Electrical Overhead Shielded Construction

Abstract

E&P Exchange 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, and performance deficiencies result. This article provides a solution to these problems. 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 the years, performance deficiencies caused by improper spacing have come to be accepted as inherent. Design errors within the construction will create flashovers, pole-top damage, carbon tracking, 60-Hz "power follow," conductor damage, breaker tripping, fuse blowing, transient over voltages, and system outages. Electricity will always follow the path of least resistance. Conductors on overhead structures are spaced to create an air gap of high resistance to ensure that arcs or flashovers will not occur between conductors. This same principle is used in the shielded power-line construction. A shielding conductor or static line is routed along the top of the structure in an overhead power-line system to attract lightning and to shield the phase conductors by producing a 45 cone of protection. The static line is grounded at each pole (see Fig. 1). Ideally, lightning will strike the static line and follow the path of least resistance through the grounding conductor down the pole and into the earth. However, because of unavoidable system impedances(resistance), not all lightning strokes can be quickly dissipated into the earth. Thus, containment in the grounding conductor is lost and a flashover may occur. At this point, the basic impulse level (BIL) of the structure has been exceeded. 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 specific breakdown voltage of the porcelain (insulators), air, and wood paths on a pole top. The dashed lines in Fig. 1 show the BIL of the various possible flashover paths for the shielded overhead power-line construction. Path A, the lower BIL voltage, is considered the BIL rating of the structure. If the lightning impulse voltage level builds up to more than about 332 kV, a flashover may occur on the structure along Path A. In practice, the draining action of the earth 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 BIL ratings (<300 kV on an 11- to 14-kV system). Under this condition, a flashover caused by lightning will create a sufficiently ionized conductive path to support 60-Hz power follow of utility power. This phenomenon may cause a power outage by short circuiting of phase conductors. Above this critical BIL, 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 common construction deficiencies observed include insufficiently elevating the static overhead line (so that the desired cone of protection is not produced), using steel crutches or steel cross-arm braces to modify existing lines, incorrectly routing the grounding conductor, using poor earth grounding techniques >5), neglecting to ground every pole, and placing grounded guy wires too close to phase conductors. In 1971, an improperly constructed project in Gaines County, TX, was plagued with only 65% electrical availability. Redesign produced 90% availability. In1974, a freak ice storm with tunnel winds toppled many of the poles. A hasty rebuild returned many of the original errors, and the availability dropped to70%. Inspection and modification enabled a return to 92% availability, which is the current level of availability. A correctly designed system can be expected to produce even better results, largely through improved earth grounding techniques for each pole. P. 656^