Best practices for motor control center protection and control

Abstract

Low-voltage motor control centers (MCCs) are numerous and consume a large portion of maintenance and operator interaction in an industrial power distribution system. The extensive human interaction with these low-voltage (less than 1,000 V) circuits makes a low-voltage MCC a location of significant potential hazard. The large number of low-voltage MCC circuits results in significantly more human interface time with low-voltage MCC equipment than with medium-voltage MCCs and switchgear. Modern protection and control systems derived from medium-voltage (1,000 V to 38 kV) and high-voltage (38 to 765 kV) power systems have much to offer low-voltage MCC systems. Proactive maintenance indicators based on load characteristics, motor start characteristics, and thermal measurements are described. Time synchronization, modern Ethernet-based protocols, sequence of events records, oscillography (COMTRADE), monitoring and alarming for protection functions, and other previously standard features in medium- and high-voltage protective relays are available in modern low-voltage MCC protective relays. Increased safety in the form of advanced protection elements and arc-flash detection are now also available. This paper focuses on the philosophy of a comprehensive low-voltage MCC protection and control system.