A critical survey of considerations in maintaining process continuity during voltage dips while protecting motors with reclosing and bus-transfer practices

Continuous-process plants, such as petro-chemical industries and paper mills with thermomechanical pulping, are being designed with fewer and larger components arranged in single-train configurations. Maintaining operating continuity with fewer components requires high equipment reliability. Motor controls for large components are using magnetic contactors equipped with automatic undervoltage reclosing. A voltage dip where contractors drop open and reclose can be very serious because of the nature of the motors and their driven loads during voltage transients. The general nature of the problem and criteria for safe reclosing or transfer of several motors on the same bus are presented. The mathematical development and modeling techniques for induction and synchronous motors are indicated, and simple easy-to-use expressions are given for determining the currents and torques due to out-of-phase reclosing. Various protective schemes against out-of-phase reclosing and transfer, as well as methods of riding through voltage dips, are discussed