Lightning problems 1

[RalphChristie]

We experience trippings (instantaneous) on some of our 66kV-transmission lines due to lightning.
Each tower consists of two wood-poles with two earth/ground conductors on top. Next to each pole there runs also a earth conducutor - this conductor is connected to the overhead conductor and burried in the ground. The overhead earth conductors are connected to each substation's earthing-grid.

Can we: (Advantages and disadvantages)

1. Disconnect the overhead earth conductor at each substation but still leave it on top of the poles? (just disconnected from the sub-grids)
2. or should we remove the total earth conductor and just leave a conductor from the ground to the top of each pole?
3. Leave it like it is?
4. Any other recomendations to solve our problem?

Thanks in advice
RCC

 

[jghrist]

Are the trippings caused by lightning induced faults on the transmission line? Or are you thinking that the surge current is getting into the station control wires and causing a misoperation of the relaying (doubtful)? I don't understand what the reason would be to disconnect the buried earth conductor (continuous counterpoise) from either the shield wires or the substation ground grid.

Disconnecting the counterpoise from the shield wire will decrease lightning performance (more lightning induced line faults). Disconnecting the counterpoise from the station grid will decrease safety in the station by forcing more station ground fault current to flow through the earth and increasing touch potentials.

 

[hibb]

I'm no lightning expert, but how about checking for sufficient spacing between the shield wires and the phase wires. We've had troubles where the lightning intensity is high enough that we've had to elevate the shield wires along several spans to avoid getting flashovers.
Check RUS or TVPPA for construction standards showing different shield wiring positions.
If shield wiring positioning doesn't work, you could always apply a few well placed 69kV LA's.
I think you had better scrap the idea of disconnecting the shield wires from the sub ground grids. You're likely to get some unexpected relay effects from that as well as affect personnel ground safety.
Good luck. We have to help each other with lightning troubles.

 

[busbar]

To add to hibb's comments — Have you considered anything like suspension arrestors that are intended to "parallel" suspension-insulator strings on towers? [They are shown available in 8.4-144kV MCOV ratings.] The described grounding already in place may somewhat limit retrofit costs.

 

[peebee]

I agree with all above posts. I can't think of any reason you'd even consider removing the ground system, that could only hurt, although you might need to increasing the spacing. LA's could only help.

 

[RalphChristie]

Thanks for all the replies.

I want to comment on some of your replies - If I am wrong, just tell me.

1.We think (suspect) that the lightning struck the earth conductor (on top of pole) and took it into the sub.
2.The earth wires and the phase wires have sufficient spacing.
3.There are LA's in the substations.
4.I have seen many other lines without an earth conductor, would it make any difference if we remove ours?
5.There will be no touch potentials on lines (wood poles and the burried earth conductor from the top), I think in the substation there would also be no high touch potentials due to the station grid.

Like I said, I'm no expert, if my interpretation is totally wrong, I will appreciate it if you will inform me.

Thanks
RCC

 

[DanDel]

RalphChristie, you WANT the lightning to strike your ground(earth) conductor(rather than the line conductors), so that it will dissipate to ground. The potential of the lightning strike is to ground.
If you believe your problem was caused by lightning going to ground, I can't agree. Perhaps you do not have an adequate path to ground from your earth conductor.
Problems from lightning are caused when your line conductors are hit by lightning, bringing their potential to ground above the rating of the line insulation, and typically causing damage. The LAs are there to provide a path to ground when the potential is above the rating of the insulation.

 

[ausphil]

RalphChristie,

Lightning can be a tough problem. Don't forget that the impedance seen by the lightning is not the impedance seen by the normal power system due to the frequencies involved. In response to a couple of your points:
1. This will occur only if the strike was close to the substation, or if your downwires (connections of your OH earthwire to your earth grid at each pole) are not in good condition. We have had part of our system where the number of lightning strikes and the corrosion from the soil have corroded nuts that connect the downwire to grid. This essentially raises the potential of the downwire to a high voltage, and can flashover thhe insulators on the phases as it passes by them. Also, you might want to check that your earth grid at each pole is of a suitable resistance. Again a high resistance can produce higher voltages where you don't want them.
2. Don't forget that the clearances must be good enough for the expected lightning potential that may occur. Again if the earthing system is not performing properly (either from degradation or poor initial planning) these clearances may not be large enough.
3. Good. Are they working properly?
4. Earth wires are run over the top of transmission lines due to a number of factors. You may also have noticed some of the lines have an earth wire that runs for about a mile (or similar rough lengths) out from each substation, because if the lightning strikes occur further out than this, the impedance of the line is great enough to attenuate the lightning strike to a level where a reasonable SA will protect the substation. You may decide that the earthwire is not required, but don't just remove it because you think it is causing problems, if you do remove it, ensure you look at the total insulation design of the susbtation and line, like incresing the BIL of equipment in the sub if requried.
5. You will have touch potentials at a wooden pole, they just may not be as great as a steel tower or concrete pole. You are dealing with large currents (short term) and these can produce large step and touch voltages, especially if your pole earth grids are not working as deisgned.

Please be careful when dealing with protective earthing, and do not take a decision about changing the condition of the system lightly. Questions I would be asking are:
Was the system designed with a certain lightning strike input, or was it designed off the shelf?
Has the performance of the system degraded (more trips) recently, or has the problem been there since as long sa everyone's memory serves? If it is more noticeable recently, then I would say look hard at your earthing system at each pole. Simple earth grid resistance checks can tell you a lot. Maybe do high current checks across the connections at the top and the bottom of the pole to look for problems.

Get a good insulation co-ordination engineer to go through insulation levels before making any removal decisions.

As with a lot of power system engineering problems, the symptoms don't always point directly towards the problem.

 

[RalphChristie]

Thanks for the responses.

It seems for my there is much more in lightning than I actually thought there were. Thanks for the detailed information, ausphil.

Every day I learn something more....
Thanks again.

RCC

 

[jghrist]

What relaying tripped the line? I assume that you think that lightning surges in the control/relaying system caused an unfaulted line to trip. Is there any evidence of this? If this is the case, you may need to consider shielded control cables, if they are not already shielded; proper grounding of the shields if they are shielded; proper single point grounding of CT neutrals; changes in routing of control cables; or additional grounding at the points where the shield wires are connected to the ground grid.

Touch potentials in the station will increase if you disconnect either the shield wires or the counterpoise from the ground grid. If these connections were assumed to be intact when the ground grid was designed, the touch potentials may be too high if they are disconnected. Only a detailed grounding system design check can determine this.

 

[Stefa]

I wonder one thing that the relay trip due to overvoltage or current discharge through LA. The tripping due to overvoltage is unlikely to happen due to the overvoltage relay has time delay normally be set at 2 or 3 sec. typically which the surge overvoltage occur in very short time in microsec. Therefore it is likely that the relay pick-up probably due to current. When consider this ,if you put LA infront of CT,it should not be a problem because the surge current would discharge to ground at LA.So the relay should not see that surge current and would not pick up.I assume that the CT are installed in front of LA then the surge current discharged to ground through LA can be seen by relay and cause relay pick up.The overhead ground wire should not be disconnected due to it help in shielding substation from lightning.If you disconnect them,the surge current may be higher than 10 kA which you may have to change LA from 10 kA to 20 kA rate and many problem would come e.g. direct lightning stroke to phase conductor.The surge coming from transmission line can be occured by many reason such as the backflashover from overhead ground wire to phase conductor due to high tower grounding resistance,low rating of suspension insulator string critical flashover voltage against the possibly occur lightning surge,induced lightning surge from overhead ground wire to phase conductor,direct lightning stroke in the span next to substation. So the best way is to have shield wire from transmission line and connected to substation ground grid.The LA should be installed in front of all equipment in substation to be protected from surge.The insulation coordination should be studied.

 

[rowbotth]

You have two items - the problem you asked about, and some grounding issues which are (according to our experiences) not part of this problem, but should be changed.

From a course I took from the folks at PTI, it appears that when you get a lightning strike and you have a wire above the phase conductors (we call this a "Sky Wire), this has the effect to raise ground potential at that pole to a very high level, and your phase closest to that conductor (usually "A" Phase) will short to ground.

If you have impedance relaying on the line, this will appear to be a dead short phase to ground fault and the relay will trip you off line immediately.

If you are using 50/5 Instantaneous / Overcurrent protection, your instantaneous protection will do the same.

Depending on your situation, you must decide whether to disable or change your protection settings or type. (How often do you get lightning strikes as opposed to other faults on the lines?) If you disable the protection that opens breakers on a lightning strike, under how many other situations are you going to possibly damage your system because the relays did not respond in a timely fashion? This is the 64k$ question, and only you can answer this.

Now about the grounding of these top ground conductors.

We use these too, but we NEVER ground these in our substations. We only ground these at least one pole away from substations. We also NEVER ground these conductors at structures where there are disconnect switches connected. We want to keep lightning from being bled off where there are substation equipment, or where there may be people working. You might be wise to consider this thought, and maybe ask about whether you should adopt the same thinking.

Connect them to earth as often as possible - lightning hates to turn corners and it hates to travel far - and make certain the conductor is as straight as possible and it runs directly to earth. And if there are people near the poles, you might want to consider putting this grounding conductor that runs down the pole in a plastic conduit.