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8/3/2019 Unit1 e72 Psg Part 1
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Over Voltages
Dr.M.GEETHANJALIDepartment of Electrical &Electronics EngineeringTCE, Madurai
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Voltage Classification Low Voltage
Up to 250 V
A/C 1 230 V & 230 DC V
Medium Voltage
Above 250 V & Up to 650 V
A/C 3 230/415 V & 500 V DC
High Voltage
Above 650 V & up to 33000V
Extra High Voltage
Above 33000 V
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Permissible Variation of VoltageMax. Permissible Limit - Voltage
For Low & Medium Voltage is 5%
For High & Extra High Voltage is 12.5%
Specified by IE Rule No.5
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What is anover-voltage? Any voltage level that is 10%
greater than the systems normal
operating AC RMS or DC voltage
level is called as Over voltage
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Voltage and Stress on Insulation Normal operating voltage of the system
do not stress the insulation severely
Voltage stresses due to over voltages can
be so high and may lead to insulation
failure
Leads to equipment damage
Protective devices are must to protect PS
Equipment from over voltages
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Causes of Over Voltages Lightning
Utility Switching Equipment / Parasitic switching
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Types of Over VoltageExternal Over voltages
Internal Over voltages
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External over Voltages Direct Lightning Strokes
Electromagnetically induced over
voltages
Due to lightning discharge taking place
near the line (Commonly known as Side
Stroke)
Voltage induced due to changing
atmospheric conditions along line length
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Contd. Electrostatically induced over voltages
due to the presence of charge clouds
nearby
Electrostatically induced over voltages
due to frictional effects of small particles
Such as dust or dry snow in the atmosphere
or due to change in the altitude of the line
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Internal Over Voltages Caused by the changes in the operating
conditions
Switching over voltages due to ON/OFF ofequipment
Temporary over voltages are the steady state
over voltage of PS arising due to line charging effect, over
compensation or loss of load
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Lightning Surges
Initial and direct strikes are the most damaging,
however, follow up pulse continue to cause
indirect problems up to 30 miles away from
strike.
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Lightning Phenomena
The discharge of charged cloud to the ground
A lightning discharge thro air occurs
When a cloud is raised to a high potential withrespect to ground or to a nearby cloud
The cloud & ground form two plates of a
gigantic capacitor Whose dielectric medium is air
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As the lower part of the cloud is negatively
charges & the ground get positively chargedby induction
Lightning discharge requires breakdown ofthe dielectric medium air between
The negatively charged cloud to positivelycharged ground
When a potential gradient of about 10kV/cm
is setup in the cloud The air immediately surrounding the cloud gets
ionized and the first process of the actuallightning discharge starts
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At this instant, a streamer called a Pilot
streamer starts from the cloud towards the
ground Which is not visible
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Utility Switching /
Switching Surges
Switching of large transformers, motors, and other
inductive load generates spike or transient impulse
that enter your facilities through the power network.
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Equipment or Parasitic Switching
Caused by equipment in your facility
These freak interferences get induced
back into your power system
Few examples of equipment that can
cause these strange transients.
Discharge lamps, Arc furnaces, Welders,
and Motors
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Transients and Causes
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Lightning Over voltages
Lightning discharges produce over
voltage surges & they are extremely
harmful Lightning over voltages can be classified
as
Induced over voltages
Over voltages due to shielding failures
Over voltages by back flashovers
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Lightning Phenomena
Lightning is a fast 1/50 wave front (few
sec)
Insulators with higher levels of fast surges canstand
Lightning surges tend to be fixed, due to the
lightning itself independent of line voltage Lightning more of a problem
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Causes of Lightning Over Voltage
Travelling wave produced due to lightning surges will
Shatter the insulators & may damage the poles
When lightning surge hit the winding of Transformer
or Generator
Cause a considerable damage to the equipment
The arc initiated in any pat of the PS due to lightning
will setup a disturbing oscillations in the line Results in damage of other equipment connected
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Klydonograph
A device attached to electric power lines forestimating certain electrical characteristics oflightning
by means of the figures produced onphotographic film by the lightning-producedsurge carried over the lines
The size of the figure is a function of thepotential and polarity of the lightningdischarge.
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Klydonograph contains a rounded electrode
connected to the line Whose surge voltage is to be measured
Electrodes rests on the emulsion side of a
photographic film or plate Which in turn rests on the smooth surface of an
insulating plate made of homogeneous insulating
material backed by a metal plate electrode
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Photographic plate or film is turned or moved
By a clockwork mechanism for bringing in theelement of time
Three assemblies are generally placed in the
same box
For simultaneously measuring the voltages on
three phases of a Tr. Line
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Lightning Protection Devices for
Utilities Shielding by earth wires / Earthing
screen
Proper earthing of transmission
towers
Lightning Arrester / Surge Arrestorlike ZnO varistor
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Ground Wire / Shielding by
Earth wires Running a conductor all along the top of the
Transmission Towers and Grounded
At Regular interval preferable at every Pole or Tower
Highly useful in protection of Tr. Line from
Direct Lightning strokes
Ground wire also provides a certain amount of
electrostatic screening As it causes the capacitance of ground to increase & so the
voltages induced in the conductors, owing to the discharge of
a neighbouring cloud to decrease
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Ground wire damp out the effect on any wave
or other disturbance travelling along the lineby its action as a short circuited secondary
Selecting the size of the ground wire is based
on the mechanical factor than the electricalconsiderations
Should have high tensile strength & be non-
corrosive
Ground resistances & Clearances between the
ground wire and Lines are important
In the design & construction of lines with ground
wires
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Limitations
It requires additional cost
Possibility of its breaking & falling on line
conductors thereby causing short-circuit faults Can be eliminated by using galvanized stranded
steel conductors as ground wires
Provides adequate strength to the ground wires
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Earthing screen
Used to protect Power Stations & Major Sub-
stations against lightning strokes
Consists of a network of copper conductorsover all the electrical equipment in the station
Earthed at least at two points
When a direct or indirect strokes occurs on thestation screen provides a low resistance path
By which lightning surges are passed to the ground
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Surge Diverters or Lightning Arrestors
A protective device which conducts/bypass
the HV surges in the PS to the ground
Ground wire provides certain amount ofprotection to the Lightning strokes
Even though Lightning surges will enter the
line needs to be protected With the help of Lightning Arrestor or Diverters
LA acts similar to a Safety Valve
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When a HV Surge reaches the surge diverter
It sparks over & provides a conducting path of
relatively low Z between the Line & Ground
As soon as the voltage attains normal valve
It stops the flow of current thro the LA
It consists of spark gap in series with a Non-
linear resistor
Resistance decrease as the voltage or current
increases and vice-versa
One end of the LA connected to the Equipment
& the other end connected to the ground
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The gap length is so set that for normal line voltage is
insufficient to cause an arc across the gap
But the a dangerously HV will breakdown the air insulation
and from an arc
Under normal operating conditions the LA
Conducts no current to the earth When over voltage occurs, the air insulation across the
gap breaks down & an arc is formed
Providing a low resistance path for the surge to the ground
Thus the excess charge on the line due to the surge is
safely diverted thro the ground
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Characteristics of an ideal Lightning
Arrestor
It must not take any current at normal system voltage
@ power frequency
Any abnormal transient voltage above the BDV must
cause a breakdown as quickly as possible
Capable of carrying the resulting discharge current
without damage to itself
Power frequency current following the breakdownmust be interrupted as soon as the transient voltage
has fallen below the Breakdown value
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Types of Lightning Arrestor
Rod Gap arrestor
Horn Gap arrestor
Electrotype arrestor
Oxide film arrestor
Thyrite arrestor
Expulsion type arrestor
Valve type arrestor
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Surge Arrestor Types
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Protection of Transmission Lines
against Direct Lightning Strokes
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Traveling Waves due to Lightning
Strike
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Protection against Traveling Waves
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Peterson Coil
A Petersen coil consists of an iron-cored reactor
connected at the star point of a three phase system
In the event of a fault, the capacitive charging current
is neutralized by the current across the reactor
which is equal in magnitude but 180 degrees out of
phase
This compensates for the leading current drawn by theline capacitances
The power factor of the fault moves closer to unity
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This facilitates the easy extinguishing of the arc
as both the voltage and current have a similar
zero-crossing IC=3I=3Vp/(1/C) =3VpC
Where IC is the resultant charging current that is three
times the charging current of each phase to ground.
The value of the inductance in the Petersen coilneeds to match the value of the line capacitance
which may vary as and when modifications in the
transmission lines are carried out. Hence, the Petersen
coil comes with a provision to vary the inductance
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Switching Over voltage
Switching causes slow 250/2500
wave fronts (few milliseconds)
Switching surges can be as high as
Twice the line volts
Switching more of a problem, whenthe operating voltage is > 300kV
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Typical voltage withstands
based on insulation ratings
Wires/Conductor: 6kV
Motors: 2.5kV
Drives: 2kV
Coil /Contactors: 1.5kV Control Transformers: 1.8 - 3.0kV
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Insulation Co-ordination
The process by which electrical equipment
insulation strength or BIL
Which is coordinated with the protective devicevoltage characteristics
so that the equipment is protected against
expected over voltages
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What type of devices are used to limit
over-voltages conditions in the facility?
MOV (Metal Oxide
Varistor) TVS (Transient Voltage
Supressor)
GDT (Gas Discharge Tubes or
Spark Gaps)
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MOV
(Metal Oxide Varistor)
Quick response time, Non-linear resistor
High Energy Capacity, inexpensive
Good for both AC/DC circuits Consistent performance
Widely used
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TVS - (Transient Voltage
Suppressor)
Linear resistor, Low energy
capacity
Expensive, Very consistent
operation
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