The creative individual who first thought up the concept of lightning protection by means of overhead ground wire( OHGW) might not appreciate what I’ m about to say. Still, in an era with focus on sustainability and given limited supplies of metal, do other means to ensure protection of transmission lines from lightning strikes not deserve consideration?
My first experience with an OHGW came when I was just a youngster. The local utility ran a new 115 kV line through my grandfather’ s hayfield and this line tapped into one that had already been there for years, constructed using Pi( ∏) type towers. The new line was similar but had an extension on one side, raised to what we now call a modified H-frame. Even though I wouldn’ t think more about it for decades, I recall wondering then why they elongated one part of the frame and added a thin wire above the conductor. Now, half a century later, I’ m studying this topic carefully and asking much the same question. However now we have green technologies such as polymer housed arresters and EGLA arresters available to us.
Is Overhead Ground Wire Headed for Extinction?
In this era of limited supply of metal, other means to ensure protection of transmission lines from lightning strike deserve serious consideration by the power delivery industry.
Woodworth on Arresters
Two years ago, at a conference for transmission line designers, I came to learn that, in the case of certain tower and line configurations, the OHGW could account for significant losses in the form of heat from induced current. I was also shocked to discover that alternatives to OHGW for lightning protection are seldom, if ever, considered by line design engineers. I asked:“ Why not use arresters for lightning protection?” and the typical response was a blank look and some remark about arrester failure. I left the event wondering if this was not the right time to conduct a serious study on superior alternatives to the OHGW.
Shortly afterwards, I submitted a proposal to a department in the U. S. government to fund a study into the economics of using transmission line arresters in place of OHGW with the goal of reducing losses. They accepted and over the past several months this topic has headed my daily to-do list. One of my first tasks was to compile and assess the comparative pros and cons of OHGW. These include:
OHGW Downside 1: Shielding Failure This is where lightning still strikes the phase conductor in spite of the shield wire above it. Although this phenomenon is well documented and much has been done to mitigate it, the fact remains that it is virtually impossible to eliminate. Even if shield wires are perfectly positioned to protect conductors from direct strike, they can still be rendered less effective whenever surrounding terrain is not level. Every degree of slope to the land( if sloping 90 ° to the line) reduces their effectiveness by the same extent. I basically confirmed this when carrying out a lightning study of a line that passes through the Appalachian Mountains. This basic line design had worked for decades with no significant outages but was now experiencing shield failures even before being fully energized. It turned out that only those spans suspended above mountainsides were affected and application of line arresters fully resolved the problem.
OHGW Downside 2: Backflash This peculiar phenomenon occurs on well-shielded lines where tower footing resistance is somewhat too high and results in significant voltage levels appearing on the down conductor. If the voltage on the down conductor( and at the base of phase insulators) rises to a level that exceeds the insulator’ s flashover voltage, lightning will‘ back flash’ the insulator from tower to phase conductor. This has a similar impact on the system as shielding failure. After the lightning strike, the power frequency voltage follows the low impedance path through air. A power arc is established and a momentary outage occurs. Here again, arresters effectively resolve this problem.
OHGW Downside 3: Energy Losses It’ s true that losses associated with shield wire are far lower than losses on the phase conductors. But does that mean we should ignore them? Segmenting is used on high voltage lines and can be quite effective, but seldom applied to systems below 345 kV. For example, the Net Present Value of losses on a highly loaded double-shielded, double circuit 230 kV system can be as much as US $ 4 million per 100 miles and only becomes higher as the cost of generation increases. Replacing the OHGW with arresters on each tower basically eliminates this as an issue.
OHGW Upside 1: Fiber Optic Carrier So far, I’ ve identified only one significant benefit of the OHGW but that one item might be enough to keep it from falling into oblivion, namely the desired function of also carrying an optical fiber. The value of such an optical fiber only increases as the level of communications in power systems grows due to more and more smart grid applications.
So, the question now becomes: how can arresters resolve the above downsides of OHGW while still not losing this desired benefit? Actually, that happens to be the topic of my paper at the upcoming 2013 INMR World Congress, this September in Vancouver, Canada.
See you there.
52 20
YEARS
Q2 2013
Jonathan Woodworth Jonathan. Woodworth @ ArresterWorks. com