Applying unbalance detection relays with motor loads

Prolonged operation under moderate to severe unbalanced supply conditions is well known to cause undesirable heating in motors. Occasionally, users are surprised that the protective relays intended to detect voltage unbalance will fail to respond. This paper reviews the definitions, calculation methods and standards of supply unbalance and compares the response of the corresponding detection techniques for various system configurations. It particularly points out the limitations of voltage unbalance measurement under certain open-phase and load configurations.