Application of composite insulators on transmission lines has increased steadily due to their performance advantages as well as lower acquisition and life cycle costs. At the same time, these insulators have also offered solutions in other areas where ceramic insulators have not proven ideal due to weight and fragility. One such application is as interphase spacers to prevent conductor galloping. In this case, composite spacers serve to improve the aerodynamic or ‘aeroelastic’ behavior of lines exposed to difficult climatic effects such as changing temperatures and wind as well as combinations of wind with ice or sleet deposits. Interactions of these have the potential to produce harmful stresses on different parts of the line through induced motion and conductor vibration – stresses that can cause extensive damage and also endanger line service performance. Insulator expert, Alajos Bognár from Hungary (now retired), reviews the problem of conductor galloping and how application of composite interphase spacers can help control it.
Because of complex physics and the highly unpredictable way conductor motions and vibrations can develop over time, it is often impossible to predict and deal with all potential future galloping problems during the design stage of an overhead line.
Finding the best solution when such problems arise first requires evaluating service experience to gain the necessary insight into all the factors that influence onset of destructive stresses and vibrations on the line. This way, there is the greatest chance of finding the most effective measures to keep these within allowable limits. Interphase spacers are among the control devices available to engineers to prevent the most dangerous kinds of vibrations, such as those caused by ‘sleet jump’. But in order to understand how these devices can help and to better assess their effectiveness, it is necessary to first review some important principles.
Development of Vibrations Along Transmission Lines
Even though transmission lines have a highly flexible form and relatively slender profile, they are continuously exposed to the forces of climate and wind. This makes them susceptible to development of sustained, cyclic conductor motions or vibrations. The specific type of vibration depends on several factors and can take different forms:
1. Aeolian Vibrations
These are caused by alternating wind forces and impart an oscillating lifting force to the conductor. This type of vibration is characterized by frequencies in the range from 5 to 100 Hz, with amplitudes of only a few mm up to the full diameter of the conductor. The phenomenon is dangerous, because fatigue-bending stress can occur at or near conductor fixation points such as clamps and cause breakage of individual strands. It is common to use special dampers (e.g. the Stockbridge damper) to minimize the level of such vibrations.
2. Wake-Induced Oscillations
Such oscillations are typical only for bundle conductors for which some sub-conductors are in the wake induced by those to the windward side. The latter then provide a shielding effect on their leeward counterparts. Four types of such wake-induced motion can be distinguished: sub-span mode (in a section between two spacers of the bundle); vertical galloping; horizontal galloping; and rolling. Basically, the oscillation is produced by the wake, which induces lower drag coefficients and creates lifting forces on leeward sub-conductors. Due to this kind of aerodynamic instability, the leeward conductor starts to move and, because of conductor spacers inside the bundle, the windward conductor is forced to participate in the movement. While this kind of vibration does not usually lead to a pronounced reduction in phase-to-phase distances, a portion of the wind’s mechanical power is absorbed by the conductors and can cause fatigue, especially if repeated over long periods of time.
Conductor galloping (also referred to as dancing) is a phenomenon where transmission line conductors vibrate with very large amplitudes. This produces violent dynamic stresses in the conductors that can lead to damaged insulator strings and tower structures. Under certain conditions, conductors having different potential can come into contact or dangerously approach one another such that a short circuit occurs. Preventing this phenomenon is therefore of fundamental importance in maintaining reliable service of the line.
Interaction of Iced Conductors & Wind
One of the most common ways galloping develops is as a result of interaction between a steady, moderately strong crosswind that acts on an asymmetrically iced conductor surface. Through this type of ‘self-excitation’ process, periodic, high amplitude vibrations (or oscillations) can occur on either single or bundle type conductors. The amplitudes can approach the value of the sag and have motion mainly in the vertical plane, with frequencies of between 0.15 and 1 Hz depending on line design and specific mode of excitation. This form of conductor galloping can affect several neighboring spans and last from only a few minutes to much longer. Most of the observed galloping in this case takes the form of standing waves that occur with one, two or sometimes as many as 10 loops in a span (although galloping events with three or less waves per span are most common). In the case of more than three loops, the galloping generally has smaller amplitudes.
When this problem develops, the oscillating motions of the phase conductor and overhead ground wire can lead to contact. Alternatively, the critical air gap breakdown distance between conductors of different phases or between the phase conductor and ground wire can be reduced. This will result in short-circuit and operation of the relevant circuit breakers to switch-off the line.
In the case of automatic reclosing, switch-on will be successful. However, contact or approach of the conductors will quickly re-occur because of the periodicity of the galloping phenomenon. The repetitive short circuit can then damage the conductors through the high energy, high temperature power arcs that develop between the conductors as they approach too close to one another or come into contact. The negative mechanical effects have to be considered as well (as mentioned above). close to one another or come into contact. The negative mechanical effects have to be considered as well (as mentioned above).