Current Transformer Selection Techniques for Low-Voltage Motor Control Centers

This paper provides a clear set of procedures and equations to follow in optimal current transformer selection for low-voltage motor control centers. Methods of using protective relay settings to minimize current transformer cost and size are also shared. The selection criteria are explained from the fundamental principles of operation of a current transformer and a protective relaying device. This paper shows how the current transformer ratio, voltage knee points, and relay protection elements can be selected together simultaneously to provide a low-cost high-performance system. This paper describes a case study in which the authors developed a simplified set of current transformer selection criteria for compact IEC low-voltage motor-control center drawers at a large oil and gas field in Central Asia.