Generator field ground protection using digital technology

Advances in digital technology have provided relay designers with the tools to increase the capabilities of modern generator relays. This allows more sophisticated protection functions to be added to digital relays to provide better generator protection within a single relay hardware platform. Functions such as field ground fault detection and brush lift-off detection are two examples of such relay functions. This paper discusses field ground protection using an injection method incorporated into a multifunction digital generator relay. The scheme presented in this paper addresses field ground detection on generators with brushes and an external source of dc field current and voltage. It has been installed on a number of generators. Though some of today´s new generators are brushless, there still exists a large population of generators where the scheme described in this paper is applicable. Developed and used in Europe for many years, the scheme lends itself to digital technology and offers many advantages over currently used field ground dc voltage methods.