SWITCHING INRUSH CURRENT

• Whenever a power transformer is just switched on without connecting load with its secondary, the primary of that transformer draws a high, peaky current from source which is transient in nature.

• There is an inrush of current at the instant of switching on a power transformer.

• Inrush current generally does not create any permanent fault in transformer.

• But inrush current can cause unwanted tripping of circuit breaker of transformer at the time of charging the transformer.

• When a transformer is just switched on the sinusoidal voltage appears across the primary winding.

• The flux induced in the core is proportional to the magnetizing current and is in quadrature with the applied voltage during steady state conditions.

• The flux wave will reach its maximum one forth cycle after the voltage wave reaching its maximum.

• This is clearly impossible as there is no flux linked the core prior to switch on.

• The steady state value of flux cannot be instantly accommodated as this would imply an infinite rate of energy transfer, therefore, this steady state value of flux can only be reached after a finite time, determine by the rate at which the circuit can accept energy.

$$ \begin{aligned} e &=E \cdot \sin \omega t=d \phi / d t \\ \Rightarrow \phi &=\int e \cdot d t=E \int \sin \omega t . d t \end{aligned} $$