OVER VOLTAGE PROTECTION CHAPTER- 9
CAUSES OF
OVER VOLTAGE
Introduction
Over voltages (or surge
voltages) on the electric power systems are due to various reasons. If the
system is working on the normal voltage satisfactorily then it will not stress
the insulation severely due to normal working temperature cause of normal
currents. But the voltage stresses due to over voltages can be so high that
they may prove dangerous to the line, equipment and accessories. The system may
be damaged. In order to avoid such happenings the equipments and lines are to
be provided with protection device and schemes to work effectively under
abnormal conditions and disconnect them for safety or divert the surge energy
to the earth or absorb the surge energy in some absorbers.
The
Voltage Surge Nature
Sudden rise in the voltage for a very small duration on the power
system is called as voltage surge or transient voltage, as shown in the figure.
These surges are generally
produced due to switching ON or OFF or lightning or fault occurrence due to
some reasons.
A steep front wave in a
short time t, when voltage reaches maximum value Em and time t1
is very short 1 to 5µ sec, Time t2 is the decay of wave to its
half the maximum value Em/2. The wave is specified in the times t1
and t2, for example if t1 =1μ sec and t2 = 60μ sec.
then surge is specified as 1/60 μ sec. surge.
Causes
of Over Voltages
These are broadly divided
into two categories:
1.
External
causes of over voltages, 2. Internal causes of over voltages.
1. External
Causes:
These are the severe cause and induce a very steep and high amplitude surge
wave and dangerous to the power supply lines and equipments.
These causes may be due to
the following:
i)
Direct
lightning strokes.
ii)
Electro-magnetically
induced due to lightning discharge taking place near the line.
iii)
Voltage
induced due to change in atmospheric conditions along the line – length.
iv)
Electro
statically induced voltages due to presence of charged clouds nearby.
v)
Electro
statically induced voltages due to the frictional effects of small particles
such as dirt, dust, snow.
2. Internal
Causes:
The surge produced due to internal causes area not so steep and do not have
much more amplitude as compared with surge due to lightning. The system voltage
hardly increases to twice the normal voltage of the power lines. Internal
causes may be:
i)
Switching
surge, ii) Insulation failure, iii) Arcing grounds, iv) Resonance.
I)
Switching surge:
a) The circuit opened and
closed causes the over voltage and produce a voltage surge of the nature
oscillatory and take the form of damped sinusoidal. When unloaded line is
connected to the voltage source a voltage wave is set up which travels along
the line. On reaching the terminal point ‘A’ it is bounced back to the supply
end without change in sign. On the system therefore double voltage id developed
(2 x √2Erms).
This is of temporary nature and line stabilizes with its normal voltage. The
same effect is produced when unloaded line is switched off.
b) In case of loaded line also
a high voltage is developed on the line. This voltage will be 2 x instantaneous
current x impedance of the line i.e. = 2
I Zn. Where line impedance (called as natural impedance) Zn
= √L/C where, L and C are line constants.
Taking
a simple case,
Zn = 1000, Lrms = 100
Amp.
Voltage
= 2iZn
= 2 x √2 x
100 x 1000 volts
2 x √2 x
100 x 1000
= --------------------------
kV
1000
= 282.8 kV
Maximum
voltage on the line is therefore,
= Vm + 282.8 kV where Vm
is maximum of the line voltage.
c)
Current chopping: It is the phenomena of
current interruption before natural current zero is reached.
This occurs mainly in air
blast CB because they retain same extinguishing power irrespective of the
magnitude of current to be interrupted.
When interrupting low
inductive current e.g. magnetizing current of transformer, a rapid de-ionizing
effect causes current, to fall to its zero value before natural current zero
this is called currents chopping.
ii) Insulation
failure
If the insulation of the
line fails due to conductor grounding i.e. conductor touches to ground a
voltage of the system increases. This fault is shown in the above figure. Take
the case of line is earthed at point x and the line has a potential of E volts.
Then two equal voltages are caused i.e. –E volts travelling along point B in the
section XB, and along XA and these sections contain currents equal to -E/Zn and +E/Zn respectively. Bothe these
currents pass through fault point x to the earth = 2E/Zn.
iii) Arcing
grounds:
The phenomenon of interminant arc taking place in line to ground fault of a
3-phase system with consequent production of transients is known as arcing
grounds. These are cumulative and cause damage to the equipment in the power
system by causing breakdown of insulation. This can be prevented by neutral
earthing.
iv) Resonance: We know that resonance is
the condition of the circuit when inductive reactance XL and
capacitive reactance XC of the line becomes equal and net reactance
becomes zero. Impedance becomes minimum and power factor of the circuit becomes
unity. In resonance voltage magnification takes place and voltage in the line
increases. In transmission lines as XL is small the resonance
condition is rare but it may be in case of cables buried under ground.
LIGHTNING
PHENOMENON
The discharge of the
charged cloud to the ground is called as Lightning Phenomenon.
Lightning is a huge spark
and it takes place when the clouds are charged to such a high potential
positive or negative w.r.t. the earth or a neighboring cloud and if this
happens then there is possibility of insulation of air breaks down (destroyed).
An electric discharge
between cloud and earth, between clouds or between the charge centers of the
same cloud is known as lightning.
Lightning is huge spark and
takes place when clouds are charged to such a high potential (positive or
negative) with respect to earth or a neighboring cloud that the dielectric
strength of neighboring medium (air) is destroyed. During the up rush of warm moist
air from earth, the friction between the air and the tiny particles of water
causes the building up of charges. When drops of water are formed, the larger
drops become positively charged and the smaller drops become negatively
charged. When the drops of water accumulate they form clouds. The charge of a
cloud may become so great that it may discharge to another cloud or to earth
and we call this discharge as lightning. The thunder which accompanies
lightning is due to the fact that lightning suddenly heats up the air, thereby
causing it to expand. The surrounding air pushes the expanded air back and
forth causing the wave motion of air which we recognize as thunder.
How
the Lightning Discharge is Produced
The charged clouds pass
over the earth and hence as per the theory of induction of the earth acquires
the equal and opposite charge. Taking the case that a positive charged cloud
passes over the portion of the earth naturally earth portion will have negative
charge on it. There exists a potential difference between the charged cloud and
the earth. As the chare on the cloud increases the p.d. will also increase a
stage may come that when potential
gradient is <5kV/cm>10kV/cm then insulation of air breaks down and
lightning stroke starts from cloud to earth.
Let us study the mechanism
of the lightning stroke in three stages.
Step
1
After the break in the air,
leader streamer/pilot streamer carries charge from he cloud towards the earth.
This process continues as long as the cloud feeds enough charge to the streamer to maintain the voltage gradient on
the tip of the leader streamer above the breaking strength of the air but if
the gradient is not maintained the process stops and the leader streamer is
unable to carry the charge to the earth. So, no lightning stroke on the earth
as shown in the figure. In his situation the current in the streamer is less
than 100 Amp. And its velocity of propagation is about 0.05% of the velocity of
light and its luminosity is very weak and lightning is not seen by the eyes.
Stage
2
If the leader streamer
moves towards the earth it makes contact with some object on the earth, it is
accompanied by points of luminescence travelling like jumping. This gives rises
to stepped leaders as shown in the figure. In such situation the velocity of
stepped leaders may exceed on sixth of that of the velocity of light and
distance travelled in one step may be 50 metres. The step leaders are producing
sufficient luminosity and give rise to first visual phenomenon of lightning
discharge.
Stage
3:
The path of leader streamer
is a path of ionization and therefore of complete breakdown of insulation. As
the leader streamer reaches near the earth, a return streamer shoots up from
the earth to the cloud, following the same path as the main channel of the
downward leader. The action can be compared with the closing of a switch
between the positive and negative terminal; the downward leader having negative
charge and return streamer the positive charge. This phenomenon causes a sudden
spark which we call “ lightning”. The negative charge and positive charge come
together and get neutralized at lightning stroke. The further lightning stroke
may be initiated from the other portion of the cloud.
Following
Points can be Noted About Lightning
1.
The
lightning which is seen by our eyes appears to be a single flash but in fact it
is made up of number of separate strokes.
2.
Strokes
are one after the other from 0.0005 sec. to 0.5 seconds and hence will see a
combined flash of these.
3.
Negatively
charged clouds are about 87% in comparison with positively charged cloud which
are only 13%.
4.
Information
collected from all over the world speaks that there are about 100 lightning
strokes produced per second.
5.
Currents
in the lightning strokes are in the range of 10kA to 90kA.
Lightning
Strokes.
We have seen how the
lightning stroke is produced. This stroke may struck the electrical system (i)
Directly or (ii) Indirectly.
i)
Directly Stroke: The lightning stroke
discharges directly from the cloud to the subject equipment like over head
lines (transmission or distribution lines). And the path of the current due to
this stroke is from the cloud, to the line the insulators ten via poles
(supports, towers) to the ground. The over voltages set up may so huge to flash
over this path directly to the ground. This stroke again may be type (A) and
(B) shown in the figure.
In stroke A, the lightning
discharge is from the cloud to the subject equipment i.e. an overhead line in
this case as shown in figure. The cloud will induce a charge of opposite sign
on the tall object (overhead line). When the potential between the cloud and
line exceeds the breakdown value of air, the lightning discharge occurs between
the cloud and the line.
In stroke B, the lightning
discharge occurs on the overhead line as a result of stroke A between the
clouds as shown in the figure. There are three clouds P, Q and R having
positive, negative and positive charges respectively. The charge on the cloud Q
is bound by the cloud R. If the cloud P shifts too near to the cloud Q then
lightning discharge will occur between them and charges on both these clouds
disappear quickly. The result is that charge on cloud R suddenly becomes fee
and it then discharges rapidly to earth, ignoring tall objects.
ii)
Indirect Stroke: Indirect strokes result
from the electro-statically induced charges on the conductors due to the
presence of charge clouds. This is illustrated in figure. A positively charged
cloud is above the line and induces a negative charge on the line by
electrostatic induction. This negative charge, however, will be only on that
portion of the line right under the cloud and the portions of the line away
from it will be positively charged as shown in figure. The induced positive
charge leaks slowly to earth via the insulator. When the cloud discharges to
earth or to another cloud, the negative charge on the wire is isolated it
cannot flow quickly to earth over the insulators. The result is that negative
charge rushes along the line in both directions in the form of travelling
waves.
PROTECTION
OF TRANSMISSION LINE SUB-STATION FROM DIRECT STROKE
The surges on the power
system may originate from switching and from other causes but the important and
dangerous surges are those caused by lightning. The lightning surges may cause
serious damage to the expensive equipment in the power system (e.g. generators,
transformers etc.) either by direct strokes on he equipment or by stroke on the
transmission lines that reach the equipment as travelling wave. It is necessary
to provide protection against both kinds of surges. Following devices are
commonly used for protection against lightning surges.
1. Earthing screen. 2. Overhead ground wires. 3. Lightning arrestors or surge diverters.
Earthing screen provides
protection to power stations and sub-station against direct stroke whereas
overhead ground wires protect the transmission lines against direct lightning
strokes. However, lightning arrestors or surge diverters protect the station
apparatus against both direct strokes and the strokes that come into the
apparatus as travelling waves.
1. The
earthing Screen:
The power stations and sub-stations generally house expensive equipment. For
their protection earthing screen is provided. It consists of a network of
copper conductors (generally called shield or screen) mounted all over the
electrical equipment in the sub-station or power station. The shield is
properly connected to earth on at least two points through a low impedance. On
the occurrence of direct stroke on he station, screen provides a low resistance
path by which lightning surges are conducted to ground. In this way station
equipment is protected against damage. The limitation of this method is that it
does not provide protection against the travelling waves which may reach the
equipment in the station.
2. Overhead
Ground Wires:
The most effective method of providing protection to transmission lines against
direct lightning strokes is by the use of overhead ground wires as shown in
fig. The ground wires are placed above the line conductors at such positions
that practically all lightning strokes are intercepted by them. The ground
wires are grounded at each tower or pole through as low resistance as possible.
Due to their proper location, the ground wires will take up all the lightning strokes instead of allowing
them to line conductors.
Footing resistance of the tower must
be kept as low as possible so that surge current immediately passes to the
ground without any chance of insulator flash-over.
Advantages
of such ground wire system are:
i)
Good
protection of line from lightning strokes.
ii)
It
reduces voltage induced in the line conductors due to discharge of neighboring
cloud.
iii)
Due
to its action of short circuiting secondary ground wire provides damping effect
on any disturbance travelling along the line.
Disadvantages:
i)
Additional
cost of ground conductor.
ii)
Possibility
of breaking of ground conductor and falling on line to create short circuit
fault.
3. Lightning
arrestors.
4. Surge
absorbers.
(3) and (4) are explained
details which give protection against lightning surges and travelling waves.
LIGHTNING
ARRESTOR
Earthing screens and ground
wires are suggested for protection against direct lightning strokes but they
are unable to protect he equipment against travelling waves. The lightning
arrestors or surge diverters take care of the terminal equipments against such
surges.
Function
The function of the
lightning arrestor or surge diverter is to conduct the high voltage, surges on
the power system to the ground, thus discharging the impulse surge to earth and
to dissipate energy in the form of heat.
Schematic
Representation and Functioning of Arrestor
Basically, the arrestor has
a spark-gap and to its series a non-linear resistor. (aprperty of non-linear
resistance is that its resistance decreases as the voltage or current increases
and vice-versa). One end of arrestor is connected to the equipment terminal and
the other is perfectly earthed.
The spark gap can be set as
per the system voltage. Its length is so selected that arc is not produced due
to normal system voltage but due to an abnormal high voltage produces a spark
(arc) in the spark-gap. The arc is formed due to breaking of insulation of air
across the gap due high surge voltage.
Working
of Arrestor/Surge Diverter
Step-1. No current jumps in the
spark-gap when system voltage is having its normal value and equipment is
working.
Step-2. When the line voltage
increases due to surges, this high voltage breaks the gap and an arc is
produced (current jumps through the gap and passes through resistance R to
ground). The arc finds the low resistance path to ground and the surge is not
sent back to the line.
Step-3. As the gap, sparks over
due to over-voltage. Since, the property of non-linear resistance is to offer
high resistance to high voltage it prevents the effect of short current.
Step-4. After the surge is over, the
resistor offers high resistance to make the gap non-conducting.
Design
of Lightning Arrestor
The design should be such
that:
1.
When
the surge is over, he arc should go out immediately otherwise the current
through gap and resistant will be maintained to flow destroying gap and
resistor.
2.
During
the surge current the voltage drop i.e. Surge current (I) x Resistance should
not exceed the breakdown strength of insulation of the equipment to be
protected against such surges.
Types
of Lightning Arrestors In
all types of arrestors the mechanism is to provide a low resistance path for
the surge currents to the ground so that the high voltage produced will not
affect the equipment and its insulation remain in tack.
There are so many types
depending upon:
(i) Equipment to be protected (ii) Type of gap produced
(iii) Chemicals used (iv) Discs used.
The main types to be
studied under this topic are:
1. Rod-gap arrestor 2. Horn-gap arrestor 3. Multi gap arrestor
4. Expulsion type arrestor 5. Thirties disc-valve type arrestor.
1.
Rod Gap Arrestor:
This is generally located across the bushings of transformer and generally used
as back up protection in case of main arrestors.
Construction:
As shown in figure the arrestor is in the form
of bend-rods having spark-gap between the two. The upper rod is connected to
the line electrically and the lower one is connected to earth to carry away
surge current.
Dimensions: 1.5 cm rods bend at 900
as shown, distance ‘d’ between gap and bushing outside, face >1/3 gap length
to avoid reaching of arc to the bushing body. The gap length is so adjusted
that breakdown should occur at 0.8 of spark over voltage.
Working: Under normal voltage
conditions the gap acts as a air insulation and non-conducting and the line
voltage is ineffective to make the gap conducting.
Now when the voltage
increases so suddenly due to surge on the line, this surge voltage breaks the
gap between the rods and spark over (arc) is produced. Surge current passes
from the line to spark-gap to second rod and finally to the earth within a very
short time and the equipment is prot4ected. After the surge the gap as a
insulator under normal voltage of the line.
Drawbacks
and Limitations:
1.
It
is expected that after the surge is over arc shall blow off but there is a
chance of remaining it in the gap due to the normal voltage also. This is due
to absence of non-linear resistance in its circuit. This may lead to short
circuit.
2.
If
not properly designed the rods may bend, melt, damage due to over heating
caused by the arc.
3.
Rain
water, humidity, temperatures may affect the gap and arrestor may not give good
service.
4.
The
polarity of the surge may also affect the performance of the arrestor.
So its uses are limited.
Applications:
i)
Transformer
protection.
ii)
Backup
protection with main arrestors.
2. Horn
Gap Arrestors:
This
type arrestor also used at the limited places due to it unreliability i.e. it
is used as back up protection with main
arrestor.
Construction: (1) and (2) are the rods
bent to give horn like construction with the air gap in between the shape of
horns widening on the top portion and narrow at the bottom portion. The horns
are supported on the insulating base of porcelain. As shown in figure, one horn
connection goes to the apparatus to be protected through a resistance ‘R’ and a
chocking coil ‘L’. The second horn connection goes to the ground for earth
connection. The resistance ‘R’ plays an important part in the circuit. It helps
in limiting the follow current to a small value. Inductance of chock coil ‘L’
also helps and protects the equipment by providing high reactance (XL
= 2π
ftL) where ft is the transient frequency which is very
high. At normal frequency the reactance XL = 2π fL is small as f is only
50 Hz. Hence, chock coil does not allow the transients to enter the apparatus
to be protected.
Working: The gap between the horns is so
adjusted that normal voltage and frequency does not produce arc between the
horns. The gap acts as an insulating media. But when over voltage occurs due to
surges or transients, the voltage is sufficient to break the insulation of the
air-gap and spark (arc) is produced. The air in the gaps heated up and hence
goes up and speeded up due to magnetic effect as shown in the figure, the arc
moves in stage 1, stage 2, stage3. At position 3, the distance is much more and
the surge voltage cannot maintain the arc and it vanishes and normal condition
is brought into the circuit. This way the equipment is protected from a voltage
surge. The excess charge is conducted through portion 2 of the horn and goes to
earth.
Advantages:
1. This is superior than
rod-type because arc is self clearing and no chance of short circuit.
2. R and L limit the current
to small values.
Drawbacks/Limitations:
1.
Time
of operation is about 2 to 3 seconds, hence not reliable.
2.
Some
materials or birds may fall in the horn gap creating troubles.
3.
Due
to long use, corrosion and pitting takes place hence setting between the horns
for proper air-gap is disturbed and hence performance is affected.
Uses: As said earlier its uses
are limited due to unreliability and used for backup protection with main
arrestors.
3.
Multi
gap Arrestor: This type is used for power lines voltage not exceeding 33
kV.
Construction: This arrestor is superior
than the rod-gap and horn type arrestor. There
are arc (air) gaps between A, B and C. And shunt resistance is connected
between B and C etc. are made from the last C connection goes to earth via
series resistance. The cylinders A, B C etc. are made of zinc alloys and are
separated by gaps and insulated from one another. Cylinder ‘A’ is connected
electrically to the H.V. line as shown 33 kV. The last ‘C’ cylinder connection
goes to earth via series resistance. By introducing series resistance the
degree of protection against travelling waves is reduced but this is
compensated by connecting shunt resistance between B and C.
Working: Under normal working
voltage (say 33kV) ’B’ is at the ground potential, hence series gaps are open and
no arcing between the gaps is possible. If now over voltage such as transients
or surges are developed this very high voltage is sufficient to produce arc
between gap A and B. This surge heavy current will follow the straight through
path to earth via shunted gap B and C instead of alternative path through shunt
resistance. When sure is over the arc B to C go out and at all if any current
persists it is limited by these two resistances which form a series circuit.
This current is too small to maintain the arc between A and B. In this way the
normal conditions are re-stored and system functions on normal working voltage.
The equipment is thus protected from over voltage.
Use: These are suitable for the
power line equipment up to 33 kV.
4. Expulsion
Type Arrestor:
The other name of the arrestor is “Protector Tube”. This arrestor is used on
the power lines up to 33 kV. As the name suggests the gases formed during its
operation are expelled from he tube through a vent.
There are two gaps provided one is
‘rod gap’ and the other gap is in the fibre tube. The first gap is external gap
and second is the internal gap. Bothe these gaps are in series. The external
gap is similar to the rod gap arrestor connected to the conductor. The lower
connection goes to ground (earth). The fibre tube encloses these two metal
electrodes in which second gap is present. For 3-phase lines such three units
are required connection of one unit is shown is the figure.
Working: Under the normal working
conditions both gaps appear and act as insulators and no arc if production in
normal conditions. But on the voltage surge a tremendous voltage is produced an
arc is strict in the upper gap between the rods. The arc is also produced
between the electrodes inside the fibre tube (i.e. in second gap). A lot of
heat is produced due to high surge current. This heat vaporizes some of the fibre
material on the tube and a neutral gas is produced. This gas is an un-ionized
mixture of water vapor and decomposition product of fibre. A very high pressure
is built up in the tube gap due to this gas formation in a very short time.
This is expelled through the lower hollow electrode. As the gas leaves the tube
violently, it carries away the ionized air around the arc. The effect is so strong;
the arc is extinguished at current zero state and does not restrict.
This arrestor is superior to the
previous types.
Advantages:
1.
Cost
is cheap.
2.
Installation
is very easy.
3.
The
re-striking possibility of the arc is nil.
Drawback/Limitations:
1.
Volt/Ampere
characteristic of this arrestor is poor and hence not used for costly
equipments.
2.
Life
is short due to consumption of fibre of the tube if the strokes are repeated.
3.
This
arrestor cannot be mounted in an enclosed equipment due to gas discharges
during operations.
Use: This is suitable for
power lines and equipment rated up to 33kiV.
These are used to prevent
flash-over of line insulators, isolators and bus-insulators..
5.
Thyrite Disc-Valve Type Arrestor:
These are suitable on High Voltage Power Lines. This is non-linear diverter.
Construction: The unit consists of
spark gaps in series and also the assembly consists of a series of electrodes,
some of which are flat and the others are specially shaped designs with pressed
out projections.
The resistance elements are
generally made up in the form of blocks compose of small crystals, silicon
carbide bound together by means of an inorganic binder, the assembly being heat
treated in an oven. Both assemblies i.e. of spark-gap and resistors are
accommodated in series within a completely tight porcelain housing. The housing
is perfectly sealed so that moisture, humidity not to enter inside. These
arrestors are classified as Ii) Station type, (ii) Line type, (iii)
Distribution type.
The voltage distribution
across the gaps is linearised by means of additional resistances dlements
called as grading resistors.
The assembly of grading
resistances, spark gaps and non-linear resistances which are housed in the air
tight porcelain are shown in the figure, whereas, connection of line, arrestor
and earth is shown in the figure.
The specialty of non-linear
resistance is such that they offer a high resistance to the current when normal
system voltage is applied but very low resistance to the high value surge
currents so that surge current is quickly passes through these resistances to
the ground.
Working: Under the normal
conditions the normal line voltage is insufficient to break the air gaps and no
arc is produce and equipments work safety. On the occurrence of very high
voltage due to transients or surges breakdown of series gaps takes place. The
non-linear resistance offers a low resistance path to the surge current and it
passes to the earth within a very short time. The arc vanishes does not
re-strike back over the line. When the surge is over, the resistances offer a
very high insulation (resistance) to the normal supply voltage of the system
and stop the flow of current.
Advantages:
1.
Provide
very reliable and effective protection to the equipment like cables,
transformers and other equipments.
Breakdown
voltage under surge condition
2.
The
impulse ratio = ---------------------------------------------------------------
is practically unity.
Breakdown voltage under low frequency condition
3.
Speed
of operation is very high (operation overs in less than one second).
4.
It
sparks over at a predetermined voltage.
5.
They
dissipate surge energy.
6.
They
discharge high frequency surges without any change in spark-over
characteristic.
Drawbacks/Limitations;
1.
They
may fail to check the surges of very steep wave front from reaching the
terminal apparatus.
2.
If
moisture enters in the loose their performance.
Uses: To protect important
equipment in power stations operation on voltage up to 22kV and higher. Used in
the station, handling voltages up to 66 kV.
SURGE
ABSORBERS
Sudden rise in the voltage
for a very short period on the power system is known as a voltage surge on transient
voltage. When lightning struck a line; the surge rushes along the line just as
flood of water along a narrow valley.
The lightning introduces a
steep fronted wave. Steeper the wave front, more rapid is the build-up of
voltage at any point in the network. The voltage surge is generally specified
in terms of rise time t1 to react and the time t2 to
decay to half of the peak value (Em/2) if the t1 = 1μsec,
and t2 = 50µsec. The surge is specified 1/50 μ-sec.
The travelling wave damages the terminal apparatus. The damage depends upon the
amplitude of the surge (i.e. Em) and also upon the steepness of the wave
front (i.e. if t1 is very very small).
If, purpose of installing a
“surge absorber” is to reduce the steepness of the wave front of the surge
wave.
Surge absorber is therefore
a protective device which reduces the steepness of wave front of a surge by
absorbing surge energy.
|
Surge Absorber
|
Surge Diverter/Lightning
Arrestor.
|
|
1.
Absorbs
the surge energy to eliminate the surge and to protect he equipment.
|
1. Diverts the surge to the earth
and eliminate the surge to protect the equipment.
|
|
2.
Construction
very simple and less costly.
|
2. Construction not so simple but
some what costly.
|
Let
us study some of the surge absorbers:
1.
Condenser
surge absorber,
2.
Chock
and resistance surge absorber.
3.
Ferranti
surge absorber.
1.
Condenser Surge Absorber:
Capacitive reactance XC = 1/2πfC is
large and hence no current flows from line to earth at normal voltage and
normal frequency of the line.
Bus when over voltage
appears due to transients or surges due to the high frequency of the surge
voltage XC becomes very very small and shorted and thus immediately
the current (energy) passes to the ground. Hence, capacitor absorbs surge
energy and allows it to dissipate in the ground and the equipment like
transformer windings are protected from surges.
2.
Chock and Resistance Surge Absorber:
In this type of absorber, a
resistance and inductor coil connected in parallel forming a series circuit of
line, R and L and the equipment (transformer).
At the normal conditions
and power frequency XL(=2πfL) is small. But in case of surge
voltage developed in the line XL becomes very large and the current passes
through the resistance where he energy is absorbed (I2Rt). Thus, the
surge energy is dissipated in ‘R’ to produce heat.
3.
Ferranti Surge Absorber:
This is the most modern type of absorber. It consists of an air-cored inductor
connected in series with each line and surrounded by a grounded (earth). This
sheet is called as dissipater. The sheet is thus magnetically coupled to
inductor but electrically isolated from it by air.
This arrangement acts as a
air-cored transformer whose primary is an inductor coil and metallic dissipater
is a short circuited secondary of a single turn. Whenever, the travelling wave
is incident on the surge absorber, the energy contained in the wave is
dissipated in the form of heat generated in the dissipater sheet (i) due to
current set up in it by ordinary transformer action and (ii) by eddy currents.
The steepness of the wave front is also reduced because of the series
inductance of the inductor coil.
HARMFUL
EFFECTS OF TRAVELLING WAVES AND PROTECTION AGAINST TRAVELLING WAVES
As seen in the previous
articles, shielding the lines and stations by overhead ground wires provided
adequate protection against direct lightning strokes and also reduces electro-statically/electromagnetically
induced over voltages. But this type of protection cannot prevent travelling
waves which may reach terminal equipments.
The travelling waves can
cause the following damages:
1.
Insulation
of the windings may be damaged due to internal flash over by the high peak (Em)
voltage of the voltage surge.
2.
Inter-turn
insulation of transformer may be damaged due to steep front of the surge wave
causing internal flash over.
3.
Insulators
of the terminal equipment may be damaged due to external flash-over by the high
peak (Em) surge wave.
4.
Resonance
and high voltages resulting from the steep fronted wave may cause
internal/external flash-over of an unpredicted nature causing building up of
oscillations in the electrical equipments.
Hence, it is absolutely
necessary to provide some protective device at the stations or substations for
the protection of the equipment against the travelling waves caused by
lightning. The protective devices used for this purpose are:
i)
Road
gap lightning arrestors.
ii)
Arcing
horns across string insulators.
iii)
Different
types of lightning arrestors or surge diverters/absorbers.
iv)
Earthing
screens.
v)
Overhead
ground wires which have been studied in the previous articles.
INSULATION
CO-ORDINATION
For each system voltage
basic impulse insulation level has been fixed by most of the National and
International Standers. The major substation equipment, viz., transformers,
breakers, isolating switches, current transformers, potential transformers, are
manufactured for the same insulation level, except where transformers may be
manufactured for a lower step of the same insulation level in consideration of
the economy possible. Sometimes, when the lightning arrestors are installed
right on the terminals of the transformers. Sometimes, when the lightning
equipment may fall outside the protective zone determined from the withstand
level of the equipment, discharge voltage of the lightning arrestor and the
distance between the equipment and the lightning arrestor and the equipment may
be arranged with one step higher B.I.L. In general, four levels of insulation
in a station are recognized; the bus insulation is highest, the post
insulators, breakers, switches etc., next lower; the transmission former next
lower; and the lightning arrestor is the lowest. The percentage of margins
between these different levels are not uniform but can be attained on a
reasonably constant basis by the use of commercially available components of
equipment. With bus insulation, for example, it is a simple matter in high
voltage station to add one or two insulator units in a strain bus without
significant increase in cost. The insulation of the disconnecting switches is
brought to a satisfactory level by usually using the next higher rated switch
above the insulation class of the system. The transformers are insulated for
the lowest level that can be protected with adequate margin of safety above the
level of the arrestor which is required to withstand 50 cycle over-voltage of
the system not only in normal service but also under fault conditions. The
co-ordination of insulation as practiced by the AG and ESC in station equipment
of various system voltages is shown in the following Table:
|
Impulse
kV
|
||||
|
Rated
System Voltage kV
|
Bus
Installation
|
Switch
and post Insulation
|
Transformers
|
Circuit
Breakers
|
|
23
34.5
69
115
138
330
|
330
420
565
865
1000
1665
|
225
235
440
590
590
1475
|
150
200
350
550/450
550/450
1175
|
150
200
350
650
650
1175
|
The value for transformers
and circuit breakers are with-stand values while those for bus and switch
insulators are critical values.
Standard
insulation levels as per practice of M.S.E.B. India.
|
System
Voltage kV rms
|
Impulse
Strength of Transformer Winding 1/50 μS Full
Wave kV peak
|
Dry
Flash Over Voltage kV rms
|
Wet
Flash Over Voltage kV rms.
|
|
230
|
900
|
530
|
385
|
|
110
|
550
|
320
|
240
|
|
66
|
350
|
260
|
180
|
|
33
|
200
|
170
|
120
|
The following
characteristics shows the basic concept of co-ordination of insulations.
(b)
Power system equipment
The lightning arrestor will
spark-over at a voltage less than the insulation withstand voltage of the
equipment i.e. to say the protective device must have a lowest protective level
(B) characteristic than that of the equipment characteristic (A) i.e.
characteristic (B) is below (A).
EXERCISE
1.
Explain
lightning phenomenon.
2.
Write
a note on insulation co-ordination.
3.
What
are harmful effects of travelling wave.
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