A lightning arrester /
lightning arrestor / lightning diverter is a device used on electric power
systems and telecommunication systems to protect the insulation and conductors
of the system from the damaging effects of lightning.
Working
Principle of LA:
The earthling screen and
ground wires can well protect the electrical system against direct lightning
strokes but they fail to provide protection against traveling waves, which may
reach the terminal apparatus. The lightning arresters or surge diverts provide
protection against such surges. A lightning arrester or a surge diverter is a
protective device, which conducts the high voltage surges on the power system
to the ground.
The earthling screen and
ground wires can well protect the electrical system against direct lightning
strokes but they fail to provide protection against traveling waves, which may
reach the terminal apparatus. The lightning arresters or surge diverters
provide protection against such surges. A lightning arrester or a surge
diverter is a protective device, which conducts the high voltage surges on the
power system to the ground.
It consists of a spark gap in series with a non-linear
resistor. One end of the diverter is connected to the terminal of the equipment
to be protected and the other end is effectively grounded. The length of the
gap is so set that normal voltage is not enough to cause an arc but a
dangerously high voltage will break down the air insulation and form an arc.
The property of the non-linear resistance is that its resistance increases as
the voltage (or current) increases and vice-versa.
The action of the lightning
arrester or surge diverter is as under:
(i) Under normal operation,
the lightning arrester is off the line i.e. it conducts no current to earth or
the gap is non-conducting
(ii) On the occurrence of
over voltage, the air insulation across the gap breaks down and an arc is
formed providing a low resistance path for the surge to the ground. In this
way, the excess charge on the line due to the surge is harmlessly conducted
through the arrester to the ground instead of being sent back over the line.
(iii) It is worthwhile to
mention the function of non-linear resistor in the operation of arrester. As
the gap sparks over due to overvoltage, the arc would be a short-circuit on
the power system and may cause power-follow current in the arrester. Since the
characteristic of the resistor is to offer low resistance to high voltage (or
current), it gives the effect of short-circuit. After the surge is over, the
resistor offers high resistance to make the gap non-conducting.
Type
of LA for Outdoor Applications:
There are several types of
lightning arresters in general use. They differ only in constructional details
but operate on the same principle, providing low resistance path for the surges
to the round.
1.
Rod Gap Arrester
2.
Horn Gap Arrester
3.
Multi Gap Arrester
4.
Expulsion Type Lightning Arrester
5.
Valve Type Lightning Arrester
(1)
Rod Gap Arrester:
It is a very simple type of
diverter and consists of two 1.5 cm rods.
One rod is connected to the
line circuit and the other rod is connected to earth. The distance between gap
and insulator (i.e. distance P) must not be less than one third of the gap
length so that the arc may not reach the insulator and damage it. Generally,
the gap length is so adjusted that breakdown should occur at 80% of
spark-voltage in order to avoid cascading of very steep wave fronts across the
insulators.
The string of insulators for
an overhead line on the bushing of transformer has frequently a rod gap across
it. Fig 8 shows the rod gap across the bushing of a transformer. Under normal
operating conditions, the gap remains non-conducting. On the occurrence of a
high voltage surge on the line, the gap sparks over and the surge current is
conducted to earth. In this way excess charge on the line due to the surge is
harmlessly conducted to earth.
Limitations:
(i) After the surge is over,
the arc in the gap is maintained by the normal supply voltage, leading to
short-circuit on the system.
(ii) The rods may melt or
get damaged due to excessive heat produced by the arc.
(iii) The climatic conditions
(e.g. rain, humidity, temperature etc.) affect the performance of rod gap
arrester.
(iv) The polarity of the f the surge also affects
the performance of this arrester.
(v) Due to the above
limitations, the rod gap arrester is only used as a back-up protection in case
of main arresters.
(2)
Horn Gap Arrester:
It consists of a horn shaped
metal rods A and B separated by a small air gap. The horns are so constructed
that distance between them gradually increases towards the top as shown.
The horns are mounted on
porcelain insulators. One end of horn is connected to the line through a
resistance and choke coil L while the other end is effectively grounded.
The resistance R helps in
limiting the follow current to a small value. The choke coil is so designed
that it offers small reactance at normal power frequency but a very high
reactance at transient frequency. Thus the choke does not allow the transients
to enter the apparatus to be protected.
The gap between the horns is
so adjusted that normal supply voltage is not enough to cause an arc across the
gap.
Under normal conditions, the
gap is non-conducting i.e. normal supply voltage is insufficient to initiate
the arc between the gap. On the occurrence of an over voltage, spark-over takes
place across the small gap G. The heated air around the arc and the magnetic
effect of the arc cause the arc to travel up the gap. The arc moves
progressively into positions 1, 2 and
3.
At some position of the arc
(position 3), the distance may be too great for the voltage to maintain the
arc; consequently, the arc is extinguished. The excess charge on the line is
thus conducted through the arrester to the ground.
(3) Multi
Gap Arrester:
It consists of a series of
metallic (generally alloy of zinc) cylinders insulated from one another and
separated by small intervals of air gaps. The first cylinder (i.e. A) in the
series is connected to the line and the others to the ground through a series
resistance. The series resistance limits the power arc. By the inclusion of
series resistance, the degree of protection against traveling waves is reduced.
In order to overcome this
difficulty, some of the gaps (B to C in Fig) are shunted by resistance. Under
normal conditions, the point B is at earth potential and the normal supply
voltage is unable to break down the series gaps. On the occurrence an over voltage,
the breakdown of series gaps A to B occurs.
The heavy current after
breakdown will choose the straight – through path to earth via the shunted gaps
B and C, instead of the alternative path through the shunt resistance.
Hence the surge is over, the
arcs B to C go out and any power current following the surge is limited by the
two resistances (shunt resistance and series resistance) which are now in
series. The current is too small to maintain the arcs in the gaps A to B and
normal conditions are restored. Such arresters can be employed where system
voltage does not exceed 33kV.
(4)
Expulsion Type Arrester:
This type of arrester is
also called ‘protector tube’ and is commonly used on system operating at
voltages up to 33kV. Fig shows the essential parts of an expulsion type
lightning arrester.
It essentially consists of a
rod gap AA’ in series with a second gap enclosed within the fiber tube. The gap
in the fiber tube is formed by two electrodes. The upper electrode is connected
to rod gap and the lower electrode to the earth. One expulsion arrester is
placed under each line conductor.
On the occurrence of an over
voltage on the line, the series gap AA’ spanned and an arc is stuck between the
electrodes in the tube. The heat of the arc vaporizes some of the fiber of tube
walls resulting in the production of neutral gas. In an extremely short time,
the gas builds up high pressure and is expelled through the lower electrode,
which is hollow. As the gas leaves the tube violently it carries away ionized
air around the arc. This de ionizing effect is generally so strong that the arc
goes out at a current zero and will not be re-established.
Advantages:
(i) They are not very
expensive.
(ii) They are improved form
of rod gap arresters as they block the flow of power frequency follow currents
(iii) They can be easily
installed.
Limitations:
(i) An expulsion type
arrester can perform only limited number of operations as during each operation
some of the fiber material is used up.
(ii) This type of arrester
cannot be mounted on enclosed equipment due to discharge of gases during
operation.
(iii) Due to the poor
volt/am characteristic of the arrester, it is not suitable for protection of
expensive equipment.
(5)
Valve Type Arrester:
Valve type arresters
incorporate non linear resistors and are extensively used on systems, operating
at high voltages. Fig shows the various parts of a valve type arrester. It
consists of two assemblies (i) series spark gaps and (ii) non-linear resistor
discs in series. The non-linear elements are connected in series with the spark
gaps. Both the assemblies are accommodated in tight porcelain container.
The spark gap is a multiple
assembly consisting of a number of identical spark gaps in series. Each gap
consists of two electrodes with fixed gap spacing. The voltage distribution
across the gap is line raised by means of additional resistance elements called
grading resistors across the gap. The spacing of the series gaps is such that
it will withstand the normal circuit voltage. However an over voltage will
cause the gap to break down causing the surge current to ground via the
non-linear resistors.
The non-linear resistor
discs are made of inorganic compound such as thyrite or metrosil. These discs
are connected in series. The non-linear resistors have the property of offering
a high resistance to current flow when normal system voltage is applied, but a
low resistance to the flow of high surge currents. In other words, the
resistance of these non-linear elements decreases with the increase in current
through them and vice-versa.
Working:
Under normal conditions, the
normal system voltage is insufficient to cause the breakdown of air gap
assembly. On the occurrence of an overvoltage, the breakdown of the series
spark gap takes place and the surge current is conducted to earth via the non-linear
resistors. Since the magnitude of surge current is very large, the non-linear
elements will offer a very low resistance to the passage of surge. The result
is that the surge will rapidly go to earth instead of being sent back over the
line. When the surge is over, the non-linear resistors assume high resistance
to stop the flow of current.
(6) Silicon Carbide Arresters:
A great number of silicon
carbide arresters are still in service. The silicon carbide arrester has some
unusual electrical characteristics. It has a very high resistance to low
voltage, but a very low resistance to high-voltage.
When lightning strikes or a
transient voltage occurs on the system, there is a sudden rise in voltage and
current. The silicon carbide resistance breaks down allowing the current to be
conducted to ground. After the surge has passed, the resistance of the silicon
carbide blocks increases allowing normal operation.
The silicon carbide arrester
uses nonlinear resistors made of bonded silicon carbide placed in series with
gaps. The function of the gaps is to isolate the resistors from the normal
steady-state system voltage. One major drawback is the gaps require elaborate
design to ensure consistent spark-over level and positive clearing (resealing)
after a surge passes. It should be recognized that over a period of operations
that melted particles of copper might form which could lead to a reduction of
the breakdown voltage due to the pinhole effect.
Over a period of time, the
arrester gap will break down at small over voltages or even at normal operating
voltages. Extreme care should be taken on arresters that have failed but the
over pressure relief valve did not operate. This pressure may cause the
arrester too.
(7)
Metal Oxide Arrester:
The MOV arrester is the
arrester usually installed today
The metal oxide arresters
are without gaps, unlike the SIC arrester. This “gap-less” design eliminates
the high heat associated with the arcing discharges.
The MOV arrester has
two-voltage rating: duty cycle and maximum continuous operating voltage, unlike
the silicon carbide that just has the duty cycle rating. A metal-oxide surge
arrester utilizing zinc-oxide blocks provides the best performance, as surge
voltage conduction starts and stops promptly at a precise voltage level,
thereby improving system protection. Failure is reduced, as there is no air gap
contamination possibility; but there is always a small value of leakage current
present at operating frequency.
It is important for the test
personnel to be aware that when a metal oxide arrester is disconnected from an
energized line a small amount of static charge can be retained by the arrester.
As a safety precaution, the tester should install a temporary ground to
discharge any stored energy.
Duty cycle rating: The
silicon carbide and MOV arrester have a duty cycle rating in KV, which is
determined by duty cycle testing. Duty cycle testing of an arrester is
performed by subjecting an arrester to an AC rms voltage equal to its rating
for 24 minutes. During which the arrester must be able to withstand lightning
surges at 1-minute intervals.
Maximum continuous operating
voltage rating: The MCOV rating is usually 80 to 90% of the duty cycle rating.
Installation
of LA:
The arrester should be
connected to ground to a low resistance for effective discharge of the surge
current.
The arrester should be
mounted close to the equipment to be protected & connected with shortest
possible lead on both the line & ground side to reduce the inductive
effects of the leads while discharging large surge current.
Maintenance
of LA:
Cleaning the outside of the
arrester housing.
The line should be
de-energized before handling the arrester.
The earth connection should
be checked periodically.
To record the readings of
the surge counter.
The line lead is securely
fastened to the line conductor and arrester
The ground lead is securely
fastened to the arrester terminal and ground.
