ABSTRACT

This chapter covers the protective relaying of low and medium voltage systems that provide the necessary auxiliary power for generating stations and industrial plants. Protective relay selectivity, or the tripping of the fewest components in order to isolate a short circuit condition, is achieved by having the lowest minimum trip value and the shortest time delay on the most downstream fault clearing device. For industrial plants, the most downstream devices would normally be bus feeder breakers or contactors that drive motors, Motor Control Centers (MCCs), and other transformers. The chapter illustrates a typical generating station auxiliary power system one-line configuration with the associated current transformers (CTs) and overcurrent relays. It also illustrates the before and after conditions of grounding a floating three-phase power source. High impedance ground schemes are typically applied in generating stations, refineries, combustibles areas, and industrial facilities where phase-to-neutral voltages are not utilized to feed loads.