Joshua Hughes - Qualus Services, LLC

In this presentation, we will analyze the Ground Differential or Restricted Earth Fault (REF) element for transformers and its performance against different levels of CT error and saturation.  Through empirical analysis, we will look to find the limits of today’s common digital relays as they experience CT error.  The impact of auxiliary CTs will also be examined to determine their influence on element performance.  Finally, we will evaluate for root cause a real-world misoperation of the REF element due to CT error.