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The optimum lightning performance of a transmission line could very likely be attained if an arrester were installed on every tower and every phase. This trivial solution would, in most cases, involve a prohibitively large capital investment. However, in many cases, it is possible to prevent most flashovers by placing fewer arresters at optimum locations.

The efficient application of TLSA to improve line performance requires the investigation of all available mitigation options and the weighing of the performance benefits of each against real cost. Estimating the effects of changes in structure design, shielding, grounding, and arresters on the lightning performance of transmission lines is a necessary step in this process. Before making these calculations, however, a basic knowledge of options to investigate in various situations is essential, for it can reduce the number of iterations and, therefore, the total design effort.

Backflashover Protection

Backflashover is an insulator flashover that results from elevated tower voltage rather than elevated phase voltage. As described in Chapter 3, the tower voltage is a function of three parameters: the magnitude and wave shape of the lightning current traveling down the tower, the tower surge impedance, and the tower ground impedance. However, because an insulator will flash over if there is a sufficiently high voltage across it for a sufficiently long time, the operating and induced voltage of the individual phase conductors is also of interest.

Phase Location of TLSA

Phase insulators are more or less susceptible to flashover from lightning surges, depending on the location of the conductor. TLSA located on some phases can help protect other phases from flashover. The following subsections describe how conductor location can be used to locate the best position(s) for TLSA on a tower.

Coupling to Overhead Groundwires

The voltage of the phase conductor does not remain constant when either the overhead groundwire or the tower top receives a lightning stroke. The overhead groundwire voltage is

Placement of Arresters for Improved Lightning Performance

coupled to the phase conductors according to the coupling factor as described in [1, Red 2006]. The voltage of the phase conductor is affected by the portion of the current flowing outward on the shield wires according to the coupling coefficient Kn. Neglecting other voltages imposed on

conductor n, such as power frequency voltages and reflections from adjacent towers or the tower base, we have: t n n t n n V Z Z Z Z V K V ⋅ + + = ⋅ = 12 11 2 1

Where: Vn is the voltage of the nth phase conductor Vt is the voltage of the tower top, Kn is the

coupling coefficient, Zn1 and Zn2 are the impedances from the phase conductor to the images of

the overhead groundwires 1 and 2, Z11 is the self surge impedance of the groundwires and Z12 is

the mutual impedance between groundwires, where Zab = 60 ln [ (distance from conductor b to

image of conductor a )/ (distance from conductor b to conductor a) ]

As Kn increases toward unity, the voltage across the insulation for conductor n decreases because

the conductor voltage more closely follows the tower voltage. The coupling coefficient Kn can be

calculated for all phase conductors on a tower. Again, from [1,Red 2006], for a tower with two shield wires at equal heights above the ground:

The equation for a single overhead groundwire is obtained by setting Zn2 and Z12 to zero. When

there are three or more shield wires, as in the case when one of the phases is protected by an arrester, then the coupling coefficient is obtained by:

• Inverting the square matrix of self (Zii) and mutual Zij surge impedances, a 4x4 matrix when

there are three shield wires

• Summing the admittance values Yn1…Ynn for the undriven conductor

• Dividing this sum by –Ynn to obtain the coupling coefficient Kn.

Appendix 12.3 of [1] and < presumably > Applet TLSA-1 carry out these computations and the help file for TLSA-1 gives further details.

In all cases, the magnitude of Kn for each unprotected phase conductor differs by the ratios of

their mutual impedance to the shield wires.

This mutual surge impedance is Zab = 60 ln [ (distance from conductor b to image of conductor a

)/ (distance from conductor b to conductor a) ]

While not rigorously proportional because the image distance, a,,, changes slightly with phase location, the mutual surge impedance, and therefore the coupling coefficient, is nearly inversely proportional to the distance between the phase conductors and the shield wires. Therefore, on a shielded line that is to be protected from backflashover:

• The phase conductors farthest from the overhead groundwires will have the highest-stressed insulation and will be statistically more prone to flashover.

Placement of Arresters for Improved Lightning Performance

• Adding arresters to the phase conductors farthest from the overhead groundwires will make the largest improvement in coupling coefficient to the unprotected phases, boxing them in a sort of Faraday cage.

This usually means that bottom-phase arresters of double-circuit lines are the best candidates for TLSA application, provided that the overhead groundwires are to be retained.

Crossarm Voltage

On towers with multiple crossarms, the voltage of a particular crossarm is a function of its height. The voltage is often calculated as the linear interpolation of the voltage between the tower top and the tower base. Figure 6-1 is a plot of the voltage as a function of time at various points on a vertical double-circuit tower [Red 2006]. In this calculation, the lower crossarm has the lowest voltage.

Figure 6-1

Plot of Voltage versus Time at Various Points on a Double-Circuit Tower using L-5 Applet and Step Waveshape (final to use CIGRE concave)

Figure 6-2 is a graph of insulator voltages for the same tower shown in Figure 6-1, but with the relative coupling from the overhead groundwires taken into account. Even though the lower crossarm has the lowest absolute voltage, the bottom phase has the greatest insulator stress because of the reduced coupling coefficient. Therefore, on this vertical double-circuit tower, an arrester should be installed on the lowest phase(s) first. Then, if line-performance calculations indicate that further performance improvement is desired, the engineer could model an arrester on the middle phase.

< Figure to be prepared from Applet >

Figure 6-2

Plot of Voltage versus Time at Various Points on Double-Circuit Tower, Taking Into Account Relative Coupling from Shield Wires (final to use CIGRE concave)

Applying arresters to the bottom phase of a tower has an additional effect on performance: A phase conductor with an arrester approximates an additional shield wire on the structure. The

Placement of Arresters for Improved Lightning Performance

lower phase-conductor voltage is constrained to be no greater than the crossarm voltage minus the PL of the arrester. The PL voltage can be relatively low in magnitude compared to the tower- top voltage. The coupling coefficient to the middle and top phases is, therefore, increased because of the proximity of the lower phase wire, which acts as an additional shield wire. As a result, the two remaining phase conductors are less likely to flash over. Similarly, applying arresters to the outer phases of a horizontal circuit increases the coupling to the middle phase, thus reducing the incidence of back flashovers on that phase.

TLSA Location for High Tower Footing Resistance

In areas of high soil resistivity it is often expensive to attain tower ground resistances low enough for acceptable lightning performance. In these areas, a high percentage of strikes to the shield wires will cause back flashovers on the line. Where the soil is rocky, or where there are exposed ridge crossings with towers on bare rock, counterpoise installation may be impractical and may not provide satisfactory results. In such situations, TLSA may provide an economical solution, for they can eliminate a relatively high percentage of outages with a relatively small investment.

Consider a section of line consisting of 10 steel lattice towers with one horizontal 138-kV circuit. The tower footing resistances, as measured with a low frequency technique, are indicated in Table 6-1.

Table 6-1

Footing Resistance at Steel Lattice Towers along Hypothetical 138-kV Transmission Line

Tower 1 2 3 4 5 6 7 8 9 10

Ohms 25 35 20 205 250 100 155 30 35 25

Fault location relays indicate that while the entire 100 mile line experiences approximately 12 flashovers/year that are associated with lightning, eight of those flashovers involve these ten towers. An inspection reveals that the tower grounds are intact, but the resistances shown in Table 6-1 are much higher than the levels reported when the line was built.

Adding counterpoise at these towers would lower the resistances, but it would involve

transporting the necessary digging equipment four miles over difficult terrain. Adding arresters to all three phases on towers 4,5,6, and 7, where resistance is highest, seems a good, economical alternative, but it would not reduce the lightning flashovers as much as desired. To achieve acceptable performance, TLSA should also be placed on towers 3 and 8, which are the first structures on each side of the ridge that have low footing resistance. The following discussion shows why placing TLSA on these structures is important.

Figure 6-3 is a qualitative depiction of traveling waves on the left span when a 50 kA stroke hits Tower 5. This simplified schematic ignores some voltage reflections and tower surge

impedances and assumes long spans.

Placement of Arresters for Improved Lightning Performance

Figure 6-3

Schematic of Traveling Waves Propagating Towards a Structure with Low Footing Resistance: If Tower 3 has no arrester, it may flashover.

The stroke current of 50 kA hitting Tower 5 divides, part going out the shield wires in both directions, part going down the tower into the ground resistance, and part flowing through the conducting arrester into the phase wire where it again splits and goes in both directions. Neglecting the influence of the tower surge impedance (which is small at the crest of a double exponential wave, anyway) the current divisions are roughly those shown. To prevent the phase insulator at Tower 5 from flashing over, the arrester brings the voltage on the phase up to within 400 kV of the crossarm potential, thereby putting a 3.1 MV surge on the phase wire. This wave and the 3.5 MV wave on the shield wire travel together to Tower 4. At Tower 4 the voltage wave on the shield wire sees a 170 ohm resistance to ground, which is much less than the shield wire 400 ohm surge impedance, so the tower top voltage at Tower 4 tries to fall to a value much lower than the incoming 3.5 MV and lower than the 3.1 MV coming in on the phase wire. This causes the arrester on Tower 4 to conduct backwards, bringing the voltages headed toward Tower 3 to nearly the same potential. At Tower 3 the voltage on the tower is pulled down nearly to zero (157,kV), whereas the voltage on the phase remains at 2.1 MV. If the BIL of the insulator is lower than 2100-157 = 1943 kV, the insulator may flash over, unless arresters are installed on this tower also. Beyond Tower 3 arresters are not needed because a Tower 3 arrester will have pulled the phase voltage down below the insulator flashover value.

When installing arresters on towers with high footing resistances, engineers should keep in mind that arresters should also be installed on the first structure in each direction that has low footing resistance (in this case Tower 3 in Figure 6-3). Line performance software can be used to analyze situations where footing resistance reduces gradually.

Unshielded Applications

The lightning performance of unshielded transmission lines can be improved dramatically by the use of TLSA. There have been numerous instances in which TLSA have replaced the overhead shield wire on the top phase of delta and vertical phase arrangements. These applications generally have been on transmission lines of 69 kV and below, but there is considerable interest

Placement of Arresters for Improved Lightning Performance

in retrofitting higher-voltage lines that were constructed without shield wires because of relatively low lightning activity.

Arrester Energy

Lightning-caused energy failures of TLSA are of highest concern on unshielded lines. This is because TLSA mounted on phase wires on unshielded lines are required to absorb much more lightning energy than arresters mounted on shielded lines. The reason is that phase wires on unshielded lines are subject to many more direct strikes than those on shielded lines thus subjecting TLSA to more lightning energy.

In 1994, Williams [15] conducted an Electromagnetic Transients Program (EMTP) computer study to find out how much more lightning energy is absorbed by TLSA on unshielded lines. He found that arresters on unshielded lines absorb energies that are approximately an order of magnitude higher than those on shielded lines. In his study, Williams tested identical TLSA, each with an MCOV of 42 kV and a switching surge energy rating of 92 kJ. The study modeled TLSA on the phase wire at a tower with a 50-Ω footing impedance and subjected both shielded and unshielded cases to an identical, simulated stroke of lightning current. With a single 50 kA stroke, Williams reported, the arrester on the shielded line absorbed approximately 9 kJ, while the arrester subjected to a stroke directly on the phase wire absorbed approximately 150 kJ.

Although the difference is dramatic, it may not be a cause for alarm. When an arrester absorbs energy above its rating, it does not necessarily fail. Although calculations indicate that a large number of arresters on unshielded lines are subjected to high energies with some frequency, the reported failure rates are low. This anecdotal evidence suggests that installing arresters on unshielded lines is a sensible approach, and many such lines are now in service. However, in order to incorporate arrester failures on unshielded lines into line-performance estimates, the issue of failure probability needs to be addressed. For that reason, data from tests performed at the EPRI Power Delivery Center - Lenox (PDC-L) in late 1996 are incorporated into Appendix B. These data were be used to generate arrester-failure probability curves that are now used the TFLSAH software.

Vertical Circuits

Phase arrangements in which the top phase conductor provides a good shielding angle for phase conductors below it are solid candidates for top phase arrester applications.

Applying arresters to the top phase of every tower effectively prevents flashovers of the top phase, and if the shielding angle is adequate, the lower phases are protected from direct strikes. Shielding angles of 20° or less are ideal, but greater shielding angles may still yield a substantial improvement in lightning performance (See Figure 6-4). If repositioning the phase wires is possible, for instance when designing a new transmission line, and top phase arresters are to be applied, then an electrogeometric model should be used to position the conductors. This process is similar to that suggested for a shielded transmission design.

Placement of Arresters for Improved Lightning Performance

Figure 6-4

A Schematic showing the Shielding Angle on an Unshielded Transmission Line with the Top Phase protected by TLSA

Performance benefits derived from the application of arresters to the top phase of a transmission line are highly dependent on the footing resistance of the towers. The arrester conducts lightning current from the phase wire to the tower in order to maintain low voltage between the top phase and tower top. This current must then flow through the tower ground, and if resistance is high, the tower potential will rise. The other two phases will then be subjected to back flashovers just as on any shielded transmission line.

The lightning performance of a line with an arrester on the top phase conductor at each tower, and with an adequate shielding position, should be comparable to or even better than a similar line with a shield wire and no arresters. There are three reasons for improved performance: 1. Lightning flashovers are virtually eliminated on the top phase conductor, thus leaving only

two phases subject to back flashovers instead of three.

2. A struck top phase conductor will have a higher potential than an equivalent shield wire due to a voltage equal to the PL of the arrester. A struck top phase conductor is also generally closer to the lowest phase wire than a shield wire would be. These two factors increase the coupling voltage to the other phase conductors and reduce the chance of back flashovers on these phases.

3. The elimination of the overhead groundwires allows the tower height to be reduced, thus reducing the line shadow width slightly and, therefore, the number of strikes to the line.

However, there are two factors that may reduce line performance in comparison to a similar line with a shield wire and no arresters:

1. The number of induced flashovers due to near misses may be higher.

2. The energy failure rate for arresters is statistically higher on unshielded lines, although the actual number of failures still may be low.

Placement of Arresters for Improved Lightning Performance

Induced flashovers are insulation failures resulting from nearby lightning strikes that terminate on trees, light poles, buildings, or the ground. Shield wires protect transmission lines from induced flashovers by conducting induced current through the tower to the tower grounds. These currents are beneficial because they couple the induced voltage to the phase wires, thereby reducing the voltage across the tower insulation. The resistance of the TLSA installed between the top phase wire and the tower top will reduce the magnitude of the shielding currents. Therefore, the number of induced flashovers caused by near-miss lightning strikes may be

higher. Induced flashovers become statistically rare when the CFO of the tower insulation system is above 500 kV. As such, induced flashovers are usually statistically rare on transmission lines of 115 kV and above.

Another advantage resulting from the replacement of shield wires with top phase arresters is the elimination of shield wire losses. These losses are negligible if the shield wires are isolated from the tower by insulators, but they can be substantial at full load if they are not isolated, as

described in (Red 2006). Shield wire losses should be compared to arrester losses, which are continuous but small. In many cases, depending on load variations, arrester losses are less than non-isolated shield wire losses.

Deciding whether to install top phase arresters or shield wires is a question of balancing the economic considerations of the two options against the resulting like performance. Engineers must compare the cost of installing and maintaining an arrester on each pole to the cost of

stringing and maintaining shield wires, along with the cost of losses for both options. In addition, they must confront the problem of predicting both maintenance costs and line performance for a line with arresters, a problem made more difficult in view of the question about failure rates noted earlier. In any case, line performance software is a valuable tool for comparing the various design options and arriving at a decision.

Horizontal Circuits

Unshielded horizontal circuits can also benefit from the application of TLSA. These differ from vertical circuits in that both outside phase wires, while exposed to direct lightning strikes, often provide the center phase with a high degree of shielding from direct strikes. Only descending leaders that are directly over the line are likely to hit the center phase. Table 6-2 shows the strike frequency calculated for a 60-foot-high transmission line at two different phase spacings. The table was generated with the electrogeometric model in the EPRI Lightning Protection Design