1.What is the effect of a lightning 2

[ckavamba]

1.What is the effect of a lightning stroke on the system voltage in an EHV network.
2.What do the following terms refer to in a power system:
i. strong source
ii. weak source

 

[Nord]

1. A lightning stroke will cause a "fast-transient" overvoltage on the system. This overvoltage will travell through the system at near the speed of light, destroying anything unprotected in its path - similar to a tidal wave.

2. A strong source is a source with a very small impedance. A weak source has a large source impedance.

 

[jack6238]

A couple of comments concerning lightning-one of my favorite topics.

-The speed of propagation of the transient wave is a function of the distributed constants of the conductor. That is the Velocity = 1/(LC)^.5 where L is in henrys and C is in Farads. The velocity in an open conductor is approximately 1,000 Ft. per microsecond( which is at the speed of light) and in a cable it is approximately 500 Ft. per microsecond-still fast but only half the spped of light. Ref. Lightning Protection for Electrical Systems, Edward Beck, pg.297.

-The lightning wave does not necessarily destroy everything in it's path. The arresters installed on the system usually safely conduct the transient safely to ground.

-The ability of an arrester to transfer a charge to ground is determined by a combination of the arrester resistance, the current discharge capability of the arrester and the maximum temperature rise that the specific arrester can withstand. Having said that, there is no standard method for measuring the joule rating of their arresters-each manufacturer appears to have a different method. The standard for surge protective devices does not specify a standard. I understand that several working groups are attempting to quantify this problem.

-Dr. Lubo Kojovic of the Thomas Edison Technical Center has recently presented a paper at the IEEE Winter Power Meeting (Feb.2001)and is suggesting that and I^2t rating may the answer. I personally like that approach.

Some interesting data on lightning:
-There is a worldwide average of 44,000 lightning storms per day.
-There is a worldwide average of 100 lightning strokes per second.
-The U.S. average is reported to be 29.5 million strokes per year which is roughly one stroke per second. The majority of these strokes occur during the months of May-Aug.
-Lightning is the principal cause of fires in rural areas.
-Average chance of being killed is 1 in 350,000.

 

[rhatcher]

jack6238 is on the money with this post (I gave him a star...). A properly shielded and protected system should have a minimal effect from a lighning strike. I will add that utility systems are almost always protected to some degree but I have seen many private industrial facilities who have a distributed power system using overhead lines which are not protected. If this is the situation that you refer to or if some other situation exists in which you are experiencing lightning strikes on an unprotected system please say so....

 

[jbartos]

Suggestion: Reference
1. Fink D.G, Beaty H.W. "Standard Handbook for Electrical Engineers," 14th Edition, McGraw-Hill, 2000, Section 27 "Lightning and Overvoltage Protection"
The lightning is considered an external surge, Reference 1, apart from internal surge due to switching. All surges must be considered. The lightning stroke can be mitigated or avoided by the following:
Mitigated by:
1. Air (Spark) Gap
2. Gapped Surge Arresters
3. Metal Oxide Varistor (MOV) Arresters
Avoided by:
1. Lightning Rods
2. Proper grounding and shielding
Temporary lightning overvoltages may have an envelope to about 3.3 p.u. of voltage rise, and duration of about 10**-4 second. Temporary switching overvoltages have an envelope 2.6 p.u. of voltage and up to 10**-2 second. Reference 1 deals with this topic on 77 pages and provides Bibliography and References.
----------------------------------------------------------------------
Strong source has a relatively small internal impedance and a relatively high short circuit MVA.
Weak source has a relatively high internal impedance and a relatively low short circuit MVA.

 

[jack6238]

Some additional information on lightning.

Magnitude of Direct Strokes Recorded on Transmission Lines.

Here is some additional information concerning ligntning strokes to transmission lines within the U.S.

1% of the strokes = or> than 111,000 amperes
5% of the strokes= or > than 63,000 amperes
10% of strokes = or > than 50,000 amperes
20% of strokes = or> than 33,000 amperes
50% of strokes = or > than 14,000 amperes
70% of strokes = or> than 9,000 amperes

The maximum recorded stroke to a transmission line was 218,000 amperes.

The maximum recorded stroke to a structure was 345,000 amperes and it was recorded at the Cathedral of Learning at the University of Pittsburgh where Westinghouse had set up an observation and measuring lab.

Wave Fronts of Direct Strokes

8% of wave fronts = or > than 6 microseconds
23% of wave fronts = or > than 4 microseconds
62% of wave fronts = or > than 2 microseconds
90% of wave fronts = or > than 1 microseconds

Wave fronts less than one half microsecond and more than 10 microseconds have been recorded.

Polarity of Strokes

Approximately 90 % of all lightning strokes have negative polarity and the maximum strokes recorded were of negative polarity. In general positive strokes are of low current magnitude

Stroke Mechanism

The typical lightning stroke si composed of three portions. The pilot leader, the stepped leader and the return streamer. The propagation velocities of these three portions are:

-pilot leader averages app. .5 ft. per microsecond
-stepped leader averages app. 30 ft. per microsecond
-return streamer averages app. 100 ft. per microsecond

And the average length of the stroke channel si 5,500 ft.

 

[SooryaShrestha]

jack6238 has posted lot of lightning information. He is worth of stars. I like to add some more. Some of above postings are need a little revisions. For example lightning can not be avoided, which some people think that it can be avoided by installing lightning rods or arresters. Arresters for transmission line phase conductors can divert the lightning surge, provided the lightning impedance to the ground is low and there is enough insulation to the line. Otherwise, there can be back flashover. With high lightning impedance, there is danger of safety both to personnel and the property. Lightning impedance is different from the impedance measured in power frequency level. The lightning impedance for grid mesh is far greater than the power frequency impedance, which is predominantly resistance. However, the lightning impedance for electrodes are lower compared to the lightning impedance for mesh. Thus it is recommended to have earthing system with electrodes for lightning protection.
One very interesting fact about lightning as studied by Dr Mousa of BC Hydro is that if the shape of the structure is conical the return stroke is not developed. Thus the less effect on such structure. Coincidentally, in most of the East and South Asia, there are idols of Buddha, where an idol of thunderbolt is kept. The mythology is: Buddha captured one of the thunderbolts and there is no possibility of lightning in such area. Even if they are located in lightning prone areas, I do not know any lightning had hit such area, but the structures are very conical!
One more interesting fact is that in some high voltage lines below 38 kV many utilities are using ground conductor at the top of the structure in order to shield the lightning. In fact the ground conductor even if it shields, is of no use as the voltage raised by lightning to the conductor is enough to have back flashover to the line conductor.
So the only way to deal with lightning is to accept it as it is and have very good ground.

 

[BJC]

Lighting seldom strikes the power conductor of an overhead line because of the counterpoise ground above the phase conductors. Lightining "strikes" are usually discharge of the static charge built up on the phase conductors. The static charge is is superimposed over the ac voltage. The charge in the line and nearby objects ( trees, ground, Budas etch) is one side of a large capicator the other side being a cloud. When lightning strikes from a cloud to a tree near a power line the charge in the AC line conductors is not held and surges both ways down the line. The resulting voltage spike is not a direct strike but is troublesome, lightning protectors can handle it most of the time. For a good description see the Westinghouse Distribution Data Book ( published by Seimens now I think.)