Download 1. Over voltage Damage Over voltage damage caused by spikes

1. Over voltage Damage
Over voltage damage 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.
2. The most common types of over voltage causes are :
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Utility Switching
Isolation arching
Electrical motor cycling.
Lightning
3. 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.
4. 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 evidenced 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.
5. 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 (25000˚c) 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 Africa’s Distribution networks are no different to other
utilities around the world and are extremely vulnerable to lightning damage. Eskom’s 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.
According to research, Lightning is said to responsible for 65% for over-voltage damage to Medium
voltage transformers. In reality the remainder of the damage is said to be 35% and is due to Human
intervention, Natural causes and Mechanical failure.
7. Rainy Seasons
South Africa’s 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%.
South Africa’s Rainfall Activity
8. Thunderstorms
South Africa’s 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.
9. Transformer losses
Surge Arrestors
These transformer losses in South Africa have been excessive for a better part of the last
decade. Research has revealed that the causes of the high losses can be attributed to the
following :
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The Fuse-Arrestor configuration where the incoming surge is not dealt with by the
surge arrestor, due to it not being positioned in the lightning’s path.
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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 95% of all over-voltage damage. The result of surge arrestors
not effectively being utilized, are - High outage times and overburdening transformer
costs.
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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 doesn’t effectively allow the tails to be blown off.
Internal transformer damage – windings burn’t
Fig. A
Healthy set of Transformer windings
Fig. B
Damaged set of Transformer windings
Fig. C
Damaged set of Transformer windings
costing about R16 000.
10. 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.