Protective relaying considerations for standby generation systems

Engine-driven generators installed primarily to serve as a standby power source during a utility outage are increasingly being used for peak shaving or load control through continuous parallel operation or a closed-transition transfer with the electric utility grid. The higher capacity requirements of mission-critical standby systems, such as those in hospitals and data centers, have led to the parallel operation of a large numbers of generators on a common standby bus. These applications require a more sophisticated electrical protection than the traditional standby operation, both to satisfy the utility interconnection requirements and to protect the generator(s) from abnormal system conditions. A great deal has already been published on the topic of generator and utility intertie protection, most of it directed primarily at turbine-driven units and utility-scale installations. This paper will discuss the protective relaying of generators and utility interconnections from the perspective of a standby application, and identify the unique considerations for selecting and setting some of the common protective relay elements, considering the issues which are not fully addressed in the traditional references. The topics covered by this paper include the accommodation of loading transients, the preservation of the standby mission, and the protection of low voltage generators.