this presentation is about protection against over voltage,its definition ,types and methods of protection against over voltage in detail.

1. Protection Against Overvoltage
QUAID-E-AWAM UNIVERSITY
OF ENGINEERNG SCIENCE AND TECHNOLOGY NAWABSHAH,
SINDH, PAKISTAN.
DEPARTMENT OF ELECTRICAL ENGINEERING
PRESENTED BY: Muhammad Arif(17EL49 )
Roll No Name
17EL27 Tarique Sahito (GL)
17EL49 Muhammad Arif (AGL)
17EL99 Syed Hussain Ali
17ELO7 Allah bux
17EL17 Ali kichi
Group Details
PRESENTED TO: Dr Aslam Pervez Memon
Date : 20/08/2020

2. What are Overvoltages?
 According to IEEE standard for Insulation Coordination, Overvoltage is defined
as:
 “ Voltage between one phase and ground or between two phases, having a
crest value exceeding the corresponding crest of maximum system voltage.”
 Overvoltages may be classified by shape and duration as either temporary or
transient.
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3. What are Overvoltages?
 Temporary Overvoltage:
 An Oscillatory phase-to-ground or phase-to phase overvoltage that is at a
given location of relatively long duration(seconds, even minute) and that is
undamped or only weakly damped.
 Temporary overvoltage usually originate from switching operation or faults
(e.g load rejection, single-phase fault, fault on a high-resistance ground or
ungrounded system) or from nonlinearities (Ferro resonance, harmonics), or
both.
 They are characterized by the amplitude, the oscillation frequencies, the
total duration or the decrement.
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4. What are Overvoltages?
 Transient Overvoltage:
 A short-duration highly damped, oscillatory, or non oscillatory overvoltage,
having duration of few milliseconds or less.
 Transient overvoltage is classified as one of the following types:
 Lightning Overvoltage
 Switching Overvoltage
 Very fast front, short duration overvoltage
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5. Transient Overvoltage:
 Lightning Overvoltage:
 A type of Transient voltage in which a fast front voltage is produced by
lightning or fault.
 Such overvoltage is usually unidirectional and of very short duration.
 A typical waveform is shown in figure.
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6. Transient Overvoltage:
 Switching Overvoltage:
 A transient overvoltage in which a slow front, short-duration, unidirectional
or oscillatory, highly damped voltage is generated (usually by switching or
fault).
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7. Transient Overvoltage:
 Very fast front, short-duration overvoltage:
 A transient overvoltage in which a short duration, usually unidirectional,
voltage is generated (often by GIS disconnect switch operation or when
switching motor).
 High frequency oscillation or often superimposed on the unidirectional wave.
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8. Couses of overvoltages
 1) Internal Causes
 (i) Switching surges
 (ii) Insulation Failure
 (iii) arcing Ground
 (iv) Resonance
 2) External Causes
 i.e. lightning
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9. 1) Internal Causes
 (i) Switching surges:
 Switching Operations on Unloaded Line:
 A switching operation produces a sudden change in the circuit conditions.
 When an open-ended line is connected to a source of voltage, travelling
waves are set up which rapidly charge the line.
 On reaching the open end of the line, these waves are totally reflected
without change of sign, thereby producing voltage-doubling at that end.
 These reflected waves travel back to the supply end, giving rise to further
reflections.
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10. 1) Internal Causes
 (i) Switching surges :
 Sudden Opening of Loaded Line:
 If a line carrying load is suddenly opened,
 A transient voltage of value given by e = i Z0 is set up,
 Where i is the instantaneous value of the current at the instant of opening
of line,
 And Z0 is the natural or surge impedance of the line.
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11. Switching surges :
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12. 1) Internal Causes
 (ii) Insulation Failure:
 The insulation failure in a power system may take place in various ways
such as:
 Between the conductors of an overhead line.
 The cores of an insulated cable.
 Between one conductor.
 Or core, and earth.
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13. 1) Internal Causes
 (iii) Arcing Grounds:
 it is experienced in insulated neutral system.
 Consider an alternator, whose one phase has been connected to a long line
which has got distributed inductance and capacitance to earth,
 As shown in Fig. The alternator winding can be imagined to be connected to
earth through its capacitance.
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14. 1) Internal Causes
 (iv) Resonance:
 Resonance in an electrical circuit implies that the impedance of the circuit
is purely resistive and the power factor is unity.
 Thus at resonance the inductive reactances and capacitive reactances cancel
out.
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15. 2) External Causes:
Lightning Phenomenon:
 Lightning is a discharge of electrical energy.
 It may occur:
 Between cells in the same storm as Inter cloud Lightning or within a cloud
as Intra cloud Lightning (80%)
 Cloud to Air (1%)
 Cloud to Ground (19%)
 The strong negative charge at the base of the thunderstorm induces a positive
charge at the surface by repulsion of electrons
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16. 2) External Causes
Lightning Phenomenon:
 If the electric field, or the difference between the negative and positive
charge regions, is large enough.
 The insulator between the charge regions (the air) “breaks down” and the
lightning discharge can occur between the regions of positive and negative
charge.
 The break down voltage for air is about 10,000 Volts/meter.
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17. Lightning Phenomenon:
 The lightning stroke begins when the electric fields exceed the break down
voltage.
 Initially streams of electrons surge from the cloud base toward the ground in
steps of 50 to 100 m.
 Start and stop steps as the stepped leader progresses toward ground.
 This occurs over a few microseconds and is relatively invisible.
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18. More On Lightning Phenomenon:
 Streamer:
 When the stepped leader gets near the ground within 100 m or so...
 Positive charge moves from the ground up toward the stepped leader --
these are called streamers.
 The streamers may come from almost any pointed object on the ground:
 Trees Antennas Grass
 Flagpoles Telephone Poles People
 Really Tall Towers
 Electric fields are stronger around pointed objects.
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19. More On Lightning Phenomenon:
 Stroke:
 An electrical current of about 20,000 Amps flows, depositing the electrons
on the ground.
 The current generated over the short time interval heats the surroundings
to approximately 3000 K (The sun’s surface ~ 6000 K).
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20. Methods of Protection Against Lightning
 These are mainly three main methods generally used for protection against
lightning. They are
 Earthing screen.
 Overhead earth wire.
 Lightning arrester or surge dividers.
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21. Earthing screen
 Earthing screen is generally used over electrical substation.
 In this arrangement a net of GI( Galvanized Iron ) wire is mounted over the
sub-station.
 The GI wires, used for earthing screen are properly grounded through
different sub-station structures.
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22. Earthing screen(cont..)
 This network of grounded GI wire over electrical sub-station, provides very
low resistance path to the ground for lightning strokes.
 This method of high voltage protection is very simple and economic but the
main drawback is, it can not protect the system from travelling wave which
may reach to the sub-station via different feeders.
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23. Overhead Earth Wire
 This method of over voltage protection is similar as earthing screen.
 The only difference is, an earthing screen is placed over an electrical sub-
station,
 whereas, overhead earth wire is placed over electrical transmission network.
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24. Overhead Earth Wire(cont..)
 One or two stranded GI wires of suitable cross-section are placed over the
transmission conductors.
 These GI wires are properly grounded at each transmission tower.
 These overhead ground wires or earth wire divert all the lightning strokes to
the ground instead of allowing them to strike directly on the transmission
conductors.
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25. Lightning Arrester
 The previously discussed two methods, i.e. earthing screen and over-head
earth wire are very suitable for protecting an electrical power system from
directed lightning strokes but system from directed lightning strokes but
these methods can not provide any protection against high voltage travelling
wave which may propagate through the line to the equipment of the sub-
station.
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26. Lightning Arrester(cont..)
 The lightning arrester is a devices which provides very low impedance path to
the ground for high voltage travelling waves.
 The concept of a lightning arrester is very simple. This device behaves like a
nonlinear electrical resistance. The resistance decreases as voltage increases
and vice-versa, after a certain level of voltage.
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27. Lightning Arrester(cont..)
 The functions of a lightning arrester or surge dividers can be listed as below.
 Under normal voltage level, these devices withstand easily the system voltage
as electrical insulator and provide no conducting path to the system current.
 On occurrence of voltage surge in the system, these devices provide very low
impedance path for the excess charge of the surge to the ground.
 After conducting the charges of surge, to the ground, the voltage becomes to
its normal level. Then lightning arrester regains its insulation properly and
prevents regains its insulation property and prevents further conduction of
current, to the ground.
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28. Lightning Arrester(cont..)
 There are different types of lightning arresters used in power system, such as
rod gap arrester, horn gap arrester, multi-gap arrester, expulsion type LA,
value type LA.
 In addition to these the most commonly used lightning arrester for over
voltage protection now-a-days gapless ZnO lightning arrester is also used.
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29. Review Questions
 Q. What are the types of over voltages?
 Ans. (i) External Over voltages
 (ii) Internal Over voltages
 Q. How are they caused?
 Ans. External over-voltages are caused due to:
 (i) Direct lightning strokes
 (ii) Electro-magnetically induced over-voltages due to lightning discharge
taking place near the line, called as 'Side Stroke'
 Internal Over-voltages are due to:
 (i) Switching operation or fault condition
 (ii) Sudden release of load in the network.
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30. Review Questions (Cont…)
 Q. What is lightning phenomenon?
 Ans. The discharge of the charged cloud to ground is called lightning
phenomenon.
 Q. What is magnetic link?
 Ans. It is an instrument for the measurement of surge currents due to
lightning.
 Q. What is a Surge Absorber?
 Ans. Surge absorber or Surge modifier is a device which absorbs energy
contained in a travelling wave and reduces the amplitude of the Surge and
steepness of its wave front. Ferranti Surge Absorber is an example.
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31. Review Questions (Cont…)
 Q. What is arcing ground?
 Ans. During a Line to Ground fault on an ungrounded system, till the fault is
cleared, there will be intermittent discharges to ground thro' the capacitance
between the healthy phases and ground. This phenomenon is called 'arcing
ground'.
 Q. On what factor, does the effectiveness of Overhead Ground wires depend
upon?
 Ans. The tower footing resistance is the factor on which the effectiveness of
the overhead ground wire depends upon.
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32. THANKS FOR
LESTENING!
GOOD BAY!
HAVE A NICE DAY!
BEST OF LUCK.
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