A Review of System Grounding Methods and Zero Sequence Current Sources

摘要

A large majority of three phase power system faults involves ground; therefore, a thorough knowledge of grounding methods and associated zero sequence current sources is essential to the effective protection of the power system. The particular method of system grounding selected will vary according to the resulting characteristics desired by the application. A grounding method can be used either to limit or entirely eliminate the level of ground fault current available so as to protect expensive equipment from damage under the most likely fault scenarios. Or, conversely, it can be used to ensure an adequate current level to aid in the prompt detection and isolation of fault conditions. The grounding method employed also may be used to reduce transient overvoltages, as well as to facilitate compliance with regulations affecting personnel safety. With retirements, new hires, reorganization, cross training, and so on, this subject, like many others, requires regular review to maintain the experience level of our protection engineers. The purpose of this paper is to provide a tutorial or "refresher" on grounding methods, sources of ground current, ground current flow in the power system, and how each impacts protective relaying. A discussion of various ground sources, such as solidly grounded and impedance grounded generators, delta/wye ground transformers, and zig-zag transformers will be included, as well as a review of symmetrical components as related to grounding issues. Traditional response of protective relays to zero sequence current will be reviewed, including loss of zero sequence current sources while a ground fault is on the system, and methods for detecting the same. Zero sequence current traps used to balance a transformer differential scheme in the presence of an unaccounted for ground source are discussed, including mathematical alternatives used in modern protection products. We begin with an analysis of power system grounding characteristics using the applicable zero sequence network modeling to address the effects of transformer and generator winding connection and neutral grounding method. The use of grounding transformers to alter the system grounding will be included in this discussion.