2. Introduction
• State the causes of over voltage.
• Explain the characteristics of Lightning Arrestor.
• Describe the Insulation coordination and basic impulse insulation Level
3. • 1. Over voltage due to external causes
• 2. Over voltage due to internal causes
When the voltage in a system, raised beyond its rated voltage, then it is known as overvoltage. This overvoltage may
be of transient or persistent nature. The main cause due to which overvoltage is produced in the power system may
be conveniently grouped into two categories, namely, internal and external. Internal overvoltage has got their origin
within the system itself, whereas external overvoltage is because of lightning on the lines.
Overvoltage Protection
4. Over voltage due to internal causes
Switching Operations on Unloaded Line:
Sudden Opening of Loaded Line
Insulation Failure
Arcing Grounds
Resonance
5. Over voltage due to external causes
Transient over voltages can be generated at high frequency (load switching and lightning), medium frequency
(capacitor energizing), or low frequency. Over voltage due to external causes:
This cause of over voltage in power system is the lightning strokes in the cloud.Now, how lightning strokes are
produced. So when electric charges get accumulated in clouds due to thunder Strom caused due to some bad
atmosphere process.
This type of over voltages originates from atmospheric disturbances, mainly due to lightning. This takes the form of a
surge and has no direct relationship with the operating voltage of the line.
It may be due to any of the following causes:
A) Direct lightning stroke
B) Electromagnetically induced over voltages due to lightning discharge taking place near the line, called ‘side
stroke’.
C) Voltages induced due to atmospheric changes along the length of the line.
D) Electrostatically induced voltages due to presence of charged clouds nearby.
E) Electrostatically induced over voltages due to the frictional effects of small particles like dust or dry snow in the
atmosphere or due to change in the altitude of the line
6. Methods of reducing over voltages
Methods of Protection Against Lightning
Earthing screen.
Overhead earth wire.
Lighning arrester or surge dividers
Some devices are available to reduce the amplitude and front steepness of surges. The following will be
described here
Rod gap
Surge Diverter
Overhead Earth Wire
7. Overhead Earth Wire
An overhead earth wire or ground wire is one of the most common
devices used to protect the lines against lightning.
It is the wire carried the line supports and runs over the phase conductors.
The purpose of the earth wire is to block the direct lightning strokes,
which would otherwise strike the phase conductors.
The waves of lightning reach the adjoining towers which pass them to
earth safely.
In case the resistance of electrical tower or ground is small, the lighting
will be raised to very high voltage, which will cause a flash over from the
tower to one or more phase conductors.
Such a flashover is known as black flash over.
Back flash over on the line can be minimized by reducing the tower
footing resistance using driven rods and counterpoise where soil
resistivity is high.
8. Rod gap
The rod gap is one of the most common frames of protective
devices. It is an air gap between the ends of two rods.
The gap setting should be such that it should break under all
conditions before the equipment to be protected is affected.
The chief merits of this device are simplicity, reliability and
cheapness.
Rod gap has a some restrictions like they are unable to prevent
the flow of power which flows in the gap after the breakdown. It
is used in the places where continuity of supply is not of much
importance. In such cases (where continuity is important) ,
automatic reclosing circuit breakers are used.
9. Surge diverters
Surge-diverters or lightning arrester is a device used for distracting abnormal high voltage to ground without
affecting the continuity of supply. Surge diverters are three types
Expulsion-type surge diverter
Valve-type surge diverter
Metal-oxide surge diverter
The name surge diverters appear to be correct than lightning arrester.
10. Operating principles, construction and applications of lightning arrester
What is Lightning Arrester?
Definition: The circuit which is protected from the strokes of lightning with the help of a protection device is known
as lightning arrester. Here the lightning strokes are nothing but surges with high transient voltage, arcs of isolation,
spark, and surge currents because of lightning, etc. These devices are used to defend the power systems by
forwarding the high voltage surges in the direction of the ground. And these power systems and over headlines can
also be protected by using ground wire or the earthing from the direct strikes of lightning.
Working Principle
Lightning arrestor working principle is, once the voltage surge travels
throughout the conductor then it reaches the location of the arrestor
where it is installed. So it will break down the insulation of the
lightning arrestor for a moment, so voltage surge can be discharged
toward the ground. Once the voltage of the system falls under the
fixed value, then the insulation will be restored among the ground &
conductor. Further, the current flow toward the ground will be
stopped.
lightning-arrester
11. Types of Lightning Arrester
Horn Gap Arresters:-As the name suggests, this arrestor has two metal rods in horn-shaped. The arrangement
of these metal rods can be done around a small air gap. The distance between these two rods can be increased
because they increase from the gap. The metal rods are located on ceramic insulators.
The connection of the horn can be done by connecting it to two dissimilar wires. One side of the horn can be
connected to the line throughout a resistance & choke coil whereas the other side is grounded efficiently.
The resistance restricts the flow of current toward a minute value. The
choke coil is used to provide less reactance at the usual power
frequency and also provides high reactance at transient frequency.
Therefore the choke coil doesn’t permit the transients to go into the
apparatus to be protected. The gap among the horns can be adjusted so
that the usual supply voltage is not sufficient to cause an arc.
12. Multi-Gap Arresters
These types of arresters are designed with a sequence of metal cylinders that are insulated and divided
through air gaps with each other. In the sequence of cylinders, the primary cylinder is connected toward
the electrical line, whereas the remaining cylinders are connected to the ground by series resistance. Some
of the gaps among the next cylinders contain a shunt resistance that grabs a surge when there is a surplus
of voltage.
13. Valve-Type Arresters
These kinds of arresters are applicable to electrical systems that are high-powered. These devices include two main
parts like a sequence of spark gaps as well as a series of non-linear resistor discs.
The working of these devices can be done whenever an extreme voltage causes the spark gaps to stroke & the non-
linear resisters hold the voltage within the ground. Whenever the surge of surplus power stops, the spark gaps can be
pushed separately by the resisters.
14. Insulation co-ordination, volt- time characteristic and basic impulse insulation level
Insulation Co-ordination
Definition: Insulation coordination is the process of knowing the insulation levels of the power system
components.
In other words, it is the process of determining the insulation strength of the equipment. The internal and
external insulation of the electrical equipment is exposed to continuous normal voltage and temporary
abnormal voltage.
The equipment insulation is designed in such a manner so that it withstands the highest power frequency
system voltage, occasional temporary power frequency overvoltage occasional lightning surges. The
equipment of power system has assigned the rated insulation level, and their capability can be approved by
applying different types of test.The requirement of insulations is determined by considering the following
factors.
1)Highest Power Frequency System Voltage
The AC power network has different nominal power frequency voltage level like 400V, 3.3KV, 6.6k, etc.
When the system is lightly loaded the power frequency voltage at the receiving end of the line rises. The
equipment of the power system is designed and tested to withstand under highest power frequency system
voltage (440 V, 3.6K, 7.2K,etc.) without internal or external insulation failure.
15. 2) Temporary Power Frequency Over voltages
The temporary overvoltage in the power system can be caused by the load throw off, faults, resonance, etc.
Their frequency is about 50 Hz and of lesser peaks, the lesser rate of rising and longer duration (second or
even minutes). The protection against temporary power frequency overvoltage is provided by IDMT relay.
The IDMT relay is connected to the secondary of the bus potential transformer and circuit breakers.
The relay and circuit breaker takes action within milliseconds and protects the system from temporary over
voltage.
16. 3) Transient Overvoltage Surge
The transient over voltage surges in the power system can be caused by lightning, switching, restrikes
travelling waves, etc.
The surge of the power system has the high peak, high rate of rising and last for a few tens/hundreds of
microseconds and are therefore called the transients.
This surge can cause spark over voltage and flash over at sharp corners, between phase and earth, at the
weakest point, the breakdown of gaseous/liquid/solid insulation, failure of a transformer and rotating electrical
machines.
17. The failure rate due to lightning and switching has been minimised by proper insulation coordination and surge
arrester. The several types of protective devices are installed on the network to intercept the lightning strokes
and minimise the peak rate of rising of surge reaching the equipment.
18. 4) Withstand Levels of the equipment
The basic insulation level is the reference level expressed in impulse crest voltage with a stranded wave not
higher than 1.2/50 μs. The apparatus or equipment should be capable of withstanding test wave above BIL.
Insulation coordination implies the selection of the insulation of equipment with regards to its application to
minimise the undesired incident due to voltage stresses (caused by the overvoltage in the system) within the
system. Insulation breakdown means the correlation of the insulation breakdown of the various components of
a power system to the insulation of the protective devices used for the protection of that equipment against
overvoltage.
For the safe operation of the equipment, it should have equal
insulation strength equal to or greater than the basic standard
insulation level. The protective equipment for a station
substation should be chosen to give the insulation good
protection corresponding to the working of these levels as
economical as possible.
19. Nominal System Voltage Indian Standards BIL British Standards BIL
11 KV 75 KV –
33 KV 170 KV 200 KV
66 KV 325 KV 450 KV
132 KV 550/650 KV 650/750 KV
220 KV 900/1050 KV 900/1050 KV
Basic Insulation Level Table
