Grounding of shield wire on transmission line 2

[KillBill7]

I am designing 240kV slack span from one substation A-frame to the other substation A-frame. Lightning protection is provided by the shield wires. The local utility practice is that they don't allow grounding of shield wire on the substation A-frame structures by installing the insulator in the shield wire deadend assembly.For typical transmission line, it won't be a problem as shield wire would be grounded on the last structure before the A-frame. However in this case, I am bit confused. My question is what would happen incase of lightning strike and shield wire not grounded? Would strike current remain on the shield wire forever or would it decay overtime? And should it be left ungrounded?

 

[waross]

It's going to flash over onto the A-frames.It won't provide much protection if it is not grounded.
The grounded A-frames will probably take the hit first.
If it does take a hit it will probably flash over the insulators.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sushilksk]

What is insulation level for shield wire?

 

[cuky2000]

Let's assume a simplified model with lightning strikes at the T. Line close to the substation were two waves will splitting in opposite direction of the fault traveling at the speed of light.
_{a) Wave traveling away from the substation: Due to the smaller impedance, this portion of the current may be larger than the other one and successively drain most to ground in adjacent towers until the current decay. Smaller current may reflect but will also decay for losses and drained into the earth.
b) Wave toward the substation: This wave will reach the insulator and reflected almost duplicating the incoming crest value. If the peak value is greater than the insulator BIL, a flashover will occur draining a large portion of the current into the ground via the tower. Otherwise, the wave will be reflected back until finding a path to ground in the adjacent towers.}

No, as described above, the reflecting traveling waves will be decay in a relatively short time frame. They may be a possibility that the system recloses if it is sensed as a ground fault and the line will be back in service to the normal operating state since a fault on a line has a self-restored characteristic. Worst case scenario, the protection system will trip the line permanently in the event of damage or large striking current.

It depends. We are assuming that somebody in the utility and the engineers of the line evaluated the pros and cons to operate the shield wire insulated (ungrounded) in the last structure. In most HV applications, there is a benefit for the protection system to have all the shield wires interconnected to the ground. Additional benefits for reducing the current injected into the ground by increasing the splitting factor of fault currents for grounding design in the substation.

 

[stevenal]

Lightning is seeking a path to ground. If the insulated static wire has sufficient BIL, the lightning will hit elsewhere: A-frames or conductors. If it's not grounded, you might as well save the cost of installing it.

 

[waross]

cuky; From the OP I understood that this was a single line between A-frames, insulated at both ends.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[KillBill7]

Thanks for the valuable input guys. Like Waross said, its a 30m slack span between two 240kV substations and utility standard asks for 25kV insulator in the shieldwire deadend assembly, hence insulated at each end.

 

[davidbeach]

Then connect that shield wire to other shield wires in the area so that it too becomes grounded outside the station.

 

[sushilksk]

for 30m slack span (insulated from both sides), there is very low probability that lightening will struck to shiled wire, it will struck on A structures instead. If anyhow lightening struck on shield wire, 25kV insulators would not withstand lightening impulse, they will fail in the first stroke.

 

[bacon4life]

Putting an insulator on just one end would limit induced steady state current flow, though I do not understand why the utility would want an insulator at both ends. The 25 kV insulator will flash at a low enough voltage to still protect a 230 kV equipment.

 

[cuky2000]

Probably the isolation is to reduce or mitigate the losses on the shield wire. This has been done in 500 kV but not sure how this will have any significant savings at 25 kV.
For a ground fault, a higher current should be expected to go into the substation ground grid due to the increase of the overall zero sequence impedance for isolated shield wire.

Hi Waross: for lightning, purposes, shield wires insulated does not make much difference than end grounded shield wires since the hight surge created by the reflecting traveling wave most likely will flashover the insulator.

 

[davidbeach]

I'm pretty sure that the goal of the original spec was to prevent standing, steady state, current from flowing into the station ground grid; to keep all that induced current out of the station. If the two ground grids are tied together, grounding both ends of the shield to the grid won't make any difference as there won't be any current induced from outside the station; but that condition wasn't envisioned in the original spec. If the two stations are adjacent but don't share a common ground grid it might be worth it to tie that shield to one or more of the incoming line shield wires and use those other wires to provide a ground away from the station.

 

[stevenal]

Relying on insulator flashover to provide lightning protection seems like a misuse of the product. If you wish to have a device conduct at a particular voltage, I would suggest using an arrester.

 

[bacon4life]

Cuky- How big are loss savings from isolating both ends? Isolating the first end saves all the inductions losses. Is the loss savings from isolating the second end mostly due reductions in corona losses, due to reductions in capacitive coupling current flow or due to some other factor?

KillBill7- If the stations ground grids are isolated, it may be worth checking what insulation level is needed ensure that the shield wire insulator(s) will not flashover due to a ground potential difference between the substations during a fault.

 

[stevenal]

Like Waross said, its a 30m slack span between two 240kV substations and utility standard asks for 25kV insulator in the shieldwire deadend assembly, hence insulated at each end.

All this speculation why the utility might engineer it this way, but KillBill provides the clear answer. The utility standard of insulating at the substations was extended to this situation where there is no other location available to ground it. Time to go up the ladder at the utility to find who has the authority to make the exception.

 

[cuky2000]

Hi Bacon4life,

Unfortunately, there are many unknowns such as loading, configuration, length, and other data to provide a specific answer regarding losses on the shield wire for a 230 kV T. Line. However, in general terms, we can say that the losses in the shield wire for the 230kV (curve in red) are expected to be low considering the reference graph below.

I hope this information provides some inside into this issue.

>>>>>>>>