A Tutorial on the Impact of Distributed Generation (DG) on Distribution Systems

Interconnect protection allows DG to operate in parallel with the utility power system and is the single most important technical issue in most DG projects. Typically, protection requirements to connect distributed generators to the utility grid had been established by each utility. These guidelines generally cover smaller distributed generators (10 MW or less), which are usually connected to the utility system at the subtransmission and distribution level. These utility circuits are designed to supply radial loads. Thus, the introduction of generation provides a source for redistribution of the fault current on the feeder circuit, which can cause the loss of relay coordination and potential overvoltages. Within the past few years, there have been efforts by the IEEE, as well as individual states, to develop standards and guidelines for the interconnection of DG. The stated goal of these standards/guidelines is to have a single document of standard technical requirements for DG interconnection rather than having to conform to local utility practices and guidelines. This paper examines how well this objective has been met.