IMPROVING LIGHTNING PERFORMANCE OF TRANSMISSION AND DISTRIBUTION LINES USING SPECIAL METHODS

Transmission and Distribution line's lightning performance can be improved by several special methods that have been used with some success. This article will give a brief review of the best known of these special methods. Designers should recognize, however, that industry experience has usually been limited to a few applications, and more experience is being accumulated.

Additional shield wires
Since the mid 1910s, it has been recognized that OHGWs on a transmission line reduce the lightning voltage
created across the insulators. This reduction comes about in the three following ways:

a) By intercepting strokes that would otherwise hit the phase conductors
b) By draining off part of the stroke current that would otherwise ├čow through the footing impedance
c) By increasing the common-mode coupling of voltage surges on the shield wires to the phase conductors,
causing the insulator voltage at the tower to be reduced

Only the first of these effects requires the grounded wire to be above the phase conductors. One or more shield wires under the phase conductors will not intercept lightning strokes, but they may improve coupling and reduce insulator lightning voltages almost as effectively as if they were above the phase conductors.


The large improvement is caused mainly by the increase in coupling coefficient from shield wires to phases rather than in the small reduction in total shield-wire surge impedance.


Guy wires on transmission towers
In some cases, towers are uprated by putting new or additional guy wires from the tower to rock or soil anchors. This treatment should also improve lightning performance in two ways. First, each new guy anchor will behave as an additional ground electrode.

The anchors may be grouted with low-resistivity material such as concrete, and bonded to any existing counterpoise or structure, to maximize the benefitt. Second, the guy wires will mitigate the tower surge response. Four widely separated guy wires may reduce the impedance of a tower from 100 W to 50 W. This factor alone may reduce the outage rate of a tall line by 30%.

Ground wire on separate structures
OHGWs may be supported by separate outboard towers or poles instead of being mounted on the same
structure that supports the phase conductors. This arrangement may give extreme negative shielding angles,
which minimize induction losses and provide excellent security from shielding failures. Tower height and wind loading may also be reduced.

While an expensive option, OHGWs on separate structures may result in excellent lightning performance. Connections from the OHGWs to towers, if required for ac fault-current management, should be designed to have a high impedance to lightning through long interconnection length to minimize risk of back flashover.

Line surge arresters
Surge arresters at every insulator location (line arresters) present an alternative to the OHGWs both for new construction and for improvements to older unshielded lines when improved lightning performance is required. For special applications such as river crossings and on one circuit of double-circuit lines, properly applied line arresters may also provide specific benefits such as reduced double-circuit outage rate.

Line arresters have been successfully used on many transmission lines. Excellent results were reported on a line that crossed mountain ridges of high ground resistivity (usually rock) and high lightning exposure, leading to frequent lightning flashovers and insulator damage.


Unbalanced insulation on double-circuit lines
Unbalanced insulation on double-circuit lines, first applied by Kawai, is a deliberate effort to force most of the flashovers onto one circuit so that the other circuit will experience few flashovers, if any. When the weaker circuit flashes over, its phase conductors are suddenly connected to the tower by the flashover path, thereby making them momentarily underbuilt shield wires until the breaker opens.

Insulator voltages on the unfaulted circuits are reduced by draining away some stroke current into the phase surge impedance. Common-mode voltage coupling is also enhanced, decreasing the normal-mode voltage appearing across the insulation. The lowest circuits have the lowest surge impedances to ground. They will also offer the greatest improvement in coupling, and would logically be selected as the weaker circuits.
return the total flashover rate to acceptable levels.

Active air terminals
In some cases, older lines were constructed with shielding angles that are now considered to be poor. Line shielding may be somewhat improved by increasing the proportion of strikes that hit the tower. This has traditionally been done through the addition of lightning masts at existing towers, although other products are now offered commercially. At this time, there is little full-scale evidence that either supports or contradicts the additional effectiveness of these devices.

Any projection will increase the effective tower height and the resulting lightning incidence, which leads to
more back flashovers. However, an advantageous trade-off may sometimes be made. Rizk describes the two important physical conditions for positive leader inception from a structure or conductor. These conditions are basically determined by structure or by wire height above ground. Under negative leader space charge, small details of the structure surface would appear to have only minor effects on the lightning incidence.



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