Electrical Overhead Shielded Construction
- Roger Hoestenbach (Paragon Engineering Services Inc.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- August 1994
- Document Type
- Journal Paper
- 656 - 656
- 1994. Society of Petroleum Engineers
- 3.1.2 Electric Submersible Pumps
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- 62 since 2007
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More petroleum engineers are being charged with facilities responsibilities.These new responsibilities cover equipment such as electric submersible pumps.Lightning protection is a key issue for these items. When problems occur,engineers with inadequate knowledge often attempt to correct them, andperformance deficiencies result. This article provides a solution to theseproblems.
The most popular type of lightning-resistant overhead power-lineconstruction in use today is the shielded construction shown in Fig. 1.Properly constructed, the design can perform satisfactorily, but over theyears, performance deficiencies caused by improper spacing have come to beaccepted as inherent. Design errors within the construction will createflashovers, pole-top damage, carbon tracking, 60-Hz "power follow,"conductor damage, breaker tripping, fuse blowing, transient overvoltages, andsystem outages.
Electricity will always follow the path of least resistance. Conductors onoverhead structures are spaced to create an air gap of high resistance toensure that arcs or flashovers will not occur between conductors. This sameprinciple is used in the shielded power-line construction. A shieldingconductor or static line is routed along the top of the structure in anoverhead power-line system to attract lightning and to shield the phaseconductors by producing a 45 cone of protection. The static line is grounded ateach pole (see Fig. 1). Ideally, lightning will strike the static line andfollow the path of least resistance through the grounding conductor down thepole and into the earth. However, because of unavoidable system impedances(resistance), not all lightning strokes can be quickly dissipated into theearth. Thus, containment in the grounding conductor is lost and a flashover mayoccur. At this point, the basic impulse level (BIL) of the structure has beenexceeded. The BIL is simply the impulse voltage level at which high-voltage,high-frequency flashover may occur on a structure. It is based on the specificbreakdown voltage of the porcelain (insulators), air, and wood paths on a poletop.
The dashed lines in Fig. 1 show the BIL of the various possible flashoverpaths for the shielded overhead power-line construction. Path A, the lower BILvoltage, is considered the BIL rating of the structure. If the lightningimpulse voltage level builds up to more than about 332 kV, a flashover mayoccur on the structure along Path A. In practice, the draining action of theearth in a properly grounded system rarely permits impulse voltages to reach300 kV, so actual flashovers are rare.
Fig. 1 shows the correct dimensions for proper shielded construction.Deviations from this construction typically result in structures with low BILratings (<300 kV on an 11- to 14-kV system). Under this condition, aflashover caused by lightning will create a sufficiently ionized conductivepath to support 60-Hz power follow of utility power. This phenomenon may causea power outage by short circuiting of phase conductors. Above this criticalBIL, the degree of ionization is insufficient to support 60-Hz current flow,which means that the 60-Hz current stays in the conductors. The most commonconstruction deficiencies observed include insufficiently elevating the staticoverhead line (so that the desired cone of protection is not produced), usingsteel crutches or steel cross-arm braces to modify existing lines, incorrectlyrouting the grounding conductor, using poor earth grounding techniques >5), neglecting to ground every pole, and placing grounded guy wires too close tophase conductors.
In 1971, an improperly constructed project in Gaines County, TX, was plaguedwith only 65% electrical availability. Redesign produced 90% availability. In1974, a freak ice storm with tunnel winds toppled many of the poles. A hastyrebuild returned many of the original errors, and the availability dropped to70%. Inspection and modification enabled a return to 92% availability, which isthe current level of availability. A correctly designed system can be expectedto produce even better results, largely through improved earth groundingtechniques for each pole.
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