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This paper provides an in-depth discussion of system grounding and ground fault protection on systems from 480 V and above. The paper also discusses modeling of ground faults, the proper design for ground-fault protection, and common problems associated with ground-fault protection. The paper will address many real-life problems associated with system grounding and ground-fault protection, including safety issues and how to avoid those problems. The topics to be included in the paper include low-voltage systems, under 600 V, through high-voltage transmission systems.
Most electrical faults in the petrochemical industry, estimated to be over 90%, are, or begin as, ground faults. (Many of these faults involve high impedances which allows the fault to exist longer than desired before being removed by standard circuit breakers). Unfortunately, few engineers and practically no plant operators totally understand ground faults and the means to protect against them. System grounding and ground fault protection in the petrochemical industry are important factors in the design, construction, and operation of a petrochemical facility. The safety of plant personnel and the reliability of the equipment are highly dependent on the type of system grounding selected and the type of ground-fault protection selected.
Ground-fault modeling is important in analyzing the effects of various types of system grounding in the petrochemical facilities and in predicting the effects of ground faults on the system. Probably the most commonly used model involves the use of symmetrical components  and, in particular, the use of the zero-sequence circuit. Historically, the use of zeta-sequence current vector 10 and zero-sequence voltage vector VO have been used with protective relays for ground-fault protection. Part of the reason for the long-time use of these components has been due to zero-sequence values are directly proportional to the sum of the three phase currents and voltages.
Ground faults also create values in the positive- and negative sequence circuits which have historically been of less value to the protection engineer than zero sequence. The reason is that the positive- and negative-sequence circuits required complex positive- or negative-sequence filtering in order to develop those sequence components. As can be seen in (3}-(6), the equations are more complex positive sequence.
|System Grounding and Ground-Fault Protection in the Petrochemical Industry A Need for a Better Understanding|