Substation Design - Securing Damping Conductor in Rigid Bus Runs

[James Purcell]

Hello,

We are having some internal debate as to whether damping conductor inside rigid bus should be secured (via tack welds, etc.) in place or left loose inside the bus. I have been taught that the damping conductor should be left loose inside the bus to allow it to break up the natural oscillation frequencies of the rigid bus. Another school of thought is that if you don't secure the damping conductor, it will coil up and shorten within the bus run, rendering it useless. I have tried to research this a bit on my own, and have not found guidance on the topic. Any guidance from the group backed by some sort of documentation (even if it's another forum I've missed) would be much appreciated!

Thanks,

James

 

[jghrist]

We have never specified securing the damping conductor. There isn't much room inside tubular bus to allow the damping conductor to coil up. Even if it shortens some, the weight will stop the vibration of the bus by changing the natural oscillation frequency.

 

[Power0020]

It is usually fixed from one end and left loose in the tube

 

[cuky2000]

RUS Bulletin 1724E-300:
a)...When installing scrap cables in the horizontal buses it is necessary that the cables be loose in the bus tubing to permit vertical movement.
b) If new cables are used, they should be straightened prior to installation to prevent the cables from jamming against the tubing sides.
c) End caps, preferably of the driven type, should be installed on the ends of the buses containing the damping cables to prevent horizontal cable movement out of the tubing.
d) To be effective, damping cables should be installed for the entire bus length for buses where excessive vibration is suspected.

 

[James Purcell]

Thanks, all! The feedback I have received from you guys is very similar to what I got from a more senior engineer I was working with a while back. Allowing the cables to sit loosely allows for the change in the bus span's natural oscillation frequency. We may tack it at one end; I will discuss further with my team to reach a decision. I suppose the RUS bulletin reference is likely the only standard that discusses an approach specifically. That kind of surprises me (you would think this would be covered by IEEE 605). If I come across any other standards along the way, I will update this thread with what I find.

 

[cuky2000]

James,

There is not much information regarding the benefit of fastening the damper conductor inside the rigid bus.
In fact, there is not clear the benefit of attaching the conductor at one or both ends.
Reviewing the formula for the natural frequency of a rigid bus the following can be observed:
.

.[highlight #FCE94F]f_b = (ΠK²/2L²)*SQRT(EJ/m)[/highlight]​

_{Were:
J= bending moment of inertia
m= mass per unit length
K= constant accounting end conditions
L= Conductor length
E=Young’s modulus}

Notice that the formula above does not have factors impacted by the fastening of the cable. The factors mainly impacted are J and m.
I found outside the ANSI/IEEE marketplace, hardware manufacturers that can provide end-caps with cable fastening. However, we are unable to find a similar device on popular hardware manufacturer in the US.
That may me wonder if there is a market for this product in the US power utility since a vast majority use lay loose damping conductor in their standard practice for HV substation applications.