Pre-Magnetizing of Transformer for Mitigating the Effect of Inrush Current

Abstract

A transformer is a key element in the electrical system, mainly used to step-up or step-down of voltage to connect two circuits in the network. When it is energized at no-load, its winding act as an inductive circuit. The magnetic flux generated will be lagging the source voltage by 90 degree electrical. During energizing, induced magnetic flux is absent hence supply voltage and induced flux start from zero instance if circuit breaker is closed at that instance. Due to this, more than twice the flux is induced in the core during the first cycle which results in transformer core saturation and a large amount of magnetizing inrush current flows through the winding. Initially, the magnitude of inrush current can be as high as short circuit current value (up to 50 %) and the effects of that inrush currents are quite severe due to its magnitude (8 to 30 times I nominal) and duration (0.2-0.5 sec). It can lead to a reduction in the lifespan of the transformer if it experiences frequent switching over a short span. It is an unsolved phenomenon in the transformer industry and needs attention to reduce/mitigate the detrimental effects of inrush current. At first, the analytical computation of inrush current and behaviour of the transformer core are analyzed during the energizing period. Then, the impact of this inrush current on the transformer is carried out with comparison with short circuit current. At last, after studying all the available mitigation methods, there are two methods on which analysis has been done with the inclusion of cost.