Mechanical oscillation of transmission lines.

1. Presented By
Anand Kishore Azad
M.Tech 1st Year (PSA)
EECE Dept .

2. WIND INDUCED OSCILLATIONS
• Wind-induced vibration of overhead conductors is common
worldwide and can cause conductor fatigue near a hardware
attachment.
• Problem created by vibration and oscillation of the very heavy
conductor arrangement required for E.H.V transmission lines.
• In the last twenty years All Aluminum Alloy Conductors
(AAAC) has been a popular choice for overhead conductors
due to advantages in both electrical and mechanical
characteristics. Unfortunately AAAC is known to be prone to
Aeolian vibration.
• Vibration dampers are widely used to control Aeolian vibration
of the conductors and earth wires including Optical Ground
Wires (OPGW).

3. • In recent years, AAAC conductor has been a popular choice
for transmission lines due to its high electrical carrying
capacity and high mechanical tension to mass ratio. The high
tension to mass ratio allows AAAC conductors to be strung at
a higher tension and longer spans than traditional ACSR
(Aluminum Conductor Steel Reinforced) conductors.
• Unfortunately the self-damping of conductor decreases as
tension increases. The wind power into the conductor increases
with span length. Hence AAAC conductors are likely to
experience more severe vibration than ACSR.
• Wind can generate three major modes of oscillation in
suspended cables.
1. AEOLIAN VIBRATION
2. GALLOPING VIBRATION
3. WAKE-INDUCED VIBRATION

4. Vibration degree of severity depending
on the various factors like :
1. Conductor tension
2. Span length
3. Conductor size
4. Type of conductor
5. Terrain of line
6. Direction of prevailing winds
7. Types of supporting clamp
8. Tower type
9. Height of towers
10. Type of spacer and damper

5. AEOLIAN VIBRATION
• Aeolian vibration (sometimes termed flutter) has amplitude of
millimeter's to centimeter's and a frequency of 3 to 150 Hz.
• Wind-induced vibration or Aeolian vibration of transmission line
conductors is a common phenomenon under smooth
wind conditions. The cause of vibration is that the vortexes shed
alternatively from the top and bottom of the conductor at the
leeward side of the conductor.
• The vortex shedding action creates an alternating pressure
imbalance, inducing the conductor to move up and down at right
angles to the direction of airflow.
• The conductor vibration results in cyclic bending of the
conductor near hardware attachments, such as suspension clamps
and consequently causes conductor fatigue and strand breakage.
• When a “smooth” stream of air passes across a cylindrical shape,
such as a conductor or OHSW, vortices (eddies) are formed on
the back side. These vortices alternate from the top and bottom
surfaces, and create alternating pressures that tend to produce
movement at right angles to the direction of the air flow. This is
the mechanism that causes Aeolian vibration.

6. • Vortex Frequency (Hertz) = 3.26 V / d
Where: V is the wind velocity component normal
to the conductor or OHSW in miles per hour
d is the conductor diameter in inches
3.26 is an empirical aerodynamic constant.
Effects
• Abrasion is the wearing away of the surface of a conductor or
OHSW and is generally associated with loose connections between
the conductor or OHSW and attachment hardware or other
conductor fittings.
• Abrasion damage can occur within the span itself at spacers Fatigue
failures are the direct result of bending a material back and forth a
sufficient amount over a sufficient number of cycles.

7. • In a circular cross-section, such as a conductor or OHSW, the bending
stress is zero at the center and increases to the maximum at the top and
bottom surfaces (assuming the bending is about the horizontal axis). This
means that the strands in the outer layer will be subjected to the highest
level of bending stress and will logically be the first to fail in fatigue.
• In standard conductors the freedom of movement (self damping) will be
reduced as the tension is increased. It is for this reason that vibration
activity is most severe in the coldest months of the year when the
tensions are the highest.
• Aeolian vibrations mostly occur at steady wind velocities from 1 to 7
m/s

8. GALLOPING VIBRATION
• Conductor gallop is the high-amplitude, low-frequency
oscillation of overhead power lines due to wind.
• The movement of the wires occurs most commonly in the
vertical plane, although horizontal or rotational motion is
also possible. The natural frequency mode tends to be
around 1 Hz, leading the often graceful periodic motion to
also be known as conductor dancing.
• The oscillations can exhibit amplitudes in excess of a meter,
and the displacement is sometimes sufficient for the phase
conductors to infringe operating clearances (coming too
close to other objects), and causing flashover.
• Galloping is induced by the winds ranging from 15 to 50
km/hour.
• Galloping is controlled by using Detuning Pendulums.

9. Wake Induced oscillation
• The wake induced oscillation is peculiar to a bundle
conductor similar to aeoline vibraton.
• The frequency of oscillation is not more than 3 Hz may be
sufficient amplitude to cause clashing with adjacent sub-
conductor which are separated by 50cm .
• Wind speed for causing this oscillation is normally in range
25 to 65 km/hour.
• Wake Induced oscillation also called as ‘Flutter instability’ is
caused when one conductor on the windward side
aerodynamically shields the leewards conductors.
• The oscillation occurs when conductor tilts 5 to 15 degree
with respect to flat ground surface.

10. WORKING OF VIBRATION DAMPER
• When the damper is placed on a vibrating conductor,
movement of the weights will produce bending of the steel
strand. The bending of the strand causes the individual
wires of the strand to rub together, thus dissipating energy.
The size and shape of the weights and the overall geometry
of the damper influence the amount of energy that will be
dissipated for specific vibration frequencies.
• Since, as presented earlier, a span of tensioned conductor
will vibrate at a number of different resonant frequencies
under the influence of a range of wind velocities, an
effective damper design must have the proper response over
the range of frequencies expected for a specific conductor
and span parameters.

11. 1. VORTX/ Stock bridge Type
• Some dampers, such as the VORTX Damper utilize
two different weights and an asymmetric placement
on the strand to provide the broadest effective
frequency range possible.
.

12. • The “Stockbridge” type vibration damper is commonly used to
control Aeoline vibration of overhead conductors and OPGW.
The vibration damper has a length of steel messenger cable.
Two metallic weights are attached to the ends of the messenger
cable.
• The centre clamp, which is attached to the messenger cable, is
used to install the vibration damper onto the overhead
conductor.
• Placement programs, such as those developed by PLP for the
VORTX Damper, take into account span and terrain
conditions, suspension types, conductor self-damping, and
other factors to provide a specific location in the span where
the dampers will be most effective.

13. 2. Spiral Vibration Damper
• For smaller diameter conductors (< 0.75”), overhead shield
wires, and optical ground wires (OPGW), a different type of
damper is available that is generally more effective than a
Stockbridge type damper.

14. • The Spiral Vibration Damper (Figure 15) has been used
successfully for over 35 years to control Aeolian vibration
on these smaller sizes of conductors and wires.
• The Spiral Vibration Damper is an “impact” type damper
made of a rugged non-metallic material that has a tight
helix on one end that grips the conductor or wire. The
remaining helixes have an inner diameter that is larger
than the conductor or wire, such that they impact during
Aeolian vibration activity. The impact pulses from the
damper disrupt and negate the motion produced by the
wind.