Transcript Overvoltages - protection

HIGH VOLTAGE ENGINEERING
FOR
B.E.(EEE) STUDENTS
OF
ANNA UNIVERSITY
Dr M A Panneerselvam, Professor,
Anna University
1
INTRODUCTION OF THE FACULTY
NAME
: Dr. M.A. PANNEERSELVAM
QUALIFICATION
: B.E., (ELECTRICAL)
M.E (HIGH VOLTAGE
ENGINEERING)
Ph.D (HIGH VOLTAGE
ENGINEERING)
AREA OF
SPECIALISATION
: ELECTRICAL MACHINES
&
HIGH VOLTAGE
Dr M A Panneerselvam, Professor, ENGINEERING
Anna University
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NO. OF PAPERS
PUBLISHED
: ABOUT 30 IN BOTH NATIONAL
& INTERNATIONAL JOURNALS
NO. OF Ph.D’s PRODUCED
: 4
CANDIDATES WORKING
FOR Ph.D. CURRENTLY
: 4
Dr M A Panneerselvam, Professor,
Anna University
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LIST OF REFERENCES
1. High Voltage Engineering -4 th
Edition- M.S. Naidu and V.KamarajuTata Mc.Graw-Hill Publishing Co.
Ltd.,- New Delhi- 2009.
2. High Voltage Engineering -3 rd
Edition- C.L. Wadhwa - New Age
International(P) Ltd. Publishers New Delhi, Bangalore …- 2010.
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Anna University
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3. High Voltage Engineering J.R.Lucas - Sri Lanka - 2001.
4. High Voltage EngineeringKuffel,E and Abdullah,M - Pergomon
Press, Oxford-1970.
5. High Voltage Engineering
Fundamentals - 2 nd Edition Kuffel,E , Zaengl,W.S and Kuffel,J Butterworths, London- 2000.
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Anna University
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6. High Voltage Measurement
Techniques - Schwab,A.J - M.I.T.
Press, Cambridge - 1972.
7. High voltage Technology Alston,L.L - Oxford University Press,
Oxford-1968.
8. High Voltage Laboratory
Techniques- Craggs, J.D. and Meek,
J.M - Butterworths, London- 1954.
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Anna University
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9. Indian Standards Specification
on High Voltage Testing of Electrical
Apparatus ( IS 1876-1961, IS 2071
Part I-1974, IS 2071 Part II-1974,
IS 2071 Part III-1976, IS 2026 Part
III- 1981, IS 3070 Part I-1985,
IS
2516 Part II/Sec.2-1965 and IS 698 ).
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Anna University
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THE SUBJECT DEALS WITH
THE FOLLOWING TOPICS:
1.OVERVOLTAGES
2.BREAKDOWN IN GASES, SOLIDS ,
LIQUIDS AND VACUUM DIELECTRICS
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Anna University
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3.GENERATION OF VERY HIGH
VOLTAGES AND CURRENTS
4.MEASUREMENT OF VERY HIGH
VOLTAGES AND CURRENTS
5.HIGH VOLTAGE TESTING &
INSULATION COORDINATION
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Anna University
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UNIT 1 : OVERVOLTAGES
1.0 NATURE OF OVERVOLTAGES
1.External overvoltages / Lightning
overvoltages
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Anna University
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2.Internal overvoltages /
Switching surges
3.Power frequency overvoltages
due to system faults
4.DC overvoltages
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Anna University
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1.1 LIGHTNING OVERVOLTAGES
Due to lightning and thunder storms
overvoltages are injected onto the
transmission lines.
CLOUD
- - - - ++++
- - - -++++
- - - - +++++
- - - +++++
++++--++++--- ++++----++++--DISCHARGE
Dr M A Panneerselvam, Professor,
Anna University
TYPE - I
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DISCHARGE
--------- - - -- - - - - - - - -- - - - - -- - - - - - - - - -- - - - - - - -- - - - - - -- - - ------
++++++++
++++++++
+++++++++
++++++++
+++++
CLOUD
CLOUD
TYPE - II
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Anna University
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--------- - - -- - - - - - - - -- - - - - -- - - - - - - - - -- - - - - - - -- - - - - - -- - - ------
++++++++
++++++++
+++++++++
++++++++
+++++
I AMPS
/2
/2
-----------------Ƶ
Ƶ
I AMPS
/2
/2
+++++++++++++++++++
Ƶ
Ƶ
TYPE - III
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PROPAGATION OF LIGHTNING CHANNEL
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Anna University
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1.1.1 Voltage developed due to
lightning stroke:
EQUIVALENT CIRCUIT
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For a lightning stroke current of
200 kA and assuming a surge
impedance of 400 Ω for overhead
line, the voltage developed is
equal to( I x Z/ 2 ) = 200 x 103 x
400/2 = 40 x 106 = 40 MV.
1.1.2 Traveling waves on
transmission lines :
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Anna University
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TRANSMISSION LINE WITH SURGE IMPEDANCE ‘Z’
The velocity of traveling
waves on overhead lines is
300 m / μs and on cables is
approximately 150 m / μs.
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Anna University
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1.1.3 Impulse voltage wave
shape:
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Anna University
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Specification for impulse
voltage: ( AS PER INDIAN STANDARDS )
t1  Time to Front  1.2 s
t2  Time to Tail  50 s
Vp  Peak voltage
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Tolerances allowed:
For Front time, t1  ± 30%
For Tail time,
t2  ± 20%
Oscillations around the peak ,Vp,
±5%
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1.1.4 Types of impulse voltages :
FULL IMPULSE
CHOPPED IMPULSE
FRONT OF WAVE
IMPULSE
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Anna University
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1.2 INTERNAL OVERVOLTAGES
(SWITCHING SURGES)
1.2.1 Reasons for switching
surge voltages:
• Sudden opening of a line
• Sudden closing of a line
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Anna University
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•Connection of inductance /
Capacitance
•Sudden connection and removal
of loads , etc.
Any sudden disturbance taking
place in a transmission line will
cause switching surge .
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Anna University
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For the range of values of the
inductance and capacitance of
overhead lines the frequency of the
switching surges are generally in
the range of kc/s and they exist for
a duration of milliseconds.
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Anna University
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1.2.2 Switching surges on
transmission lines:
Ex.1 Opening of an unloaded OH
line:
Simply opening of an unloaded line
transmission line may result in
switching surge as shown in the fig.
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Anna University
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Assume the switch ‘AB’ is opened
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Anna University
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at time, t = 0, when the AC voltage
is at its peak. During the next half
cycle the voltage at terminal ‘A’
changes to negative peak of AC
voltage , wheras the voltage at
terminal ‘B’ remains at positive
peak. Hence the voltage across the
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Anna University
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B
2 Vp
t=0
A
switch becomes 2 Vp .If the switch
is unable withstand this voltage it
breaks down and a switching surge
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Anna University
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of magnitude 2 Vp travels on the
line. At the terminations it gets
reflected and refracted and builds
up further to a higher level.
Another example for generation
of switching surge is the
operation of a circuit breaker as
shown in the next figure.
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Anna University
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Ex.2 Operation of a circuit
breaker:
RE STRIKING
VOLTAGE
2Vp
RECOVERY
VOLTAGE
ARC VOLTAGE
FAULT CURRENT
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Anna University
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RESTRIKING VOLTAGE
ACROSS A CIRCUIT BREAKER
The maximum voltage across the
breaker contacts = 2 Vp =2√2 VRMS
The voltage after reflection and
refraction at the terminals of the
transmission line may reach a
maximum of 5 t0 6 times the
system voltage.
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Anna University
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1.3 PF OVERVOLTAGES DUE
TO LOCAL SYSTEM FAULTS
1.3.1 Local faults in the systems
are :

Line to ground fault (3)
 Double line to ground fault (3)
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Anna University
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 Double line fault (3)
 Triple line fault (1)
 Triple line to ground fault (1)
Of the total 11 faults above, a
double line to ground fault is
more dangerous with respect to
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Anna University
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overvoltages developed.
Coefficient of earthing (COE) of a
system is defined as the ratio of the
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Anna University
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voltage of the healthy phase to
ground to that of the line voltage
in the event of a double line to
ground fault.
The value of COE varies between
1/√3 to 1.0(i.e., 0.59 to 1.0) depending
upon the neutral impedance.
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Anna University
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When the value of ‘COE” is less
than 70 % ,the system is said to be
an effectively or solidly earthed
system.
When the ‘COE’ is more than 70 %,
the system is said to be a non
effectively earthed system.
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Anna University
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Systems above 230 kV are generally
effectively earthed.
For System ratings above 230 kV
the Switching surge voltages attain
very high values and become more
severe than impulse voltages.
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Anna University
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Hence,the insulation design (i.e.,
insulation coordination) is based
on switching surges rather than
impulse voltages.
1.4 DC OVERVOLTAGES
During the past 2 to 3 decades
HVDC systems came into existence.
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Anna University
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HVDC systems have converters
and inverters at the sending end
and receiving end respectively
employing thyristers.
Switching surges are produced
due to thyristers’ operation.
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Anna University
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1.5 TRAVELLING WAVES ON
TRANSMISSION LINES:
LONG TRANSMISSION LINE
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Anna University
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Assuming a lossless line (i.e.,
R=0,G=0) when the wave has travelled
a distance ‘x’ after a time ‘t’, the
electrostatic flux associated with the
voltage wave is, q = CxV ------------(1)
The current is given by the rate of
charge flow , I = dq/dt = VC dx/dt---(2)
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Anna University
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Here dx/dt is the velocity of the
travelling wave represented by,
I = VC v
-------------------(3)
Similarly, the electromagnetic flux
associated with the current wave,
Φ = Lx I
---------------------(4)
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Anna University
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The voltage is the rate of change of
flux linkages,
V = LI dx/dt = LIv --------------------(5)
Dividing Eqn.(5) by (3),
V/I= LIv/VCv = LI/CV
V2/I2=L/C. i.e.,V/I=Z=√(L/C) --------(6)
Next multiplying Eqn. (5) and (6),
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Anna University
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VI = VCv x LIv = VILC v2
v2 = VI / VILC = 1/ (LC)
v = 1 / √ (LC) -----------------------(7)
Substituting the values for ‘L’ and
‘C’ of overhead lines we get,
v = 1 / ((2x10-7 ln d/r x 2πε/(ln d/r))
= 3x108 m/sec. = 300 m/μ sec.
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Anna University
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which is the velocity of light.
Hence, travelling waves travel with
velocity of light on overhead lines.
In cables, since εr >1, the velocity of
travelling waves is lesser than
overhead lines and is approximately
150 m/ μ sec.
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Open ended line:
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Anna University
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The voltage wave and current
waves travelling towards the open
end are related by, V / I = Z.
Since the current at the open end
is zero, the electromagnetic energy
vanishes and is transformed into
electrostatic energy:
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Anna University
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i.e.,½ L(dx) I2 = ½ C(dx)e2
i.e.,(e/I)2 = L/C = Z2 . i.e., e=IZ=V.
Hence, the potential at the open end
is raised by ‘V’ volts and becomes
V+V=2V.
The
incident wave = V, the reflected wave
= V and the refracted (transmitted)
wave = V+V= 2V
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Anna University
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The refracted(transmitted) wave =
Incident wave + Reflected wave.
For an open ended line the reflection
coefficient for voltage wave is +1
and the reflection coefficient for
current wave is -1.
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Anna University
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VOLTAGE AND CURRENT WAVES OPEN ENDED LINE
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Anna University
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Short circuited line:
For a short circuited line , the
reflection coefficient for voltage
wave is -1 and for current wave is +1.
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Anna University
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VOLTAGE AND CURRENT WAVES FOR SHORT CIRCUITED LINE
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Anna University
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Reflection and transmission
coefficients for line terminated with
impedance ‘R’ :
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Anna University
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Let the incident voltage and current
waves be V and I, the reflected waves
V’ and I’ and the transmitted waves V’’
and I’’.
It is
seen in the earlier sections that
whatever be the value of terminating
impedance,whether it is open or short
circuited , either the current wave or
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Anna University
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voltage wave is reflected back with
negative sign, i.e., I’ = - V’/Z
I=V/Z , I’=-V’/Z and I’’=V’’/ R.
Since I’’=I+I’ and V’’= V+V’,
we
have,
V’’/R = V/Z – V’/Z =V/Z – (V’’-V)/Z
= 2V/Z – V’’/Z.
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Anna University
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V’’(1/R +1/Z)= V’’(R+Z)/RZ = 2V/Z
V’’= V 2R/(R+Z) and
I’’ = I 2Z/(R+Z)
Hence , the refraction coefficients
for voltage and current waves for
open ended line respectively are:
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Anna University
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2R / R+Z
and 2Z / R+Z
Similarly, the reflection
coefficients for voltage and
current for open ended line are
respectively:
(R-Z) / (R+Z) and - (R-Z) / (R+Z)
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Anna University
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Bewley’s Lattice Diagram:
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Anna University
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1.6 COMPARISON OF DIFFERENT
TYPES OF OVERVOLTAGES
1 Lightning overvoltage :
Lightning overvoltage is an external
overvoltage as it is independent of
the system parameters. It injects
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Anna University
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current on to the transmission
lines producing a voltage ranging
from kV to MV . It has a wave
shape of 1.2/50 μs and exists for a
period of microseconds. The very
high rate of rise of the impulse
voltage striking the line is
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Anna University
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equivalent to applying a voltage
at very high frequency of the
order of Mc/s.
2 Switching Surge :
Switching surges are internal
overvoltages as they are
dependant upon the system
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Anna University
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parameters (i.e., the voltage level,
the values of R,L and C of the line).
Their magnitudes range from 4 to 6
times the system voltage and they
have damped oscillations of kc/s
and exist for durations of
milliseconds.
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Anna University
63
3 Power frequency overvoltage :
Due to local system faults such as
‘double line to ground faults’ the
voltage of the healthy phase to ground
will increase from phase voltage to line
voltage depending upon the neural
earthing impedance of the system.
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Anna University
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1.7 PROTECTION OF
TRANSMISSION LINES AGAINST
OVERVOLTAGES
1.5.0 Transmission lines are
protected from lightning and
switching surges by adopting
the following methods :
1.Use of shielding wires
2. Reduction of tower footing
resistance and use of counter poises
3.Using spark gaps ( sphere gap
and horn gap )
4.Connection of surge absorbers
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Anna University
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5.Overhead lines connected to
cables
6.Using protector tubes ( Expulsion
Arresters )
7.Using non-linear resister
lightning arresters ( Valve
Arresters)
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Anna University
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1.7.1 Shielding wires:
 Shielding wires are ground wires
connected above phase wires.
 The shielding angle should be
less than 300 for effective
protection of the transmission line
against lightning stroke.
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Anna University
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SHIELDING ARRANGEMENT
OF TRANSMISSION LINES
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Anna University
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1.7.2 Reduction of tower footing
resistance and use of counter
poises :
Dr M SHOWING
A Panneerselvam,
Professor,
ARRANGEMENT
COUNTER
POISES
Anna University
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1.7.3 Spark gaps :
When Spark gaps are connected
between phase to ground the gaps
breakdown due to lightning
overvoltage and lightning energy is
diverted to ground through gaps.
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Anna University
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Spark gaps are of the
following types:
Rod gaps
Horn gaps
Sphere gaps
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Anna University
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Sphere gaps are generally
preferred as they have ,
Consistency in breakdown
Less affected by humidity and
other atmospheric conditions.
Lesser impulse ratio
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Anna University
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ROD GAP
HORN GAP
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Anna University
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IMPULSE HORN GAP
SPHERE GAP
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Anna University
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IMPULSE RATIO:
Impulse ratio is defined as
the ratio of peak impulse
breakdown voltage to that of
peak
power
frequency
breakdown voltage of a given
insulation.
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Anna University
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Sphere gaps have impulse ratio
around unity and hence they offer
better protection against lightning
overvoltages and helps in
reduction of insulation
of
equipment connected in the system.
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Anna University
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1.7.4 Surge absorbers :
•Power loss takes place due to corona
at excess overvoltages and helps in
the reduction of such overvoltages.
•In addition the front time of the
impulse voltage is increased resulting
in reduced stress on the equipment.
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Anna University
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IMPULSE VOLTAGE AT DIFFERENT TIMES ON A TRANSMISSION
LINE
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Anna University
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Connection of resistance in series
and Ferranti’s surge absorber :
FERRANTI’S
SURGE ABSORBER
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Anna University
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1.7.5 Connection of UG cable
to overhead line :
The reflection coefficient,
R = ZC – ZL / ZC + ZL . Taking ZL as
400 ohms and Zc as 60 ohms
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Anna University
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The reflection coefficient , R
= 60 – 400 / 60 + 400 = -340 / 460 =
-0.739
The voltage transmitted into the
cable 1.0 - 0.739 = 0.261 pu = 26 %
of the incident voltage.
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Anna University
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1.7.6 Protector tubes ( Expulsion
Arresters ) :
Spark gaps have the following
draw backs:
 They offer protection against
overvoltages by diverting the
lightning energy to ground but
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Anna University
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they cannot arrest the power follow
currents.
 They are always used as secondary
protection except for very small
system voltages.
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Anna University
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The drawbacks of expulsion
arrestors are:
They require certain minimum
energy to produce gas to quench the
arc.
For very high current values they
may explode due to very high
pressure of gas generated.
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Anna University
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EXPULSION ARRESTER
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Anna University
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1.7.7 Non linear resister lightning
arresters( Valve Arresters ) :
These arresters act as valve in the
sense that they offer very low
impedance for lightning voltages and
offer very high impedance for power
frequency currents.
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Anna University
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VALVE ARRESTER
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Anna University
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CHARACTERISTIC CURVES
FOR VALVE ARRESTORS
VOLTAGE TIME CHARACTERISTICS
RESIDUAL
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Anna University
VOLTAGE
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REPRESENTATIVE
PHOTOGRAPHS
OF
LIGHTNING DISCHARGE
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Anna University
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Anna University
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Anna University
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Anna University
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