tripping of Transformer due to high Magnetizing Inrush Current. In modern Numerical Relay, 2nd Harmonics blocking feature is provided which blocks the 2ndHarmonics when it is more than set value, thereby don’t issue trip command to Transformer Primary side Breaker due to Magnetizing Inrush Current. 

Normally the setting of 2nd Harmonics blocking is set to 20% which means if 2ndHarmonic component is more than 20% of fundamental frequency value of current then Relay will understand that it is because of Magnetizing Inrush Current and hence won’t issue trip command but if it is less than 20% fundamental frequency value of current then Relay will treat it due to fault and will issue trip command to the primary side Circuit Breaker.

Is there no way to minimize Magnetizing Inrush Current of Transformer?

Point on Wave Switch is used to minimize the Magnetizing Inrush Current of Transformer. If you want to know more about Point on Wave Switch, please read “Point on Wave Switch

Thank you!

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