Performance Improvement of Transformer Differential Protection During Cross-Country Fault Using Hyperbolic S-Transform
Nassim Shahbazi, Sajad Bagheri, Gevork B. Gharehpetian
Abstract
A cross-country fault is a fault occurring at two different locations of the same circuit, simultaneously or with a short time delay and possibly involving different phases, which is a challenge for the protection system. On the other hand, one of the main problems in transformers’ differential protection is the distinction between inrush current and other faults, such as cross-country faults, internal faults, and saturation of current transformers (CTs) during external faults. In this article, for the first time, external electrical faults and internal mechanical faults, such as radial deformation (RD) and axial displacement (AD) of winding, were considered as cross-country faults in a real winding. The transformers’ winding is modeled based on the detailed model, and the parameters were practically validated in the laboratory. Differential currents are sampled during different disturbances simulations by EMTP software, and feature extraction is performed using hyperbolic S-transform (HST) in MATLAB environment. As a final point, cross-country faults were distinguished from other faults and events by calculating various S-transform indices, including mean square sum absolute deviation (MSSAD), correlation coefficient (CC), and minimum–maximum (MM). The obtained results confirmed that the proposed protection algorithm is very efficient in detecting and distinguishing cross-country faults, even in the presence of noise.