Avaliação de esquemas de proteção para faltas próximas ao ponto neutro em transformadores delta-estrela

Abstract

This work evaluates protection schemes applied to delta-wye transformers during single-phase faults near the neutral point. Differential (87T), timed neutral overcurrent (51N residual and measured), and restricted earth fault (87N REF) protection functions were analyzed and compared. The methodology was based on simulations of a fictitious system with a 26.6 MVA transformer in the PS Simul software, complemented by detailed parameterization in a Siemens 7UT82 relay, following manufacturer recommendations. Various fault conditions were simulated, varying the fault position in the winding, fault resistance, inception angle, and load unbalance. From the simulated fault situations, the respective analog signals were generated and injected into the relay using the Conprove CE-7012 relay test set. The results demonstrated that the 87T function has sensitivity limitations for faults very close to the neutral. The 87N REF proved to be the most accurate for detecting faults very close to the neutral, although its performance can be affected by system imbalances and requires careful parameterization. The study reinforces the need for special protection functions.