Analysis of Transformer Inrush Current and Comparison of Harmonic Restraint Methods in Transformer Protection

It is well known that differential protection is most suitable for transformer protection. However, inrush current due to transformer energization can appear as fault to the protective relay. To improve the security while maintaining the required levels of sensitivity, many restraint methods have been proposed to inhibit operation of the differential element. This paper first will analyze the magnetizing inrush current during transformer energization with a simplified excitation curve. It derives mathematical equations to compute the inrush current based on the residual flux and saturation flux for the worst case energization event. This paper reviews several popular restraint methods used today. A conceptual logic diagram is provided for each method. Advantages and disadvantages of different methods are analyzed for various systems. Finally, using data recorded from real life events and data generated by digital simulations, a performance comparison of different methods is provided.