anna university /EEE/ 7 th semester subject .it deals with unit1 : causes of over voltage in power system. internal over voltage external over voltage

1. EE8701-High Voltage
Engineering

2. Introduction
• What is high voltage Engineering ?
• High voltage(above 1000 V AC and above 1500V DC)
• Application of high voltage under different medium (gas
,liquid and solid) or
• Behaviour of high voltage in different medium like gas,
liquid and solid
• Applied design insulator (dielectrics ),protective devices

3. Application of HVE
• Power System
• HVAC reduce transmission losses increase
transmission effciency
• Industries
• Electrostatic Precipitator(ESP) Pollution control
• Electrostatic painting painting electrical and
mechanical machines
• Electrostatic printing designing PCB
• Research labs
• Van de graff generator

4. UNIT -1
OVER VOLTAGES IN ELECTRICAL POWER SYSTEMS

5. Causes of over voltages
• Examination of over voltages on the power system
includes a study of their magnitudes, shapes,
durations, and frequency of occurrence. The study
should be performed at all points along the
transmission network to which the surges may
travel.
• Types of over voltage occur in power system
• External voltage or lightning
• Internal overvoltage's
• Temporary over voltages- power frequency oscillation and
harmonics
• Switching over voltages

6. Internal Causes of over voltage
and its effect on power system
The over voltage causes may be broadly divided in to two
main categories:
i) INTERNAL OVER VOLTAGES CAUSES
Switching surges.
Insulation Failure.
Arcing Ground.
Resonance.
ii) EXTERNALOVER VOLTAGES CAUSES
Lightning.

7. INTERNAL CAUSES OF OVER
VOLTAGE:
• Internal causes of over voltage on the power
system are primarily due to oscillations set up by
the sudden changes in the circuit conditions.
a) Switching surges:
The over voltage, produced on the power system due
to switching operations are known as switching
surges. the few causes will be discussed here.
• Case of open line
• Case of loaded line
• Current chopping

8. Case of open line:
• During switching operations of unloaded line,
travelling waves are set up to produce over voltage
on the line. When the unloaded line is connected
to the voltage source a voltage wave is setup which
travels along the line.

12. Lightning Over voltages
• Lightning is produced in an attempt by nature to
maintain dynamic balance between the positively
charged ionosphere and the negatively charged
earth.
• Lightning over voltage is natural phenomena, its is
peak discharge in which charge accumulated in
clouds, discharges in neighbour clouds or to the
ground

13. • Electrode separation
• Cloud to cloud
• Cloud to ground is very large ( more than 10KM)

14. Charge formation in cloud
• During thunder storm , positive and negative
charges becomes separated by heavy air currents
with ice crystals in upper parts and rain in lower
parts
• This charge separation depends upon heights of the
clouds, which ranges from 0.2km to 10km, with
their charge centres probably at distance about 0.3
to 2Km

16. • Upper region of the clouds are usually positively
charge where as lower region of the clouds are
negatively charged except at the local region near
the base and head which is positive

18. Theories based on charge
separation
• Wilsons Theory of charge separation
• Simpons Theory
• Reynolds and Mason theory

19. Wilsons Theory of charge separation
• It is based only on assumption
• A large no.of ions are present in the atmosphere
• Many of these ions attach themselves to small dust
particles and water particles
• A normal electric field exists in the atmosphere under
fair-weather conditions which is generally directed
downwards towards the earth.
• The intensity of the field is approximately 1 volt/cm at
the surface of the earth and decreases gradually with
height [at 9500m,it is 0.02V/cm]
• A relatively large rain drop (0.1 cm radius) falling this
field becomes polarised, the upper side acquires a -ve
charge and lower side +ve charge.

20. • Then the lower part of the drop attracts negative
charges from the atmosphere.
• .

21. • The upwards motion of air currents tends to carry up the
top of the cloud, the +ve air and smaller drops that the
wind can blow against gravity.
• Meanwhile the falling heavier raindrops which are
negatively charged settle on the base of the cloud.
• It is to be noted that the selective action of capturing –
ve charges from the atmosphere by the lower surface of
the drop is possible.
• No such selective action occurs at the upper surface.
Thus in the original system, both the positive and
negative charges which were mixed up, producing
essentially a neutral space charge, are now separated.

22. Simpson’s and Scarse Theory

24. Reynolds and Mason proposed
modification,
• According to which the thunder clouds are
developed at heights 1 to 2 km above the ground
level and may extend up to 12 to 14 km above the
ground.
• For thunder clouds and charge formation air
currents, moisture and specific temperature range
are required.

25. • The temperature is 0ᵒC at 4km & may reach -50ᵒC at
12km.
• Water droplets do not freeze at 0ᵒC & freeze only when
temperature is below -40ᵒC & form solid particles on
which crystalline ice patterns develop & grow.
• Thundercloud consisting supercooled water droplets
moving upwards and large hail stones moving
downwards.
• The ice splinters should carry only +ve charge upwards.
• Water has H+ &OH-ions, the ion density depends on
temperature.
• Lower portion has a net –ve charge density(OH-)&
upper portion has a net +ve charge density(H+).

26. Mechanism of Lightning Stroke
• The cloud and the ground form two plates of
capacitor and dielectric medium is air. Since the
lower part of the cloud is negatively charged the
earth is positively charged by inductions. Lightning
discharge will require for break down the Air
Electric field required is 30 kv/Cm (peak)
Electric field required is 10 Kv/cm( If the
moisture content of the air is large)

27. Type of lightning streamer:
• Pilot streamer
• Stepped Leader
• Return Stroke
• Second Charge center
• Dart leader
• Heavy return Streamer

28. Pilot Streamer:

29. • After the gradient of approximately 10Kv/cm is set
up in the clouds the air surrounding gets ionized. In
this condition a streamer starts from clouds
towards earth.
• The current in the streamer is 100 A. and speed is
0.16m/µsec. This streamer is known as pilot
streamer. This leads to the lighting phenomenon.

30. Stepped Leader:
• Depending upon the state of ionization of the air
surrounding the streamer it‘s branched to several
paths and this is known as stepped leader.

31. Return Stroke:
• Once stepped layer contact with earth a power
return stroke moves very fast up towards the
clouds through the already ionized path by the
leader.

32. Second charge center:

33. • Negative charge of the cloud is being neutralized by
the positive induced charge on the earth.
• This instant gives rise to lighting flash which we
observes with our naked eyes.
• There may be another cell of charges in the cloud
near the neutralize charged cell

34. Dart Leader:
• This charged cell will try to neutralize through this
ionized path. This streamer is known as dart leader.
The velocity of dart leader is about 3% of the
velocity of light. The effect of the dart leader is
much more severe that of the return stroke.

35. Heavy return stroke:
• The second charge centre is discharging to ground
through the dart leader. positive streamers are
going up from ground. This is called heavy return
stroke. This begins to discharge negative charge
under the cloud and the second charge centre in
the cloud.

37. Rate of charging of thunder
clouds

39. Parameters and Characteristics of
Lightning Strokes:
• The parameters and Characteristics of Lightning Strokes
include the amplitude of the currents, the rate of rise,
the probability distribution of the above, and the wave
shapes of the lightning voltages and currents.
• Typical oscillograms of the lightning current and voltage
wave shapes on a transmission line are shown in Figs
8.11 and 8.12. The lightning current oscillograms
indicate and initial high current portion which is
characterized by short front times up to 10 μs. The high
current peak may last for some tens of microseconds
followed by a long duration low current portion lasting
for several milliseconds. This last portion is normally
responsible for damages (thermal damage).

41. • Lightning currents are usually measured either directly from
high towers or buildings or from the transmission tower
legs. The former gives high values and does not represent
typical currents that occur on electrical transmission lines,
and the latter gives inaccurate values due to non-uniform
division of current in legs and the presence of ground wires
and adjacent towers.
• Measurements made by several investigators and
committees indicated the large strokes of currents (> 100
kA) are possible (Fig. 8.7). It was shown earlier that tall
objects attract a large portion of high current strokes, and
this would explain the shift of the frequency distribution
curves towards higher currents.

43. • Other important Characteristics of Lightning Strokes are
time to peak value and its rate of rise. From the field data, it
was indicated that 50% of lightning stroke currents have a
rate of rise greater than 7.5 kA/μs, and for 10% strokes it
exceeded 25 kA/μs. The duration of the stroke currents
above half the value is more than 30 μs.
• Measurements of surge voltages indicated that a maximum
voltage, as high as 5,000 kV, is possible on transmission
lines, but on the average, most of the Characteristics of
Lightning Strokes give rise to voltage surges less than 1000
kV on lines. The time to front of these waves varies from 2
to 10 μs and tail times usually vary from 20 to 100 μs. The
rate of rise of voltage, during rising of the wave may be
typically about 1 MV/μs.

44. • Characteristics of Lightning Strokes on
transmission lines are classified into two groups:
• Direct strokes and
• Inducted strokes.

45. Direct strokes
• When a thunder cloud directly discharges onto a
transmission line tower or line wires it is called a
direct stroke. This is the most severe form of the
stroke. However, for bulk of the transmission
systems the direct strokes are rare and only the
induced strokes occur.

47. Inducted strokes

48. • Consider the three clouds, clouds 1 and 3 are positively
charged, and cloud 2 is negatively charged as shown in
the figure above.
• The potential of cloud 3 is reduced due to the presence
of the charged cloud 2.
• On the flash over from Cloud 1 to Cloud 2, both these
clouds are discharged rapidly, and class 3 assumes a
much potential and flashes to earth very rapidly.
• It is the most dangerous strokes because it ignores
taller building and reaches directly to the ground. This
stroke is called the induces strokes.

49. Mathematical Model for lightning
discharge
• During the charge formation process, the cloud may be
considered to be a non conductor.
• Hence, various potentials may be assumed at different parts
of the cloud.
• If charging process continues , it is probable that the
gradient at certain parts of the charged region exceeds the
breakdown strength of the air or moist air in the cloud.
• Hence, local breakdown takes place within the cloud. This
local discharge may finally lead to a situation where in a
large reservoir of charges involving a considerable mass of
cloud hangs over the ground, with the air between the
cloud and the ground as a dielectric.

50. • When a streamer discharge occurs to ground by
first a leader stroke, followed by main strokes with
considerable currents flowing, the lightning stroke
may be thought to be a current source of value I0
with a source impedance Z0 discharging to earth.

53. • In case a direct stroke occurs over the top of an
unshielded transmission line, the current wave tries
to divide into two branches and travel on either
side of the line.

54. Isokeraunic level or thunderstorm
days
• Thunder storm days (TD) (is known as the Iso
Keraunic level) is defined as the number of days in a
year when thunder is heard or recorded in a
particular location,
• The incidence of lightning strikes on Tr. Line / substation
in related to T.D.
• T.D is =5 to 10 in Brittan
• 30 to 50 in USA
• 30 t0 50 in India

55. Model for Lightning

57. Switching Surges and temporary
overvoltage
• For transmission voltages (400 KV and above) the
dis advantages generated due to switching is same
as that of the magnitude of lightning over voltages.
This over voltages exists for a long time so it‘s
dangerous to the system.
• Switching over voltages increases as the system
voltage increases. In extra high voltage line,
switching over voltages determine the insulation
levels of the lines and their dimensional and cost.

58. Source (or) Origin of switching surges:
• Open and closing the switch gears
• High natural frequency of the system
• Damped normal frequency voltage components
• Restriking and recovery voltage with successive
reflective wave form terminations
• Repeated restriking of the arc between the
contacts off

59. Characteristics of switching surges:
Switching surges arise from any one of the following sources.
• De energizing of lines, cables, and shunt capacitor bank etc.
• Disconnection of unloaded transformer, reactors etc
• Opening and closing of protective devices connected to
lines and reactive loads
• Switch off the loads suddenly
• Short circuit due to insulation failure, line to ground
contact, line to line contact, L-L-G contacts, three phase to
ground contacts etc
• Clearing of the faults
• Arcing ground.

60. Shape of switching surges:
Irregular Power frequency with its harmonics
Relative magnitude-2.4 p.u for transformer
energizing
1.4 to 2.0 p.u for switching transmission line.

62. Temporary over voltage

63. • Temporary over voltages represent a threat to
equipment as well as to any surge protective
devices that may have been provided for the
mitigation of surges.
• The scope of this Guide includes temporary over
voltages only as a threat to the survival of
SPDs(surge protection devices), and therefore
includes considerations on the selection of suitable
SPDs.

64. Following considerations are necessary to reach the goal of
practical surge immunity:
• Desired protection
• Hardware integrity
• Process immunity
• Specific equipment sensitivities
• The power environment
• Surge characteristics
• Electrical system Performance of surge protective devices
• Protection Lifetime The test environment Cost effectiveness

65. Measure to control overvoltage due
to switching and power frequency
• In EHV or UHV lines we should control the
switching voltages less than 2.5 p.u the following
measures are taken to reduce over voltage.
• One or multi-step energization of lines by inserting
resistors.
• Phase controlled closing of circuit breaker with proper
sensors.
• Drain the trapped charges before reclosing of the lines
Using shunt reactors
• By using lightning arresters or surge diverters

66. One or multistep energization of
lines by inserting resistors
• During switching of circuit breaker, inserting a
series resistance in series with circuit breaker
contacts and short circuiting this resistance after a
few cycles
• By using inserting resistance the transients due to
switching reduces. If the resistance is inserted for a
long time, successive reflections takes place and
the over voltage reaches high value. therefore using
the pre-inserting resistor limit the over voltage

68. causes for switching and power
frequency over voltage
• In the lines (400 KV and above) power frequency
over voltages occurs are caused during tap
changing operations in transformer.
Causes for power frequency over voltages;
• Sudden load rejection
• Disconnection of inductive loads
• Ferranti effect
• Unsymmetrical faults
• Saturation in transformer etc.
• Tap changing operations.

69. Sudden load rejections:
• When sudden load rejection in the system cause
the speeding up of generator prime movers hence
the system frequency will raise.
• The speed governing system will respond by reducing
the mechanical power generated by the turbines. But
initially both the frequency and voltage increases.
• The approximate voltage rise is given by

72. Tap changing transformer:
Tap changing operations are required when the
voltages changes due to load variations so during
these operations power frequency over voltages
occurs.

73. Ground wire for protecting power
conductor against direct lightning
stroke

74. Ground wires:
• Ground wire is a conductor run parallel to the main
conductor of the transmission line, supported on
the same tower and earthed at every equally and
regularly spaced towers. The different arrangement
of ground wires is as shown in below fig.

75. Important considerations of
ground wires are:
• Ground wire selection should be based on
mechanical considerations rather than electrical
considerations.
• It should have high strength and non – corrosive.
• Ground resistance, insulation and clearances
between the ground wire and the lines are
important in the design.

78. Using Counter – Poise Wires
• Counter- Poise Wires are buried in the ground at a
depth of 0.5 to 1m, running parallel to the
transmission line conductors and connected to the
tower legs. Wire length may be 50 to 100 m long.
The arrangement of counter – poise is as shown

80. Requirements of a Lightning
Arrester or Surge Diverter

81. Rod Gap
• Rod Gap is used to protect the system from
lightning or thunderstorm activity is less.
• A plain air gap usually between 1 inch square rods
cut at right angles at the ends, connected between
line and earth.
• The rod gap arrangement is shown.

82. Rod Gap

83. • It is a very simple type of diverter and consists of two
1.5 cm rods, which are bent at right angles with a gap in
between as shown in Fig.
• One rod is connected to the line circuit and the other
rod is connected to earth. The distance between gap
and insulator (i.e. distance P) must not be less than one
third of the gap length so that the arc may not reach
the insulator and damage it.
• Generally, the gap length is so adjusted that breakdown
should occur at 80% of spark-voltage in order to avoid
cascading of very steep wave fronts across the
insulators.

84. • The string of insulators for an overhead line on the
bushing of transformer has frequently a rod gap
across it. Fig 8 shows the rod gap across the
bushing of a transformer.
• Under normal operating conditions, the gap
remains non-conducting. On the occurrence of a
high voltage surge on the line, the gap sparks over
and the surge current is conducted to earth. In this
way excess charge on the line due to the surge is
harmlessly conducted to earth

85. Advantages
• Simple in construction.
• Cheap.
• Rugged construction.
Disadvantages
It does not interrupt the power frequency follow current.
Every operation of the rod gap results in L –G fault and
the breakers must operate to isolate the faulty section.
Uses
It is used as back – up protection.

86. Expulsion Type Lightning Arrester
(Protector Tube)

87. • This type of arrester is also called ‘protector tube’
and is commonly used on system operating at
voltages up to 33kV.
• The above Fig shows the essential parts of
an expulsion type lightning arrester.

88. • It essentially consists of a rod gap AA’ in series with a second gap
enclosed within the fiber tube.
• The gap in the fiber tube is formed by two electrodes. The upper
electrode is connected to rod gap and the lower electrode to the
earth. One expulsion arrester is placed under each line conductor.
• On the occurrence of an over voltage on the line, the series gap
AA’ spanned and an arc is stuck between the electrodes in the
tube. The heat of the arc vaporizes some of the fiber of tube walls
resulting in the production of neutral gas. In an extremely short
time, the gas builds up high pressure and is expelled through the
lower electrode, which is hollow. As the gas leaves the tube
violently it carries away ionized air around the arc.
• This deionizing effect is generally so strong that the arc goes out
at a current zero and will not be re-established

90. Value Type Lightning Arrester
(Non – Linear Type)
• Value Type Lightning Arresters are used to protect
substations and at line terminations to discharge
the lightning over voltages and short duration
switching surges. A value type arrester is shown
below.

92. • Valve type arresters incorporate non linear
resistors and are extensively used on systems,
operating at high voltages. Fig shows the various
parts of a valve type arrester. It consists of two
assemblies (i) series spark gaps and (ii) non-linear
resistor discs in series.
• The non-linear elements are connected in series
with the spark gaps. Both the assemblies are
accommodated in tight porcelain container.

93. • The spark gap is a multiple assembly consisting of a
number of identical spark gaps in series. Each gap
consists of two electrodes with fixed gap spacing.
The voltage distribution across the gap is line raised
by means of additional resistance elements called
grading resistors across the gap.
• The spacing of the series gaps is such that it will
withstand the normal circuit voltage. However an
over voltage will cause the gap to break down
causing the surge current to ground via the non-
linear resistors.

94. • The non-linear resistor discs are made of inorganic
compound such as thyrite or metrosil. These discs
are connected in series.
• The non-linear resistors have the property of
offering a high resistance to current flow when
normal system voltage is applied, but a low
resistance to the flow of high surge currents.
• In other words, the resistance of these non-linear
elements decreases with the increase in current
through them and vice-versa.

95. Working of value type surge
arrester
• Under normal conditions, the normal system voltage is
insufficient to cause the break down of air gap
assembly. On the occurrence of an over voltage, the
breakdown of the series spark gap takes place and the
surge current is conducted to earth via the non-linear
resistors.
• Since the magnitude of surge current is very large, the
non-linear elements will offer a very low resistance to
the passage of surge. The result is that the surge will
rapidly go to earth instead of being sent back over the
line. When the surge is over, the non-linear resistors
assume high resistance to stop the flow of current.

97. Merits:
• To protect station equipment rated 400 KV and
above.
• To protect motors and generators
• To protect distribution transformer.

98. CORONA AND ITS EFFECTS
Definition
• If the field is uniform , then an increase in voltage(A.C.)
directly leads to breakdown without any preliminary
discharge.
• However in non-uniform geometry, the increase in a.c.
voltage will cause a luminous discharge with the
production of hissing noise at points with highest
electric field intensity.
• Ionization of surrounding air around the conductor , a
hissing noise and production of ozone gas in over head
transmission line is known as corona effect,

100. Theory of corona formation

101. • Some ionization is always present in the air due to
cosmic, ultraviolet rays and radioactivity. So under
the normal conditions, the air contains some free
electrons, negative ions, and neutral molecules.
• But when the potential difference between the
conductors exceeds than a certain value a potential
gradient is set up on the conductor’s surface.
• When the potential gradient reaches up to 30kv/cm
which is sufficient for the free electron to strike
with a neutral molecule with enough force to emit
an electron.

102. • This process occurs in other molecules which emit
more free electrons.
• This process is cumulative that is why large of
electrons jumped into the air surrounding the
conductors, the air is now ionized and the spark
occurs in between the conductors.
• Now the corona effect can be divided into two
parts, one is the sound and another one is the
visual part.

103. • Critical disruptive voltage: It is the minimum
phase-neutral voltage at which corona occurs.
consider two conductors of radii r and space b/w
them is d. if V has applied voltage then the
potential gradient at the surface of conductors are,

104. • Visual critical voltage: It is the minimum phase-
neutral voltage at which a glow appears around all
along the conductors. It has been seen that in the
case of parallel conductors, the corona glow does
not begin at the disruptive voltage Vc but at a
higher voltage, Vv called visual critical voltage. The
phase neutral effective value of visual critical
voltage is given by the following formula,

105. Factors affecting corona
• The phenomenon of corona can be affected by the
physical state of the atmosphere as well as by the
condition of the line. The following are the factors upon
which corona depends.
• Atmosphere: As corona formed due to the ionization of the
air surrounding the conductors, therefore corona is affected
by the physical state of the atmosphere. In stormy weather,
the number of ions is more than the normal, so the corona
can occur at much less voltage as compared to the fair
weather.
• Conductor size & shape: The corona can be affected by the
physical shape and size of the conductor as well. If the surface
of the conductor is irregular then it will give rise to the corona
compare to the solid conductor. Because the unevenness of
the conductor produces more chances of corona than a
smoothly surfaced conductor.

106. • Spacing b/w the conductors: The spacing between the
transmission line conductors must be greater than the
diameter of the conductors because if the spacing b/w
the conductors are less the air surrounding the
conductors can be ionized at low voltage.
• Line voltage: The line voltage greatly affects the
corona. If the line voltage is low then there will be no
change in the air surrounding and hence no corona is
formed. However, if the line voltage exceeds than a
certain value the electrostatic stresses developed at the
conductors surface which ionized the air and corona is
formed.

107. Methods of Reducing Corona
Effect
• It has been observed that intense corona effects
are seen at working voltage of 33kv or above;
therefore the care should be taken while designing
the transmission lines and substations to avoid this
kind of enormous and destructive effects of a
corona.
• The following are the methods of reducing the
effects of corona discharge.

108. • By increasing the conductor size: By increasing the
size of the conductor, the voltage at which corona
occurs is raised and hence corona effects are
considerably reduced.
• By increasing conductors spacing: By increasing the
space between the conductors of transmission lines
can considerably reduce the corona effect. We
should increase the space b/w conductors from the
space at which corona occurs. Increasing space
accommodates more particles b/w the conductors.

109. Merits and demerits of corona effect
Corona has many advantages and disadvantages. In the
correct design of a high voltage overhead line, the following
merits and demerits are considered the most important.
Merits
• Due to corona the space b/w conductors is ionized and become
conducting path, so the virtual diameter of the conductor is
increased.
• Corona reduces the effects of transients produced by surges.
Demerits
• Corona is accompanied by a loss of energy. This affects the
transmission lines efficiency.
• Ozone is also produced in the corona and may cause corrosion.
• The current drawn by the line due to corona is non-sinusoidal and
hence non-sinusoidal voltage drop occurs in the line.

110. • This form of discharge is termed as Corona
discharge and is accompanied by the formation of
ozone , as is indicated by the characteristic order of
this gas. If the voltage is d.c., then the appearance
will be different .
• The positive wire will be having a uniform glow and
negative wire has a more patchy glow often
accompanied by streamers.
• An important point in connection with corona that
it is accompanied by a loss of power and this means
that there is a flow of current to the wire.

111. • The current waveform is non-sinusoidal and the
non-sinusoidal drop of volts caused by it may be
more important than loss of power. It gives rise to
radio interference.
• Attenuation due to corona:
• The effect of corona is to reduce the crest of the voltage
wave under propagation, limiting the peak value to the
critical corona voltage.
• Hence, the excess voltage above the critical voltage will
cause power loss by ionizing the surrounding air.

112. Practical Importance of Corona:
• 1.)Under normal conditions the loss of power due to corona is of no
good importance , and consequently corona calculations do not enter
directly into transmission line design. The basis of such design is entirely
financially the most economical line being the most acceptable.
• 2.)The non-sinusoidal coronal current causes a non-sinusoidal drop of
volts and these may cause some interference with neighbouring
communication circuits due to electromagnetic and electrostatic
induction .The current contains large third harmonic.
• 3.)Average corona loss on several lines from 345 KV to 750 KV gave 1 to
20 KW/Km in fair weather the higher values referring to higher voltages .
In foul-weather the losses can go upto 300 KW/Km.
• 4)When a line is energized and no corona is present , the current is a
pure sine wave and capacitive