Attenuation of Transformer Inrush Current Using Controlled Switching System on Delta-Star Transformer

In this paper, a simple and easily applicable approach of controlled switching is used to reduce the mechanical and electrical stresses due to high inrush current and steep voltage front. This work focuses on attenuation of transformer inrush current using controlled switching technique. The main idea of the proposed technique is to mitigate switching transients by using controlled transformer energization based on a preceding controlled de-energization in order to achieve a defined and repeatable residual flux with its lowest possible level for individual phases. In addition, the optimal instant of energization is determined considering the core residual flux and the effect of mechanical scatter of the breaker's operating time. By using this approach, no harmful transients will be generated and the last steady state condition of the transformers will be immediately occurred. Moreover, controlled switching system is being promoted due to its added advantage of higher reliability and accuracy. Since there are no many extensive researches using controlled switching technique for the case of delta side energization of a delta-star transformer, this work will focus on applying the suggested method to Egyptian 66/11.5 kV, 25 MVA power transformer with delta connected winding. The alternative transient program (ATP) software is used to perform various simulation studies of delta side energization of the Egyptian delta-star transformer and results have been presented. Simulation results that were extracted from this work insure a very good agreement of the proposed technique to mitigate the inrush current for the case of delta side energization of a delta-star transformer. The paper also investigates the impact of various influencing factors affecting the duration and magnitude of the inrush current including switching-on angle, switching resistance, residual flux density and load conditions through theoretical analysis and simulation.