Protective relaying considerations for standby generation systems

Increasingly, engine-driven generators installed primarily to serve as a standby power source during a utility outage are being used for peak shaving or load control through continuous parallel operation or closed-transition transfer with the electric utility grid. Higher capacity requirements of mission-critical standby systems, such as those in hospitals and data centers, have led to the parallel operation of large numbers of generators on a common standby bus. These applications require more sophisticated electrical protection than traditional standby operation, both to satisfy 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 standby application, and identify unique considerations for selecting and setting some of the common protective relay elements, considering issues not fully addressed in the traditional references. Topics covered include accommodating loading transients, preservation of the standby mission, and protection of low voltage generators.