Vibration in Transmission Line:
Overhead transmission lines are designed to carry electrical power over long distances by suspending conductors between towers or poles. When the wind blows, it causes the conductors to swing back and forth in the vertical plane, which is known as "swaying." However, in addition to swaying, transmission lines also experience two types of vibration: aeolian vibrations and galloping vibrations.
What are Aeolian vibrations?
Aeolian vibrations occur when the wind speed is between 5-20 km/hr, and there is a vortex phenomenon that creates a spiral movement of air around the conductor. This causes the conductor to vibrate in a sinusoidal loop across its span with a magnitude of 20-50 mm and a frequency of 50-100 Hz. If the vibration is not controlled, it can lead to the breakdown of conductors from supports and clamps. The length and frequency of the vibration loop can be calculated using the formula:
Length L = 1/2f √(T/W)
where T is the tension on the conductor, W is the weight of the conductor, and f is the frequency. The frequency is determined using the formula:
f = 50(V/d)
where V is the wind velocity and d is the diameter of the conductor.
Method to Damp Out Aeoline Vibrations:
The aeolian vibrations in transmission lines can be dampened by using vibration dampers. Vibration dampers are devices that are designed to absorb the vibrational energy of the conductor and prevent vibrations. They are typically installed at points where the vibrations are restricted or cause problems.
One type of vibration damper that is commonly used to damp out aeoline vibrations is the stock bridge damper. The stock bridge damper is named after its inventor, George H. Stockbridge. The damper consists of a stranded cable that is between 0.3 to 0.5 meters long. Two hollow weights are attached at either end of the cable, and the damper is clamped to the line conductor.
The weights are made of galvanized iron, and the clamp is attached to the conductor using one bolt. The stranded cable of the damper dissipates the vibrational energy of the conductor by hysteresis and inter-strand friction, which reduces the amplitude of the vibration.
The stock bridge damper is a highly effective method of dampening aeoline vibrations, and it can be installed on the line conductor even when the line is carrying current. The dampers are typically spaced at regular intervals along the transmission line, and the spacing depends on the diameter of the conductor and the wind speed in the area.
Vibration Dampers in Transmission Line:
Vibration dampers are devices used to reduce the amplitude of the vibrations in overhead transmission lines. The use of dampers is particularly important in areas where there are high wind speeds, ice loading, or other environmental factors that can cause vibrations in the line conductors. There are different types of vibration dampers available, and each is designed to target a specific type of vibration.
The Stockbridge damper is one of the most commonly used vibration dampers in transmission lines. This type of damper is a specially designed device that helps to reduce aeoline vibrations. It consists of a stranded cable 0.3 to 0.50 m long to which two hollow weights are attached at either end. The weights are usually made up of galvanized iron, and the clamp is attached to the conductor using one bolt.
When the conductor vibrates due to the aeoline effect, the weights on the damper oscillate in opposite directions, causing the cable to vibrate. The vibration of the cable absorbs the energy of the conductor's vibration, thereby reducing the amplitude of the vibration. The stranded cable is designed in such a way that it dissipates the vibrational energy of the conductor through hysteresis and inter-strand friction, thus damping out the vibrations.
The Stockbridge damper is very effective in damping out aeoline vibrations, and it can be attached to the line conductor even when the line is carrying current. It is a reliable and cost-effective solution to reduce vibrations in overhead transmission lines, and it is widely used in transmission line installations across the world.
Galloping or Dancing Vibrations:
Galloping or dancing vibrations are low frequency and high amplitude vibrations that occur in overhead transmission lines. These vibrations are characterized by a frequency of about one hertz and an amplitude of about 6 meters. The vibrations are self-excited, which means that once they start, they continue to build within themselves and become very large.
The main cause of these vibrations is the irregular coating of sleet on the line conductors. When the sleet accumulates on the conductor, it can cause the conductor to become unbalanced, resulting in horizontal and vertical vibrations with large amplitude in an irregular manner in two or more loops. Sometimes the two loops appear superimposed on one loop.
These vibrations can have a number of negative effects on the transmission line. For example, they can lead to contact between phases or between the phase conductor and ground wire, which can result in a short circuit. In addition, they can cause mechanical damage at the supports, which can lead to the failure of the transmission line. The worst effect of galloping may be a flashover between two phases, which can cause power outages and damage to the transmission equipment.
To prevent galloping, vibration dampers are used on transmission lines. These dampers are designed to absorb the energy of the vibrations and prevent them from building up within the transmission line. They work by introducing damping forces that reduce the vibrations' amplitude and dissipate the vibration's energy.
Method to Damp Out Galloping Vibrations:
Circular conductors are less susceptible to galloping vibrations as they have a smooth and uniform surface that reduces the chance of irregular sleet coating. The use of PVC cable wrapping is a simple and effective method to convert stranded conductors into circular ones. PVC wrapping is wrapped around the conductor at regular intervals to make them circular. This wrapping increases the diameter of the conductor and makes it smooth and uniform.
However, this method may not be effective if there is heavy sleet formation on the conductors. In this case, the I2R losses can be utilized to reduce the sleet coating. When an electric current flows through a conductor, it produces heat due to the resistance of the conductor, known as the I2R losses. By increasing the current in the conductor, the heat produced can be used to melt the sleet coating on the conductor.
The increased current can be achieved by using a series reactor in the transmission line. The series reactor will cause a voltage drop in the line, resulting in an increase in current. The increased current will produce heat, which will melt the sleet coating on the conductor. However, this method can only be used for a short duration as the increased current will also increase the power losses in the line, which is not economical for a long duration.
Another method to reduce sleet formation is to use de-icing equipment like heaters and hot air blowers. These devices can melt the sleet coating on the conductor and prevent the formation of ice. However, these methods require a continuous power supply, which may not be available in remote areas.