Influence of Grading Capacitance on Sympathetic Inrush Current of Parallel Power Transformers

This article presents research on the phenomenon of sympathetic inrush current triggered by the energization of a third transformer in parallel with two others already connected, considering the impact of circuit-breaker grading capacitance, which is an emerging in the literature on the subject. The aim is to propose recommendations to mitigate the intensity of this phenomenon to reduce the risk of outages or system failures during transformer energization. The study focuses on a common configuration in the central Mexican electrical grid, which may include power transformers connected in parallel, which can be energized through their 230 or 85 kV windings. The research employs electrical power system modeling using the Alternative Transients Program software to simulate typical scenarios involving substation switch operations during grid activity. It incorporates transformer models with manufacturer-specified saturation characteristics and a certain level of remanent magnetization. With the results obtained, it is inferred that the magnitude, waveform, and duration of the sympathetic inrush currents can cause imbalances and affect the normal operation of the system. Transformer outages can occur due to malfunctioning of its differential and overcurrent relays, as well as power quality problems. Remanent magnetization is not a determining factor for the appearance of the phenomenon. However, the magnitude of the sympathetic inrush current is strongly related to the closing time of the circuit breaker. Furthermore, the findings indicate that a higher capacitance makes the sympathetic inrush current phenomenon more evident.