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State of the art of conductor galloping

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State of the art of conductor galloping

    8. Risks, prevention, anti-galloping devices and design guide 8.1 Protection methods

    The reader who would like to have access to more detailed discussion may go back to a recent publication published by the CIGRE TFG into ELECTRA [Cigre, 2000b].

    We reproduce here the conclusions:

    ( The complexity of galloping is such that control techniques cannot be adequately tested in the laboratory and must be

    evaluated in the field on trial lines. This testing takes years and may be inconclusive.

    ( Analytical tools and field test lines with artificial ice are useful in evaluation of galloping risk and appropriate design

    methods.

    ( No control method can guarantee it will prevent galloping under all conditions.

    ( Interphase spacers virtually ensure galloping faults will not occur, but do not necessarily prevent galloping. Their

    usage is growing and their design is undergoing further development.

    ( Mechanical dampers to stop vertical motion are still being pursued but to only a very limited extent. ( Torsional devices, which either detune or increase torsional damping or both, are being pursued and actively evaluated. ( Techniques which disrupt either the uniformity of ice accretion by presenting a varying conductor cross-section or the

    uniformity of the aerodynamics by inducing conductor rotation are being actively pursued.

    ( Methods of ice removal or prevention are not widely used as specific anti-galloping practices.

    ( Despacering or using rotating-clamp spacers is still used extensively in a number of parts of Europe subject to wet

    snow accretions.

    ( For bundled conductors, the influence of the design of suspension and anchoring dead-end arrangements on the

    torsional characteristics of the bundle and on the occurrence of vertical/torsional flutter type galloping has been

    recognized.

    8.1.1 Galloping control methods for existing lines

    A survey of the various known galloping control methods was recently completed under the aegis of CIGRE and published in ELECTRA [Cigre, 2000b]. The various control approaches were classified as “retrofit” or “design” systems. This

    chapter will provide descriptions of “retrofit” devices. The ELECTRA paper also includes a list of discontinued methods. This chapter will focus on control devices which are considered to be practical, and in use, at least on a trial basis, on operating lines. Whenever possible, practical issues relating to ease of installation and side effects attributable to the devices will be summarized. A table forms the final section of this chapter, combining the key information about the application of each of the devices in current use. The devices will be discussed in this chapter including, where possible, the following aspects:

    • For which type(s) of weather exposure and line construction has each device been tested and applied.

    Galloping can be caused by a range of different conditions, namely the type, density and adhesion of the ice, be it

    glaze, wet snow, or hoar frost, and the speed, direction, and turbulence of the wind. Most of the North American

    experience is with galloping due to wind acting on glaze ice accretions. Galloping due to wind acting on wet

    snow has received more attention in Japan and parts of Europe. The type of icing under which each device has

    been evaluated will be included along with known practical details. It is also different on small versus large single

    conductors, on bundle conductors versus single conductors, and on deadend spans versus suspension spans.

    There are even rare conditions, with wind but without ice, in which other mechanisms create galloping-like

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    motions. The common feature of all galloping is the excitation of the lowest natural frequencies of the spans and

    the resulting large amplitude, low frequency motions.

    • What are the proper locations for each galloping control device.

    The number of devices required for control, or the physical design of the devices, or the manner of application of

    the devices may also differ according to the expected type of ice accretion and the physical details of the

    conductor span. Where there are alternative practices, these are identified. While application practices for some of

    the devices are public knowledge, for some devices these are considered proprietary by the suppliers. • What are the limitations and precautions required with each galloping control device.

    The performance of a control device may be acceptable in one range of sizes of conductor while less acceptable

    in another size range. Also the effectiveness in one weather condition may or may not indicate effectiveness in a

    different form of icing.

    • Observed motions without and with each control device.

    Data from tests on scaled or full size test lines, sometimes with airfoils to represent ice are included where

    available. More weight should be given to information obtained from observations on actual operating lines,

    especially where there are systematic trials including untreated phases similar to the phases with the control

    devices, and such results are included where possible. When galloping does occur in a span of an overhead line,

    the individual conductors are frequently moving at different amplitudes and in different modes under nominally

    the same exposure to ice and wind. During an ice storm the galloping amplitudes change as the speed and

    direction of the wind, as well as the amount of ice deposited changes. This randomness and variability are

    inherent in the galloping phenomenon. Conclusions on the overall performance of a device need to be based on a

    number of separate galloping events. The greatest confidence can be placed on the devices that have been the

    subjects of the widest exposure and evaluations. At the same time the control device needs to be installed on one

    or more phases in the same span as nominally identical phases without controls. Galloping motions on all the

    phases needs to be documented to enable proper statistically supportable conclusions on performance of the

    control devices to be obtained.

    8.1.2 Interphase spacers

    The most widely use galloping control device is the interphase spacer. Stiff and flexible interphase spacers have been used on 115, 230 and 500 kV lines since the 1970s [Edwards and Ko, 1979]. The earliest stiff spacers were assembled from ceramic insulator sections joined with an aluminum tube, and attached to the conductors using standard suspension clamps. These spacers were heavy and difficult to handle and install (Figure 8.1). Some early rigid spacers suffered breakages of the insulating sections due to the high compressive forces occurring during galloping, and there were failures of the welded joints at the ends of the central aluminum tube. Later polymeric insulators were substituted for the ceramic sections creating a lighter and more manageable, but still rigid, assembly (Figure 8.2). Flexible clamps were also used, but special means were needed to avoid arcing at the sliding surfaces. More recently armor grip suspension (AGS) clamps have also been used to reduce local stresses in the conductors at the points of attachment. Subsequent designs were made more flexible through joints within the length of the spacers, initially retaining the metal middle section (Figure 8.3). Later designs substituted silicone rubber covered fibreglass rods for the metal sections. These changes effectively created a chain of insulated links between the phases. Figure 8.4 and Figure 8.5 show designs of such interphase spacers used at 230 kV and 500 kV respectively, in the CEA sponsored field trials of galloping controls for bundled conductors [Pon and Havard, 1994]. The joints are bridged with flexible metal bonding straps to eliminate arcing from movements of the loose joints. Corona rings are mounted at the high voltage ends of the sheds of the polymeric insulators to reduce the electric field gradient and minimize arcing damage to the sheds. A sample installation of a flexible interphase spacer on a Manitoba Hydro 500 kV triple bundle line is shown in Figure 8.6.

    While most of the applications are to vertical or near vertical circuit arrangements, interphase spacers have also been applied to horizontally arranged circuits with galloping problems. One such design for a two conductor bundle line in northern Norway which has experienced frequent winter damage, is shown in Figure 8.7. This rigid design uses composite insulators and has a tubular steel central section. It is underslung to ensure that the bundle stays in its normal orientation.

    Some cases of damage to interphase spacers have occurred with this design.

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    Figure 8.1: Installation of a rigid interphase spacer [Havard, 1978]

    Figure 8.2: Stiff hybrid spacer for 115 kV lines [Pon et al., 1982]

     Figure 8.3: Flexible spacer for use at 500 kV [Pon et al., 1982]

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     Figure 8.4: Flexible polymeric interphase spacer for use at 230 kV [Pon and Havard, 1994]

     Figure 8.5: Flexible polymeric interphase spacer for use at 500 kV [Pon and Havard, 1994]

     Figure 8.6: Flexible polymeric interphase spacer installed on a triple bundle 500 kV line in Manitoba Hydro [Pon and Havard, 1994]

     Figure 8.7: Rigid interphase spacer for a horizontally aligned two conductor bundle circuit in northern Norway [Loudon,

    2003]

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    The proper location scheme for the interphase spacers in the span is important to ensure that the flashovers due to conductor clashing are avoided. Some interphase spacers have been installed at the point of largest galloping amplitude, namely the mid point of the spans. While this should avoid clashing and reduce the motions during single loop galloping, this point has the least impact on two-loop galloping.

    Overhead line circuits in Germany can be vertical, horizontal or in a delta configuration. Interphase spacer location schemes, that are intended to be economical while still establishing an adequate level of control, have been proposed for each of these circuit designs [Schmidt and Jürdens, 1989]. The scheme for the delta circuit arrangement is shown in Figure 8.8. This spacing scheme was installed on over 100 spans of a twin bundle line in southern Germany, but the report does not include any field experience during galloping events. They also note that spans which include a transition from one configuration to another, or which serve to rotate the phases for electrical load balance, are at particular risk of flashovers during galloping. They recommend use of interphase spacers at the closest point of approach of the conductors. Figure 8.9 illustrates recommended use of two or four interphase spacers per span of a vertically oriented circuit [Edwards and Ko, 1979]. In either of these arrangements the interphase spacers could be effective in both the single and two-loop modes. These arrangements were used in the field evaluations, but the alternative using four spacers is preferred, because there is still a possibility of contact between the phases at the quarter points in the span during mixed mode galloping with only two spacers. A diagram of this type of mixed mode motion, observed during a galloping event on Ontario Hydro lines, is shown in Figure 8.10. The upper and lower phase conductors are moving in a single loop mode, while the middle phase is in a two-loop mode. The middle phase conductor can approach the other conductors at the top of the left hand and at the bottom of the right hand interphase spacer.

    Figure 8.11 shows a double exposure of a more usual type of galloping motion on a span of a vertical circuit fitted with four interphase spacers. This shows that galloping motion can occur but the spacers maintain the phase separation and minimize the likelihood of phase to phase contacts.

    Field trials of interphase spacers were in place on Ontario Hydro lines during the 1970s [Pon et al., 1982]. In that period a number of manufacturers’ products were installed, and most of the installations were on single conductor lines with stiff spacers. The field results from single conductor lines only are presented in summary form in Table 8.1. The field data from single conductors are also presented graphically as a plot of peak-to-peak amplitude versus the fraction of the observations in Figure 8.12. The x-axis scale is based on the Weibull statistical analysis of values of extreme events (such as flood levels of rivers), and allows linear projection to give predictions of behavior beyond the plotted data. The figure includes all values of peak-to-peak amplitude on the untreated phases and all those with interphase spacers recorded during 10 separate galloping events. This figure shows that there is, on average, a reduction in the reported galloping amplitudes, but there are still large amplitudes of motion on the lines with interphase spacers.

    In Figure 8.13 the same data are divided by sag and plotted against number of data points on the same scale as in the previous figure. This form of presentation compares directly with the design guides in which the galloping clearance envelopes are scaled to the sag of the conductor. The maximum amplitude is reduced from 0.52 x sag to 0.38 x sag, a reduction of 27%.

    Results from Ontario Hydro’s trials of interphase spacers on bundled conductors are included in Table 8.2, as part of the results of the CEA field trials.

    The CEA field trials of galloping control devices for bundled conductor lines [Pon and Havard, 1994] included four sites with flexible interphase spacers on twin, triple and quad bundle lines. Figure 8.6 shows an installation on a 500 kV triple bundle line in Manitoba Hydro from that field program.

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     Figure 8.8: Mounting scheme for interphase spacers on a delta circuit [Schmidt and Jürdens, 1989]

     Figure 8.9: Alternative arrangements of interphase spacers in a span of a vertical circuit [Edwards and Ko, 1979]

     Figure 8.10: Forced motion of the middle phase conductor during mixed mode galloping with two interphase spacers [Pon et al., 1982]

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     Figure 8.11: Double exposure photo of a span equipped with rigid interphase spacers during galloping showing two-loop motion [Pon et al., 1982]

     Figure 8.12: Effect of interphase spacers on peak-to-peak galloping amplitude based on 10 observations on single conductors [Pon et al., 1982]

     Figure 8.13: Effect of interphase spacers on peak-to-peak galloping amplitude/sag based on 10 observations on single conductors [Pon et al., 1982]

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    Table 8.1: Summary of performance of interphase spacers during eight galloping events on Ontario Hydro’s single

    conductor lines [Pon et al., 1982]

    LOCATION & CONDUCTOR DATA AVERAGE PEAK-TO-PEAK AVERAGE PEAK-TO-PEAK

    GALLOPING AMPLITUDE GALLOPING AMPLITUDE VOLTAGE

    WITH INTERPHASE SPACERS WITHOUT INTERPHASE

    (m) SPACERS (m)

    Kleinburg - 2332.8 kcmil ACSR 2 0

    45.7 mm diam 190 m Richview 230 kV

    (620 ft) span

    Kleinburg - 2332.8 kcmil ACSR 1.5 0

    45.7 mm diam 259 m Richview 230 kV

    (850 ft) span

    Kleinburg - 2332.8 kcmil ACSR 0 2.2

    45.7 mm diam 259 m Richview 230 kV

    (850 ft) span

    Kleinburg - 2332.8 kcmil ACSR 2.7 1.4

    45.7 mm diam 259 m Richview 230 kV

    (850 ft) span

    Major Mackenzie 2332.8 kcmil ACSR 0 4.8

     Nashville 230 45.7 mm diam 213 m

    kV (700 ft) span

    Kleinburg 2332.8 kcmil ACSR 1 1.7

    45.7 mm diam 259 m Richview 230 kV

    (850 ft) span

    Kleinburg - 2332.8 kcmil ACSR 1.4 2.1

    45.7 mm diam 259 m Richview 230 kV

    (850 ft) span

    Kleinburg - 2332.8 kcmil ACSR 1.7 1.5

    45.7 mm diam 259 m Richview 230 kV

    (850 ft) span

    The field trials of interphase spacers on bundled lines produced four documented galloping observations and these are summarized in Table 8.2. The results include three events in which there were no visible motions on the phases linked by the interphase spacers and small amplitude motions on the reference untreated phases. One event included significant motions on both the treated and untreated phases. These four results were not considered sufficient to draw conclusions about the overall performance of these devices under the range of ice and wind conditions conducive to galloping. A world wide survey in the 1990s [Cigre, 1992] showed data from 32 utilities in 13 countries with nearly 13,000 installed interphase spacers. The survey reported that these are used on lines at voltages from 11 kV to 420 kV. The questionnaire investigated the opinion of the utilities with regards to both performance as a control device during galloping, and the experience with respect to damage and maintenance required of the interphase spacers.

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    Table 8.2: Summary of performance of interphase spacers during four galloping events on bundled conductor lines [Pon et

    Havard, 1994]

    LOCATION & CONDUCTOR DATA AVERAGE PEAK-TO-PEAK AVERAGE PEAK-TO-PEAK

    GALLOPING AMPLITUDE GALLOPING AMPLITUDE VOLTAGE

    WITH INTERPHASE SPACERS WITHOUT INTERPHASE

    (m) SPACERS (m)

    Manitoba 3 x 1271 kcmil ACSR 6.1 7.6

    35.1 mm diam 377 m Hydro 500 kV

    (1237 ft) span

    Ontario Hydro 4 x 585 kcmil ACSR 24.1 0 1.5

    mm diam 255 m (837 ft) 500 kV

    span

    Ontario Hydro 4 x 585 kcmil ACSR 24.1 0 1.5

    mm diam 274 m (899 ft) 500 kV

    span

    Ontario Hydro 4 x 585 kcmil ACSR 24.1 0 0.6

    mm diam 274 m (899 ft) 500 kV

    span

Solely from the performance point of view the survey indicated:

    • Many survey responses indicated that there were no phase to phase or phase to ground flashovers after installing the

    interphase spacers

    • Some low amplitude galloping was seen after spacer installation, but large amplitude motions appear to be eliminated • Clashing was prevented but galloping continued at a lower level

    • Wear and conductor damage occurrences were reduced

    Reported side effects of using the interphase spacers included:

    • Some mechanical damage to the insulator sections of the spacers in the form of cracking of the sections with sheds

    • Electrical and mechanical breakdown in some urban areas, due to tracking attributed to pollution.

    • A few cases of compression failures during galloping

    • Some spacers damaged by birds pecking at the insulator sheds

    • Some porcelain insulator sections were replaced by polymer insulators.

    Spacers have proved effective at eliminating phase to phase contacts during galloping but there can still be conductor motions and dynamic loads on the support structures.

    Recently, studies of interphase spacer behaviour during simulated galloping have been carried out at IREQ, Hydro Quebec’s research facility [Van Dyke and Laneville, 2004]. A “D’ section foil was attached to the conductors to produce

    galloping at any time of year providing the winds were adequate. Tests were conducted on full scale test line with a vertical conductor arrangement simulating a 120 kV line with and without interphase spacers. Interphase spacers were efficient in preventing conductors from coming close together, however, higher galloping amplitudes were reached with it.

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    It contradicts some observations from the field which indicate that conductors are generally less prone to galloping when equipped with interphase spacers. This may be attributed to the fact that in the field, the natural ice accretion may be different on each conductor and one conductor may act as a damper while the other one alone would experience severe galloping. Moreover, when there are no interphase spacers, conductors tilt under the effect of drag. Consequently, instead of remaining vertical, the initial angle of incidence on the D-section varies between 0º and 20º, depending on the perpendicular component of the wind speed. This may explain why the amplitude does not increase much with the wind speed when there are no interphase spacers since the angle of incidence exceeds the range of galloping instability. However, when there are interphase spacers, even for high wind speed, the conductors will remain mainly vertical because they are linked together at two points along the span. The D-sections were installed vertically and their initial angle of incidence remained the same regardless of wind speed. The higher torsional flexibility of the configuration without interphase spacers may facilitate the initiation of galloping but it may also set a lower bound for galloping amplitudes since the conductor torsion added to the apparent angle of attack (ratio of conductor speed over wind velocity) may bring the conductor out of its range of instability at lower amplitudes. Since the range of instability (in term of angle of attack)

    of a natural ice accretion is not necessarily located around the at-rest position of the conductor, its susceptibility to gallop

    may also be reduced by interphase spacers while it was increased for the D-section.

    In summary, the interphase spacers have a good track record for eliminating flashovers during galloping but they do not prevent the galloping motions. Observations in the field show that motions still occur with interphase spacers in place, especially when the galloping conditions are such that high levels of motion can occur. The side effects of galloping such as high loads on the support structures and damage to the conductors at the suspension clamps can still be a problem with interphase spacers. Interphase spacers are also subject to breakage if they are not designed well enough for the dynamic loads applied to them.

    8.1.3 Aerodynamic control devices

    8.1.3.1 Air Flow Spoilers

    Galloping can be suppressed by modifying the aerodynamics along the line with alternating profiles, as by the air flow spoiler and the twisted pair conductor described earlier. The standard Air Flow Spoilers are mainly used on single conductors up to 230 kV [Douglas and Roche, 1985; Whapham, 1982]. Above 230 kV, the standard PVC material can degrade in the high electrical gradient. For bundled conductors above 230 kV, a special PVC material is used to survive the high electrical gradient. The air flow spoiler is comprised of a circular cross section, ultra violet light resistant, PVC

    rod with a diameter almost as large as the conductor and length of 4.3 m (14 feet). It is formed to create gripping sections at each end and the middle section is wrapped around the conductor as shown in Figure 8.14. The air flow spoilers amounting to about 25% of the span length are applied in two groups around the quarter and three quarter points in the spans.

     Figure 8.14: Air flow spoiler for galloping control on smaller conductors and overhead ground wires [Whapham, 1982] Field trials of the air flow spoiler were conducted in the 1980-1986 period and these showed that galloping could be suppressed except when the ice covering is excessive and overwhelms the shape effect of the twisted profile. The field data obtained are summarized in Figure 8.15. This figure contains plots of field records of peak-to-peak galloping amplitude divided by sag versus percent of the data, using the Weibull extreme values probability scale. The left figure is from conductors without the air flow spoilers, and the right figure is from identical conductors with air flow spoilers within the same spans of operating lines. Division by sag allows comparisons with the galloping ellipses sizes used in design of clearances between conductors in structure profiles. The observations were obtained by field staff of cooperating US and Canadian utilities, during 31 different galloping events. They show that the air flow spoilers reduce the maximum galloping amplitudes from 1.3 to 0.5 times the sag, a reduction to 38 percent. The figures also show that there were no observable galloping motions during 20 of the 31 events when the air flow spoilers were installed.

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