Selecting Current Transformers and Relay Settings for Low Impedance Differential Relaying

Two common questions are raised when applying differential protection: how do I select the right class and ratio of Current Transformers (CTs)? What pickup, slope, and other settings should be used in the differential relay? Several traditional methods of selecting current transformers often fall short by recommending CTs that are impractically large to fit in the available space, or too small to reproduce the fault current waveforms. This paper demonstrates how to use waveform analysis to optimize CT selection without using CTs larger than necessary for the application. The paper also shows how waveform analysis is used to calculate and validate pickup and slope settings.