Unit1 e72 Psg Part 1

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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Unit1 e72 Psg Part 1

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