New protective relay modeling scheme and analysis for AC electric railway feeding systems with Scott transformer

Abstract

The Scott transformer is used in substations of the Korean AC electric railway system to convert a three-phase source to a single-phase source. The Scott transformer consists of two single-phase transformers and protects the M-phase and T-phase transformers with a single-phase protective relay. Percentage differential and overcurrent relays are used to provide primary and back-up protection to the Scott transformer. In this study, an overcurrent relay is used, and the protective relay is modeled using the Power System Computer Aided Design/Electromagnetic Transients program including DC (PSCAD/EMTDC). The inverse-time overcurrent relay present in the existing PSCAD/EMTDC will only change the coefficients in a given equation or use limited specifications with the default model. Unlike the existing element, which is limited when configuring the protection operating time, it is possible to obtain the operating time using a specific desired equation and coefficients through the modeled overcurrent relay. This overcurrent relay can be used to model the actual Korean AC electric railway system, and it is verified that the modeling approach that generates a fault in the feeding system is set based on comparison with numerical calculation. Based on this study, we developed a new modeling methodology for the overcurrent relay and for the Korean AC electric railway feeding system. In addition, no national or international standards exist that govern the protection standards for Scott transformers. Therefore, this study is expected to contribute considerably to the design and establishment of protection regulations for Scott transformers.