Paralleling of generators is the synchronous operation of two or more generator sets connected together on a common bus in order to provide power to a common load. Paralleling equipment makes two or more generator sets function that allows them to act as a single large power source. The capacities of the individual equipment directly add together while maintaining nominal bus frequency and voltage.
Paralleling of generator sets can be a valuable tool in the drive to improve power system reliability in critical installations. Use of multiple generator sets capable of serving a common load is often considered to be more reliable than single level backup and support systems. For applications that depend on power for life safety, and applications that depend on power to assure the financial viability of a business, paralleling is often considered to increase the probability that critical loads are served.
The paralleling application guide from Cummins provides a comprehensive discussion on why paralleling is required and how to implement generator parallel operations. The paralleling application guide can be downloaded here.