Lightning Statistics

Over voltage Damage

Over voltage damage is caused by spikes and transients occurring in the form of lightning, utility switching, isolation arcing, electrical motor cycling, or any other sudden change in electrical power flow on AC power lines is said to be responsible for a third of all outages. The damage is caused by the sudden shock to the system which has not been designed for the higher voltage under the current design parameters and constraints.

The sharp rise in voltage moves across the surface of the conductor, creating orifices in the insulation, breaking down the insulation co-ordination between phases. The breakdown in insulation, causes the system voltage to move across the electrical fields allowing the system frequency to cause a deep, prolonged type of damage to equipment associated with burning.

The most common types of over voltage causes are :

• Utility Switching
• Isolation arching
• Electrical motor cycling
• Lightning

Heavy Motor Cycling

Voltage spikes or power transients are sharp, very brief electrical impulses in excess of the normal voltage. Typically lasting less than 200 millionths of a second. Spikes are often 50 times the normal voltage level. They can wipe out data stored in memory, alter data in progress, produce output errors and even cause equipment damage.

Spikes are commonly caused by the on and off switching of heavy electrical motors, such as air conditioners, electric power tools, furnace igniters, welders, office photocopiers or other large business machines and elevators. Capacitor switching by both utility and customer is also a common cause of spikes.

Switching surges

Switching surges, like spikes, are momentary voltage increases caused by frequent on and off switching of equipment. They last more than 10 milliseconds and less than one second. Unlike sharp spikes, generally exceed the normal voltage level supplied to the building by only 20 percent. They are often by the brightening of lights.

Surges can affect the performance of electronic equipment causing data errors, memory losses, equipment shutdowns and equipment damage. Life time of equipment can be detrimentally affected by surges.

Lightning damage

Lightning damage accounts for about 65% of all over-voltage damage to Electrical Distribution networks in South Africa. Distant strikes and even indirect strikes within 40 meters of a line can induce over-voltages around 250kv that can cause flashovers and over-voltage damage to Distribution lines. Lightning contains billions of volts and tens of thousands of amps. The temperature of a lightning strike is 3 times the heat of the suns surface (250000c) and consists of a core that is the about 15mm-20mm in diameter (the width of your thumb) whilst the sheath can vary anything from 5 meters to 15 metres in diameter. When a lightning bolt terminates on any Distribution line, the lightning energy is powerful enough to penetrate the insulation of any hardware (eg. a transformer) and allow the system frequency (50/60hz) to follow through and cause damage. The average Lightning bolt could light up a 100w bulb for 3 months if the energy could be contained. It is this type of energy that can cause extreme damage and result in degradation of a Distribution lines performance. Hardware losses, outages and excessive downtimes are the main contributors to the loss of revenue where Lightning activity is prevalent. South Africas Distribution networks are no different to other utilities around the world and are extremely vulnerable to lightning damage. Eskoms Distribution networks operate mainly on 11kv and 22kv but range from as little as 3kv up to 33kv. Surge Arrestors are (ZnO) MOV-Oxide based devices designed to protect electrical equipment from the damaging effects of spikes and transients caused by lightning, utility switching, isolation arcing, electrical motor cycling, or any other sudden change in electrical power flow on incoming AC power.

Interesting Facts about lightning

On average lightning strikes the earth 100 times per second. 3 p.m. is the most dangerous time of day, it is five times more dangerous than 9 a.m. Men account for 84 percent of lightning deaths, women 16 % Number of thunderstorms occurring at any given moment: 2000 Number of lightning strikes round the world per day: 8 Million Number of VOLTS in a lightning flash: 1 Billion Number of AMPS in a lighting flash: between 10,000 and 200,000 The average lightning flash would light a 100 watt light bulb for 3 months The heat generated by the electrical discharge is about 20,000 0C (3 times the temperature of the surface of the sun). Days of week of most incidents = Sun./Wed./Sat. Time of the day for most lightning incidents = 2 PM to 6 PM. Number of victims = One (91%), two or more (9%). Lightning tests confirms that even indirect strikes can cause surges on lines with horizontal arcing in excess of 40m. The chances of being struck by lightning is estimated at 1 in 700,000

Rainy Seasons

South Africas greatest rainfall would typically occur in the seven month period between October through to April. This period accounting for 82 % of all rainfall, whilst the remaining 5 month period from May through to Sep only 18%.

Thunderstorms

South Africas Highveld experiences intense Lightning activity. Lightning density and intensity in this region of the country, is between 7-10 flashes/km/year. Kwa-Zulu Natal also experiences intense lightning with many areas reaching the maximum, 14 flashes/km/year. Some lightning transients have been measured to be as high as 100ka. Traditionally lightning transients are in order of 20ka up to 60ka.

Highveld thunderstorms are fierce, sporadic, and usually short outbursts with intense downpours of rain relatively short in duration. The average duration of a thunderstorm is from 30min to several hours at most.

The coastline experiences thunderstorms but rainfall is usually more predictable, dense, less violent but longer lasting. Periods of rainfall on the coastline last from several days to weeks on end.

Transformer losses

Transformer losses have been excessive for a decade. Research has revealed that the causes of the high losses can be attributed to the following : The Fuse-Arrestor configuration at pole mounted transformers where the incoming surge is not dealt with by the surge arrestor, due to it not being positioned in the lightnings path. Sub-standard maintenance of surge arrestors at pole mounted transformers means that there is not always the sufficient protection available against over-voltage damage. Lightning is responsible for the majority of all over-voltage damage. The result of surge arrestors not effectively being utilized, are - High outage times and overburdening transformer costs. Spent Surge Arrestors are not always identified or known by ground staff, during routine maintenance. The Surge Arrestor blows off the line, but is still bolted onto the line. Sometimes the lead is still connected to the arrestor and is connected in such a way that it doesnt effectively allow the tails to be blown off.

Pole damage

Prevention is better than cure

The placement of live line surge arrestors on these lines would enable these overhead line to behave as a shielded lines, by improving their insulation co-ordination, and can prevent such damage and ultimately the replacement of these poles.
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E – sales@liveline.co.za

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