earhting guard cable by mean inner steel cable of a concrete pole 1

[aherbas]

Dear experts

We are using reinforced concrete poles in 69 kV transmission line, and to connect to earth the cable guard, we are using an inner cable that travel by mean of the pole, and it´s in contact with metal frame of the pole.

Normally we connect teh cable guard by an external cable, not by an inner cable that was build with the pole.

Do you any have experience about it?. Do this can produce some problem with lightnings? what about induction?

Thank you

 

[vic3fan]

First, do not consider me an expert; I just want to share something from my experience.

You should be concerned about the induction only if you have an ungrounded guard conductor. This can be a problem when performing maintenance or other works made without power shutdown, due to dangerous potentials this conductor might ‘gather’. But it’s not the case here, as you mentioned the grounding of the reinforced poles.

If you want to analyze what happens during abnormal conditions, I think you should take into consideration not only the overvoltages due to lightings, but also the distribution of the currents in the transmission line elements during the grounded faults. This can be important to determine the correct cross-section of the guard conductor within the 0-25% range of the line length close to the transformer station(s). Here is how you can make the theoretical model for an estimation of the fault currents. Each pole can be represented by the impedance of the guard conductor existing from one pole to another – usually the typical values of ohm/km or ohm/mile are considered, multiplied with the span lengths – and the impedance of the grounding conductor and the grounding installation.


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Now connect in series all these Gamma-type quadripoles, and take into consideration the MVAsc calculated for the bus your line is connected to (ask it from control room people, they have the software that calculates the short-circuit regimes). This way you can compute the current values through each element and see if the cross-section of the guard conductor supports the corresponding current. For a line supplied from both ends, use the superposition theorem: calculate the currents for each bus your line is connected to, and add them to obtain the total effect. Usually, problems occur within the first 25% of the line length, considered from each substation.

About 10 years ago, while I was working with a utility company, we had to reinforce the guard conductors for several transmission lines, due to the increased MVAsc of some buses.

Regarding the grounding procedure you described, I don’t think you should worry too much. In almost all cases, the metal frame impedance covers all the fault current values; in other words, there is no danger the fault currents will melt the metal frame components. But the metal frame is built with poor electrical connections between its elements and you cannot count 100% on a proper response. That’s why the inner cable is used, to make sure the guard conductor is grounded. Usually, this inner conductor is chosen to support the thermal effect of the fault currents by itself; and if it is connected together with the metal frame to the guard conductor and the grounding installation, this should be enough. You can check it to make sure there is no danger of underestimation of the total cross-section needed. In addition, an external conductor can be installed at any time if the inner conductor is suspected of malfunction. I think a continuity test of this inner conductor can be easily performed before mounting the pole.

 

[cuky2000]

QUESTION: Do this can produce some problem with lightning?
The concrete pole is a supplementary ground electrode that helps to dissipate the energy of lightning hit directly to the line as well as a nearby lightning stroke. Additional lightning protection performance could be achieved if combined with lightning arresters.

QUESTION: what about induction?
Several studies lead to a conclusion that the surge impedance of the grounding conductor was affected by the existence of the steel reinforced elements due to an electromagnetic field induction. Concern of cracking concrete pole during lightning stroke was considered in special cases. However, not significant negative effect was reported.

The enclose article contain useful information of the grounding effect using reinforced concrete poles connected to the shield grounding cable.

http://www.ipst.org/TechPapers/2001/IPST01Paper028.pdf

http://www.iee.or.jp/honbu/back_number/journal/index_back_number/2004/2004_01b_18.pdf

http://www.ewh.ieee.org/soc/pes/lpdl/archive/4_Bill_Chisholm_paper.pdf

http://www.iee.or.jp/honbu/back_number/journal/index_back_number/1999/1999_7b6.gif

 

[aherbas]

Dear Friends

First of all, thank you for your answers.

Question?

Dou you think it´s enough to use grounding poles to connect the cable guard of the transmission line.

Thanks

 

[k2panman]

Thanks to all for the interesting and good information. I have a question regarding the terms used in the post: Is the "cable guard" and "guard conductor" also known as a "shield wire" or "static wire"?

 

[cuky2000]

I also some use the term "sky wire", "overhead ground wire", cable de guarda (Guard cable), etc. All this terms are referring to the same overhead shield whire conductor to protect facilities against lightning strike.

Shield Wire is the IEEE official definition consistent with the transmission-working group. See the enclose site for additional reference.

http://www.ewh.ieee.org/soc/pes/lpdl/D_minutes/wgmin.htm