I was recently invited to Tsinghua University in Beijing to give lectures and, while there, I was searching for photos of applications of transmission line arresters in China to support my topic. Then – lo and behold – in my email inbox came the Photo of the Week in INMR’s eNews #60. Aside from giving me exactly what I was looking for, it also provided a way to alert my audience to the technical value of the Chinese-language version of INMR’s web site (www.inmr.com).
Another site – Linked In – allows members to exchange views on various specialized areas. A recent posting and response thread in the “High voltage lines/overhead transmission lines” group came from someone who asked: how many years can a space-damper be utilized on a line? Spacer-dampers hold the conductors of a bundle apart and provide mechanical damping, usually using an elastomeric element. According to experts, a properly designed, well-applied device should provide somewhere between 30 and 50 years’ service but must be tested for these requirements according to IEC 61854. Counter-examples cautioning an only 12-month service life were also posted.
The idea of a 30 to 50 year service life for mechanical components on a power line perhaps seems reasonable, given that it is possible to change out an exhausted spacer-damper if the underlying conductor remains in good condition. One problem, however, is that mechanical damage to phase conductors leads first to fatigue breaks in the aluminum strands that lie just below the outer layer. Such damage cannot easily be discerned visually although it can be repaired using armor rods – assuming it is discovered early enough.
Inspection of the eNews #60 image (photo 1) shows two possible areas of concern in this particular installation of externally-gapped line arresters. Both relate to measures to control conductor vibration. The conductors here have two complementary forms of protection against damage from long-term fatigue. Improved resistance to such damage is given by the armor rods wrapped around the phase conductors near the insulator clamps to increase outside diameter. Also, aeolian vibration dampers, consisting of two metal weights wired to a single clamp, are fitted about 2 m to the left of each double-chain insulator string. The choice of these special damper weights, their distance from the clamp and the overall distance between the assembly and the suspension clamp are all intended to absorb mechanical energy from the protected span at specific frequencies. The purpose of the vibration damper is to limit the deflection of the phase conductor, measured at 89 mm from the suspension insulator clamp, thereby reducing repetitive flexing that can lead to long-term fatigue after millions of cycles.
The double-chain insulator string at the right of photo 1 is electrically protected by a metal oxide resistor (MOR) body, hanging from a steel member that has been bolted to the tower. If you look carefully (photo 3), you will see that a small metal ring has been added to the phase conductor to provide the second electrode in the EGLA design. The air gap between the ring and oval electrodes on the bottom of the MOR is such that it will flash over under lightning impulse conditions but will withstand switching surge overvoltages. An oval clamp is used at the bottom of the MOR to keep air gap distance constant even when the conductor sways sideways.
The small metal ring is mounted just to the left of the aeolian vibration damper. This is a location that should have relatively low mechanical vibration level, compared for example to the anti-node location centered between the damper and clamp. However, the incremental weight of the ring may interfere with the performance of the damper by shifting its resonant frequencies.
The double-chain insulator string at the left (photo 2) is also electrically protected by an EGLA but, in this case, a short insulator with corona rings at each end is used to ensure that the gap remains constant under all conditions of conductor swing. The close-up shows that the short insulator is attached to the conductor about halfway between the insulator clamp and the Aeolian vibration damper. This clamping location, combined with the considerable weight of the insulator and the MOR may have more serious effects on damper performance. In this case, it would have been preferable to move the damper to a different location when installing the EGLA.
At present, there is still no industry guidance on what to do with vibration dampers when mounting arresters of this kind. For some time, damper manufacturers have mostly stood ideally by, even though voicing concern that applying arresters between dampers is basically a mistake. At the recent CIGRE meeting in New Zealand, a new multi-disciplinary task force (B2-TF007) was formed to better define the problem. Within two years, it is expected to provide preliminary guidance on how to protect vibration control systems from damage due to arresters and, at the same time, conductor-mounted arresters from damage due to vibration.
William A. Chisholm