Sympathetic inrush current phenomena in parallel connected transformers considering grading capacitance

Abstract

The article presents the results of a research study focused on the phenomenon of sympathetic inrush current caused by the sequential energization of two transformers connected in parallel with another transformer already in operation, introducing the novelty of considering the effect of the circuit breaker's grading capacitance. The aim is to propose an operational practice that reduces the intensity of this phenomenon and minimizes the risk of blackouts or equipment failures during the energization or re-energization of the transformers.The studied system is common in the central Mexican electrical grid and consists of a substation with three 100 MVA transformers. Through these transformers, it is possible to transfer energy between the 230 kV and 85 kV networks. The research employs electrical system modeling using the ATP (Alternative Transients Program) software to simulate common practical cases involving the manipulation of substation circuit breakers during the operation of the power grid. The program allows for controlling the closing time of the circuit breakers to create critical scenarios during the energization process. It also uses a transformer model with saturation characteristics provided by the manufacturer, including a certain level of remanent magnetization. The results show that the magnitudes, waveforms, and durations of both the magnetizing inrush current and the sympathetic inrush current can cause imbalances and affect the normal operation of the electrical system. This may lead to transformer outages due to the incorrect functioning of differential and overcurrent relays or problems with the quality of power within the system. However, the results also suggest that the proper handling of certain circuit breakers can have beneficial effects, reducing the intensity of the phenomenon and improving the quality and reliability of the power supply.