Evaluation of Natural de-Magnetization of Power Transformer Due to Capacitor Interaction with Area Under the Curve Method

Abstract

Transformer energization at random point presents inrush phenomena and is unfavorable for equipment due to high magnitude asymmetric current flow. These asymmetric currents are largely affected by level and polarity of residual flux presents in transformer core post its de-energization. Controlled switching technology is widely used to limit inrush current. Moreover, ignorance of residual flux would yield only moderate level of inrush current. Also, inaccurate estimation of de-magnetization status leads to improper estimation of energization instants during independent pole switching and the resulting inrush currents could not be reduced below desired level. Therefore it is quite important to know the exact level of residual flux post every de-energization operation of power transformer. Presence of capacitor bank, filter banks and connected XLPE cables offer natural de-magnetization of power transformers. The de-magnetization is categorized in no, partial and full de-magnetization. This paper presents novel approach for detection of natural de-magnetization of Power Transformer using load side measurements. Numerical integration has been used for the evaluation. Suggested method has been verified from field data collected from power transformers of different design and connection configuration across globe used for various grid voltage levels. Lastly, the effect of variation in sampling rate and level of residual fluxes is analyzed and the suggested method is found to be equally effective for said variations. The categorization of natural de-magnetization play important role in deciding energization instant for next switching of transformer