Calidad De La Energia

An overview of relationships between currents, imedances, and potentials.

BY EDWARD S. THOMAS, JOHN B. DAGENHART, RICHARD A. BARBER, & ALLEN L. CLAPP

T

HIS ARTICLE IS AN OVERVIEW OF THE VARIIEEE INDUSTRY APPLICATIONS MAGAZINE • SEPT|OCT 2005 • WWW.IEEE.ORG/IAS

ous relationships between neutral currents, ground currents, electrode impedances, and voltage potentials that are encounteredin the operation of

multigrounded wye distribution systems. This system configuration is the most commonly used configuration among U.S. domestic utilities. Voltages range from 4.16/2.4 kV to 34.5/19.9 kV. The most common system voltages are 15- and 25-kV class systems with nominal operating voltages on rural systems generally being 12.47/7.2 kV and 24.9/14.4 kV. The article aims to review therelationships that might be encountered due to system grounding and provide an overview of common instal© DIGITAL VISION

lations and their relative effectiveness. The National Electric Safety Code (NESC) (ANSI C2) [1] requires multi-

grounded distribution system neutrals to be effectively grounded (Rule 96C). The definition of “effectively grounded” is to be intentionally connected to earththrough a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to limit the buildup of voltages to levels below that which may result in undue hazard to persons or to connected equipment. Effective grounding, or earthing, of the distribution system neutral is necessary to achieve several objectives, the most important of which is thesafety of the public and utility personnel. The effectiveness of the grounding system also affects system reliability, power quality,
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IEEE INDUSTRY APPLICATIONS MAGAZINE • SEPT|OCT 2005 • WWW.IEEE.ORG/IAS

and the longevity of both utility and customer equipment. Effective grounding and bonding reduces voltages between adjacentgrounded facilities within utility and public/customer installations. For all of these objectives, the general method to achieve maximum effectiveness of the utility grounding system is to establish the best practical connection between the neutral conductor and the earth. Decreasing the resistance in this connection reduces both: ■ the effect of lightning discharges on or near utility or customerfacilities ■ the effect of neutral-to-earth (NTE) voltages that may exist between the neutral and earth. A low neutral-to-earth impedance is particularly important when the distribution system neutral is connected to metallic objects that are accessible to the public. These objects include guy wires, pole grounds, and the customer-owned wiring and plumbing within a residence or other building. If theseclasses of metallic objects are not interconnected to the distribution system neutral, there could be a strong local voltage difference between these objects, either on the utility facilities or within the customer premises. Bonding of grounded conductors of circuits entering customer facilities is required to assure the safety and reliability of customer equipment. The importance of effectivegrounding and bonding is recognized by both NESC and the National Electrical Code (NFPA 70) [2]. Both codes require interconnection of the power, telephone, CATV, and customer grounding conductors at the served installation, in order to limit voltage potentials that may be hazardous to personnel or equipment. Another area where the interconnection of system neutrals is important is with the otherutilities, principally communication utilities, that occupy joint-use structures and enter the same customer premises as the electric system neutral. In order for joint-use occupants to minimize utility structure size adjacent grounded facilities must be bonded. Otherwise, workers could be exposed to steady-state and/or transient voltages between metallic objects within close proximity to each...