Overhead Line Lightning Strike Severity and Probability

Lightning Phenomenon:

Lightning Strike is the discharge of electric charge accumulated in the clouds to the ground. Clouds accumulate typically the negative charge, and in response the ground produces the counter charge: the positive charge. Electric discharge happens between the two, i.e. clouds and the ground, under certain electric discharge conducive conditions, effectively creating the shortest electrical path.  These conditions include accumulated charge density, humidity in the air (enabling faster dielectric break down), ground elevation (buildings, mountains, tall living things) etc.

Physics behind this phenomenon is much elaborated in several books and articles found in both libraries and internet, and their perusal is recommended.

Lightning Strike Frequency:

Although the frequency of lightning strikes might vary year to year, long term (usually many years to decades) maintained records give a statistical approximate number of expected strikes each year. This is usually referred as Isokeraunic Level [1]. Reference [2] and [3] give typical Isokeraunic Levels around the world, and gives a geography dependent empirical formulae to calculate Ground-Flash Density (or – Strike Density). The empirical relationship between Isokeraunic Level and Ground-Flash Density for UK is given as:

Where, GFD is the Ground-Flash Density in ,  is a factor that varies between , b is a factor that varies between , and  is the Isokeraunic Level in . Isokeraunic Level for Ireland, north UK, mid-west UK  and southern-west UK is typically between 5-10, while for middle-east UK and southern-east UK is between 10-20.

Overhead Hit Frequency:

Strike radius is the scope of the ground elevated structures or ground itself that will be exposed to lightning. This, usually calculated in meters, is very much dependent of lightning strike peak current magnitude.

Lightning strike distance in meters [3],

Where, is the Lightning Strike peak magnitude in kA.

Now, the scope of Lightning Strike area that will hit the OHL will depend on the tower structure. For a un-shielded wood pole structure (assuming all conductors are same height from the ground level), this is given as following:

Strike Area,

Where,  is the sum of strike radius of left most conductor, distance between the farthest conductors, and strike radius of the right most conductor respectively. $latex L$ is the length of the exposed overhead line.

Total estimated OHL Flashes per Year if every flash was as chosen Strike Current, , is given as,

For a given strike current, Cumulative Lightning Strike Probability is given as [3],

For a given strike current, Estimated Annual OHL Lightning Strikes in (Flashes/Yr) =

Now plot a bar chart between various strike current versus expected annual lightning strike frequency, and you see typical decline of Flashes/Yr with increasing strike current magnitude. And if you invert the Flashes/Yr you get expected number of years before you get see a lightning strike for a given strike current peak.

References:

1. Lightningtech Website. [online], Available: http://www.lightningtech.com/d~ta/faq2.html, Accessed: Oct 2009.
2. Chowddhuri, P., Electromagnetic Transients in Power Systems, Exeter: Research Strudies Press Ltd., 1996.
3. Grigsby, L. L., Power Systems, Boca Raton: CRC Press, 2007.

Lightning and Switching Surge Model

Power Systems equipment, directly or in-directly connected, is expensive and needs protection against both external factors and normal every day power system operation. Protection assessment of these equipment, especially outdoor, against voltage surges is almost mandatory due to their exposure to lightning and general switching operations. Insulation material’s strenght is usually expressed using Basic Insulation Level (BIL) in kV that indicates voltage withstand capability of the material before it breaks down, providing a path for fault current. In order to assess equipment performance to a real environment, actual equipment or prototypes are tested to Lightning and switching surge. This is where surge generators come handy that generator desired surge wave fronts and tails.

Various standards (IEEE, IEC, EN etc.) provide rules and guidelines on how testing is to be performed. Typical surge-wave shapes recommended by standards is a double exponential curve functions with rise time to reach the peak and then gradual decay fall time. Irrespective of the equations that define this curve, what’s of real importance is the voltage peak rise reached at the end of rise time and the total time (also the fall time) it takes to reach half the peak value during its decay. There are two variations in surge reference curve depending on how the rise time is defined, while the fall time remains the same.

• Type 1, where the rise time is the time reached by the surge peak to 90% of its value. Also shown in Figure.
• Type 2, where the rise time is the time reached by the surge peak to 100% of its value.

As I understand, Type 1 is used for fast-front surge waveshapes (e.g. Lightning), while Type 2 is used for slow-front surge waveshapes (e.g. Switching).

Now you know the basics, here’s a simple Linear Lightning Surge Generator Model in PSCAD based on [1]. Download. If you are looking for a double-exponential surge generator model, there’s one in PSCAD examples folder in c:\program files for MS Windows.

Connect the current source output to the point of surge injection in the test system. It’s simple to use: Enter the current peak (say 50kA), surge front time (say 20μs) and tail time (say 80μs), initial time (say 0.1s) when you want the surge in the simulation and you are done.

An opportunity to test equipment to lightning and switching surges without frying ourselves up.

Have fun! ^__^

References:

1. TVS Diode Application Note , Semtech Corp., 2001. Link: http://www.semtech.com/pc/downloadDocument.do?id=535. Accessed: Aug 2009.
2. P. Chowdhuri, Electromagnetic Transients in Power Systems, Edition 1. Exeter: Research Studies Press LTD, 1996, pp 4-6.