FAQs on Galloping Control
FAQs on Galloping Control & Anti Galloping Devices
Galloping is a phenomenon of low-frequency, high-amplitude wind-induced vibration with a single or several loops per span. These waves are self-excited and sustained by the interaction of aerodynamic, inertia, and elastic forces.
Galloping usually occurs in moderately strong, steady wind, acting upon an asymmetrically iced conductor surface. Wind direction causing the galloping has to be more than 45° to the line axis.
Galloping is not to be confused with aeolian vibrations which are characterized by high-frequency, low-amplitude, multiple loop modes.
Wind and ice cause power lines to gallop. Depending upon the wind direction and speed it can create uplift on the iced-up conductor. Consequently, a galloping, or jumping, motion occurs.
While power lines can sway in high winds, it's the combination of wind and ice that causes them to gallop more forcefully.
Effects of wind velocity, ice shape, angle of attack, wind shear, turbulence, hysteresis, etc. are all elements in the mechanism of galloping.
CIGRE 322 advises on the following three protection methods as countermeasures to galloping:
- Removal or prevention of ice formation on conductors.
- Interrupting the galloping mechanisms to prevent galloping from escalating or attaining high amplitudes.
- Making lines tolerant of galloping through:
- ruggedness in design
- increased phase clearances
- controlling the mode of galloping with interphase ties.
Galloping can get started in any span and is prone to travel forward and back. Trying to control galloping in one span may not decrease the likelihood of galloping since it can get started in any span and travel in both directions. The objective should be to control the galloping amplitude to within safe limits (<5ft).
A conductor can gallop up to 100% of sag, which means that decreasing galloping amplitude by a certain factor (i.e. 20%) will not provide an adequate galloping solution.
Dampers that simply decrease the amplitude by a certain factor leave the conductor at risk for flash-over and outages.
AR uses proprietary software to evaluate a transmission line and to recommend the appropriate application of anti-galloping dampers. Our software calculates:
- The likelihood for flash-over or phase clashing,
- Critical wind speed where galloping initiates and where the aerodynamic lift is dumped,
- The predicted ellipses in galloping conditions.
All AR products offer galloping control by twisting the conductor. This methodology operates on two key principles – acceleration and inertia.
Experience has shown that twisting the conductor will arrest galloping, not simply decrease galloping amplitude. Generally, a conductor will gallop up to 100% of sag and gallop down a percentage as well.
A twisting mechanism controls galloping by interrupting the cycle before it gets going. As the angle of wind attack creates galloping conditions, the gravitational force on the AR damper causes the conductor to start twisting thereby dumping off the aerodynamic lift.
All AR Products are designed to interrupt (stop) galloping to prevent higher amplitudes.
When galloping control systems are used on low and medium voltage lines (115kV-230kV), corona is not a factor.
When voltages are higher (345kV, 500kV), the AR Products are corona protected.
To further protect the conductor most AR products are installed with line guards or Armor Rods.
These are two different phenomena:
- Galloping is a steady-state of large amplitude motion, however, there is little data or experimental evidence on the actual number of galloping cycles. The AR Twister is designed for a low-frequency conductor, you get only one cycle every 5-7 seconds depending upon the span length. Because twisting interrupts the conditions that lead to galloping, there are no cycles to get underway.
- When ice sheds in large chunks, it causes the conductor to jump or gallop. Ice shedding is an intermittent phenomenon and its effects have a negligible effect in causing galloping.
Galloping is a low-frequency, high-amplitude wind-induced motion of a single conductor. Uncontrolled galloping at high-amplitudes can cause flashover and outages
Aeolian vibration is a high-frequency, low-amplitude motion caused by smooth laminar winds flowing across a cable. Vibration that is unchecked can cause wear and damage to structures, hardware, and the conductor.
Yes, AR Twisters are designed to control Aeolian vibration and galloping at the same time, by the same unit.
This is a key element of the damper.
The MOD2 Spacer Damper controls galloping by the twisting mechanism in the clamps. The installation methodology is also a key factor in controlling high-amplitude galloping.
When the conductor experiences high-frequency vibration, the conductor moves against the clamp. The clamp itself creates an impact with the steel hoop, thereby absorbing the transferred energy.
This motion then creates movement of the coil springs, which also impacts the hoop, adding more energy dissipation.
Low-Frequency Galloping Control Systems all deliver an initial twist of the conductor at installation. These product solutions include:
High-Frequency Vibration Control Systems offer control for aeolian vibration. These product solutions include:
AR Dual Dampers and AR Twisters control galloping in addition to aeolian vibration.
In most instances, AR dampers will last the life of the transmission line.
The average service life value of the AR Twister is 12-15 years or more.
Many WINDAMPERS have been in service for more than 30 years.
MOD2 Spacer Dampers have been in service for more than 12 years.
If routine maintenance plans are followed for Sandampers, the service life value can be 25 years or more.
The key difference between interphase spacers and the AR product line is the clamp.
For example, the AR Twister clamp is responsible for rotating the conductor through large angles so that galloping never gets going. The interphase spacers are designed to keep the conductors separated to avoid flashover.
The net result of using the interphase spacer is that the two lines tend to gallop in synchrony.
The AR Twister is a galloping control damper that also protects the line from aeolian vibration because of how it is installed.
Dog-bone dampers are designed to protect the line from vibration and the structural damage caused by unchecked vibration. Dog-bone dampers do not mitigate galloping.
In a publication issued by a CIGRE workgroup in 2005, it was reported that during a field test by EPRI, the WINDAMPER was reported to reduce galloping amplitude by 75%.
When AR Products are installed as a system, across at least 10 consecutive spans, it is possible to achieve a reduction in peak-to-peak galloping amplitude of more than 50%.