Paralleling CTs for line current differential applications: Problems and solutions

Paralleling current transformers (CTs) is a common practice in differential or line protection applications where the number of CTs exceeds the number of relay current inputs. This is especially true of line current differential applications applied to breaker-and-a-half and ring-bus configurations. This paper discusses complications that can arise from this practice and highlights relay design improvements that have been made to add security to these applications. Real-world event reports are shown to illustrate problems and solutions.