HVE unit 1.pptx

Dr. N.G.P. INSTITUTE OF TECHNOLOGY
Approved by AICTE-New Delhi & Affiliated to Anna University, Chennai
Recognized by UGC & Accredited to NAAC with ‘A’ & NBA( BME, CSE, ECE, EEE & MECH)
Dr.NGP Nagar, Kalapatti, Coimbatore – 641048
DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
EE8701 HIGH VOLTAGE ENGINEERING
COURSE HANDING FACULTY
DR.V.RANGANAYAKI
CLASS: II/III EEE
TOTAL PERIODS:45
Dr.NGPITEEE 1

PREREQUISITES
Transmission and Distribution
Power System Analysis
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COURSE OBJECTIVES
• To impart knowledge on the following Topics
Various types of over voltages in power system and
protection methods.
Generation of over voltages in laboratories.
Measurement of over voltages.
Nature of Breakdown mechanism in solid, liquid and
gaseous dielectrics.
Testing of power apparatus and insulation coordination

COURSE OUTCOMES
Explain the causes of over voltage and protection
methods in power system.
Describe the nature of breakdown mechanism in solid,
liquid and gaseous dielectrics
Illustrate the methods used in generation of high voltages
and Currents
Use appropriate methods for measuring high AC & DC
voltages and currents
Explain various high voltage testing methods and Insulation
coordination.

SYLLABUS
UNIT I OVER VOLTAGES IN ELECTRICAL POWER SYSTEMS
• Causes of over voltages and its effects on power system – Lightning, switching surges and
temporary over voltages, Corona and its effects – Bewley lattice diagram- Protection against
over voltages.
UNIT II DIELECTRIC BREAKDOWN
• Properties of Dielectric materials - Gaseous breakdown in uniform and non-uniform fields –
Corona discharges – Vacuum breakdown – Conduction and breakdown in pure and
commercial liquids, Maintenance of oil Quality – Breakdown mechanisms in solid and
composite dielectrics- Applications of insulating materials in electrical equipment.
UNIT III GENERATION OF HIGH VOLTAGES AND HIGH CURRENTS
• Generation of High DC voltage: Rectifiers, voltage multipliers, vandigraff generator:
generation of high impulse voltage: single and multistage Marx circuits – generation of high
AC voltages: cascaded transformers, resonant transformer and tesla coil- generation of
switching surges – generation of impulse currents - Triggering and control of impulse
generators.
UNIT IV MEASUREMENT OF HIGH VOLTAGES AND HIGH CURRENTS
• High Resistance with series ammeter – Dividers, Resistance, Capacitance and Mixed dividers
- Peak Voltmeter, Generating Voltmeters - Capacitance Voltage Transformers, Electrostatic
Voltmeters – Sphere Gaps - High current shunts- Digital techniques in high voltage
measurement.
UNIT V HIGH VOLTAGE TESTING & INSULATION COORDINATION
• High voltage testing of electrical power apparatus as per International and Indian
standards – Power frequency, impulse voltage and DC testing of Insulators, circuit breakers,
bushing, isolators and transformers- Insulation Coordination& testing of capabilities.

Over voltage Vs. High voltage
7/26/2023
• When the voltage in a circuit or part of it is raised
above its upper design limit, this is known as
overvoltage.
Eg., Charging mobile directly with 230VAC instead
of a charger 5 to 12V DC
• High voltage means voltage above a particular
threshold value
• British Standard BS 7671:2008 defines high voltage as
any voltage difference between conductors that is
higher than 1000 VAC or 1500 V ripple-free DC, or any
voltage difference between a conductor and Earth
that is higher than 600 VAC or 900 V ripple-free DC.
Dr.NGPITEEE 11

Range of voltage
• Below 11kV : Low voltage
• 11kV – 100kV : HV (High Voltage)
• 100kV – 400kV : VHV (Very High Voltage)
• 400kV -1000kV : EHV ( Extra High Voltage)
• 1000kV and above: UHV (Ultra High Voltage)

Power System

Single loop generator

Why 132kV?
• Many people cite Form Factor a reason for so.
• Form Factor is defined as the ratio of RMS value
to Average value of a given AC voltage and it is
different for different waveforms.
• The commonly used AC waveform is Sine Wave
which has form factor of 1.11.

Why 132kV?...
• So, the reason given is that the transmitted voltage of
10kV, 20kV, 60kV etc. is multiplied to this form factor
to obtain such results.
- For 10kV → 10 x 1.11 = 11.1 kV (Okay! It is
approximately correct)
- For 20kV → 20 x 1.11 = 22.2 kV (Okay!
approximately 22kV)
- For 60kV → 60 x 1.11 = 66.6 kV (Error! it is 66kV)
Like that
- 120 x 1.11 = 133.2kV (A big error of about +1.2kV because it
is 132kV as used)

Why 132kV?...
• Generation companies add 10% more in their actual
target which neutralizes the line losses and the
receiving end gets the targeted result. So,
Net Voltage = Target Voltage + 10% of Target Voltage
→ 132kV = 120kV + 12kV (10% of 120kV)
→ 66kV = 60kV + 6kV
→ 11kV = 10kV + 1kV

Why 132kV?...
Distance in km Upto
15
15
to
30
30
To
80
80 to
120
120 to
160
160 to
240
240
to
400
Transmission
voltage in kV
11 11-
33
33-
66
66-
110
110-
132
110-
154
132-
220
Short Medium Long
• Short Transmission line - length less than 80kms
• Medium Transmission line – length between 80kms and
200kms
• Long Transmission line - length above 200kms

7/26/2023
OVER VOLTAGES IN ELECTRICAL POWER SYSTEM
Different Kinds of Overvoltages:
External or Lightning Overvoltages:
Internal Overvoltages
They are generated internally by connecting or disconnecting the
system, or due to the systems fault initiation or extinction.
Internal overvoltages are subdivided into:
 Temporary overvoltages - Power frequency oscillations or harmonics
 Switching overvoltages.
 The magnitude of the external overvoltages remains independent of
the system's design, whereas that of internal overvoltages
increases with increasing the operating voltage of the system.
 So, internal overvoltage effect has to be taken into, account in the
design of high voltage system insulation
 Lightning phenomenon is a peak discharge in which charge
accumulated in the clouds discharge into a neighboring cloud or to
the ground
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EXTERNAL OVERVOLTAGE :LIGHTNING
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Charge Formation in the Cloud
During thunderstorms, positive and negative charges become
separated by the heavy air currents with ice crystals in the upper
part and rain in the lower part of the cloud.
This charge separation depends on the height of the clouds, which
ranges 0.2 to 10 km, with their charge
Centres probably at a distance of about 0.3 to 2km as shown in
Fig.
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7/26/2023
Charge inside the cloud - 1 to l 00 coloumb
.
Charge Formation in the Clouds
• Theories based on charge separation
 Wilsons Theory of charge separation
 Simpons Theory
 Reynolds and Mason theory
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7/26/2023
Charge Distribution in the cloud
The upper region of the clouds are usually positively charged,
whereas the lower region of the clouds are negatively charged
except the local region , near the base and head, which is
positive as shown in Fig.
Fair weather condition:Maximum gradient = l V /cm
Bad Weather condition = 300 V/cm
Dr.NGPITEEE 24

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 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
Wilson theory
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• Then the lower part of the drop attracts negative charges
from the atmosphere.
• Thus both the positive and negative charges which were
mixed up and are now separated.

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Simpson's Theory
 3 essential region-A,B,C
 Below region A, air currents travel above 800cm/s, & no
rain drops fall through.
 In region A, air velocity is high enough to break the
falling raindrops causing +ve charge spray in the cloud &
-ve charge in the air.
 The spray is blown upwards, as the velocity decreases,
the positively charged water drops recombine with
larger drops & fall again.
 Thus region A becomes +vely charged & region B
becomes –vely charged by air currents.
 In upper region of cloud, the temperature is low & only
ice crystals exists.
Dr.NGPITEEE 27

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• The impact of air on these crystals makes them –vely
charged , thus the distribution of charge within the cloud is
as shown.
Fig. Charge formation in the cloud according to simpson’s Theory
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REYNOLD AND MASON'S THEORY
 According to this theory, thunder clouds are developed at
heights 1 to 2km above ground level& they go upto 14km.
 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 particles should carry only +ve charge upwards.
 Water has H+ & OH- ions, the ion density depends on
temperature.
 Lower portionhas a net –ve charge density (OH-)&upper
portion has a net +ve charge density (H+).
Dr.NGPITEEE 29

7/26/2023
Mechanism of Lightning
Stepped Leader or
pilot streamer
pilot streamer
contact with earth
Return Stroke
Discharge Starts
from second
discharge
centre
Dart Leader
Heavy return Streamer
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Rate of charging of Thunder Clouds
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7/26/2023
Mathematical model for lightning
The Mathematical Model of Lightning Stroke may be thought to be a current
source of value I0 with a source impedance Z0 discharging to earth. If the stroke
strikes an object of impedance Z, the voltage built across it may be taken as
The source impedance of the lightning channels = 1000 to 3000 Ω.
have surge impedances less than = < 500 Ω
(overhead lines 300 to 500 Ω, ground wires 100 to 150 Ω, towers 10 to 50 Ω, etc.).
the value Z/Z0 will usually be less than 0.1 may be neglected.
V = I0Z
Direct stroke occurs on top of an unshielded line , Then the current wave divided
into two branches,
xZ
I
V
2
0

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Thunderstorm days
The incidence of lightning strikes on transmission lines and sub-
stations is related to the degree of thunderstorm activity. It is based on
the level of “Thunderstorm days” (TD) known as “Isokeraunic Level”
defined as the number of days in a year when thunder is heard or
recorded in a particular location.
The factors that influence the lightning induced voltages on transmission
lines are
 the ground conductivity,
 the leader stroke current, and
 corona.
Dr.NGPITEEE 34

7/26/2023
Mathematical model of lightning
L = length of line,
R0,R1 = terminating resistance
L′,C = inductance and capacitance of line per unit length (dynamic)
u’ (x,t) = voltage developed at any distance x
i’ (x,t) = current through the line at any distance x
E’ (x,h,t) = Electric field in x direction at a height h (transmission line height)
Zg = equivalent ground impedance.
Dr.NGPITEEE 35

7/26/2023
SWITCHING SURGES AND TEMPORARY OVERVOLTAGES
Sources Origin of Switching Surges
Opening and closing of switchgears.
 In circuit breaker operation, switching surges with a high
rate of rise of voltage may cause repeated restriking of the
arc between the contacts of the C.B.
High natural frequencies of the system.
 Damped normal frequency voltage components.
Restriking and recovery voltage with successive reflected
waves from terminations.
Dr.NGPITEEE 36

7/26/2023
Characteristics of Switching Surges
Switching surges arise from any one of the following
Sources:
Deenergizing of lines, cables and shunt capacitor banks, etc.
Disconnection of unloaded transformers, 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 contact, 30 to ground contact etc.
Clearing of the faults.
 Arcing ground.
Dr.NGPITEEE 37

7/26/2023
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 lines
Switching Overvoltages In EHV and UHV System
Interruption of low inductive currents by high speed circuit breakers.
 Interruption of small capacitive currents by switching off the
unloaded lines.
Ferro-resonance condition.
Energization of long EHV or UHV lines.
Interruption of fault current when the fault is cleared.
➢ Single pole closing of C.B.
 Resistance switching used in C.B.
Switching operation of series capacitor connected to line for
compensation.
Sparking of the lightning arrester located at receiving end of line.
Dr.NGPITEEE 38

7/26/2023
Measures to Control Overvoltage due to
Switching and Power Frequency
 One or Multi-step energization of lines by inserting resistors.
 Phase controlled closing of C.B. with proper sensors.
 Drain the trapped charges before reclosing of the lines.
 Using shunt reactors.
 By using lightning arresters or surge diverters.
Power Frequency Overvoltages in Power Systems
Causes for Power Frequency Overvoltages
 Sudden load rejection (loss of loads)
 Disconnection of inductive loads or connection of capacitive loads
 Ferranti effect.
 Unsymmetrical faults.
 Saturation in transformers, etc.
 Tap charging operations
Dr.NGPITEEE 39

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Corona
Dr.NGPITEEE 40
Corona
When the potential difference is increased, potential gradient is set
up. If the potential gradient is above 30 kV/cm, the conductor gets
ionised. The phenomenon of faint violet glow, hissing noise and
production of ozone gas is known as corona.
 Corona discharge characteristics
 Critical Disruptive voltage
 Visual Critical voltage
 Corona power loss
 Factors affecting corona
 Advantages of corona
 Disadvantage or corona
 Methods of reducing corona
 Effects of Corona

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PROTECTION AGAINST OVER VOLTAGES
Dr.NGPITEEE 41
Types of faults may occur in power lines
 Symmetrical faults - 3ϕ fault
 Unsymmetrical faults
 L-G fault
 L-L fault
 L-L-G fault
Protection of equipment in the power system from over voltages
due to lightning can be done by:
 Using ground wires above the phase wires.
 Using ground rods.
 Using counter-poise wires.
 Using protective devices like rod gap, expulsion type and valve
type surge arrester, etc.
Ground wire
 It is a conductor run parallel to the main conductor of the
transmission line, supported on the same tower and earth
regularly spaced towers,

7/26/2023
Arrangement of Ground wire
Dr.NGPITEEE 42
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.

7/26/2023
Shielding Angle or Protection Angle ϴs
Dr.NGPITEEE 43
 The angle between the vertical line drawn through the vertical of
tower and a line through the ground wire and the shielded conductor
is as shown
Protection of Line Using Ground Wire
Assuming positively negative charge near charged cloud is present above
the line, it induces a the line conductors and ground wire. Ground wire is
earthed at regular intervals, so that the negative charges drained to the earth As
the ground wire is nearer to the line conductor, the induced charge on it will be
much less the potential rise is small.
A single ground wire reduces the induced voltage to one half of that with
ground wire. For two ground wires, the reduction is one-third of that with
ground wire.

7/26/2023
Ground Rods
Dr.NGPITEEE 44
 Effective protection depends on:
 h (height)
 ϴs (shielding angle) 30°
 Material used: Galvanized stranded steel conductors
 Uses.
 It is used for direct stroke protection of lines for voltages of 110 kV a
above.
 To protect lines from attenuation of travelling waves set up in the lines.
 Using Ground Rods
Ground rods are used to reduce the tower footing resistance. These are buried
into the ground surrounding the tower structure.
Ground rods are a number of rods about 15 mm diameter and 3 m long drive:
into the ground. The tower footing resistance can be varied by:
 Varying the spacing's of the rod.
 Varying the number of rods.
 Varying the depth to which they are driven.
 Material used: Galvanized iron or copper bearing steel.

7/26/2023
Arrangement of counter poise
Dr.NGPITEEE 45
Using Counter-Poise Wires
Counter-poise wires are buried in the ground at a depth of 0.5 to 1
m, 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

7/26/2023
Lightning arrester and Shunt protected devices
Dr.NGPITEEE 46
 Shunt Protected Devices
 Rod Gap
Rod gap is used to protect the system from lightning
or thunderstorm activity is less.
A plain air gap usually between1 inch square rods cut
at right angles at the ends, connected between line and
earth. The rod When the magnitude of an incoming
wave exceeds the gap setting of the rod-gap, a spark-
over occurs and the surge is diverted.
Basic requirements of a lightning arrester or surge arrester
 It should not pass any current to the system component which is to be
protected at abnormal conditions.
 It should break down as quick as possible when abnormal conditions
occurs.
 It should discharge the surge current without damaging it.
 It should interrupt the power frequency follow current after the surge
is discharged to ground.

7/26/2023
Expulsion type Lightning arrester
Dr.NGPITEEE 47
Expulsion type arrester is an improvement over the rod gap in that
it seals the flow of power frequency follows the current. This
arrester consists of a tube made up of fibre which is very effective,
isolating spark gap and an interrupting spark gap inside the fibre
tube.
During operation, the arc due to
the impulse spark over inside the
fibrous tube causes some fibrous
material of the tube to volatile in
the form of the gas, which is
expelled through a vent from the
bottom of the tube. Thus,
extinguishing the arc just like in
circuit breakers.

7/26/2023
Valve Type Lightning Arrester
Dr.NGPITEEE 48
Such type of resistor is called nonlinear diverter. It essentially consists a
divided spark gap in series with a resistance element having the nonlinear
characteristic.
The divided spark gap consists of some identical elements coupled in series.
Each of them consists two electrodes with the pre-ionization device.
Between each element, a grading resistor of high ohmic value is connected
in parallel.

Reflection and Refraction of travelling waves
 Bewley lattice diagram from which motion reflected and
transmitted waves and their positions at every instant can be
obtained.
 It over comes the difficulty of keeping track of the multiplicity of
successive reflection at the various junctions
Attenuation factor
α = e
𝑒
−𝑎𝑡𝑡𝑒𝑛𝑢𝑎𝑡𝑖𝑜𝑛 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑋𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑙𝑖𝑛𝑒
Propagation constant γ=
zy

7/26/2023
Reflection and Refraction Coefficient
Dr.NGPITEEE 50

7/26/2023
Procedure to draw lattice diagram
Dr.NGPITEEE 56

Bewley lattice diagram

Problems

Solution 1:

Solution 2

REFERENCES
TEXT BOOKS:
1. S.Naidu and V. Kamaraju, ‘High Voltage Engineering’, Tata McGraw Hill,
Fifth Edition, 2013.
2. E. Kuffel and W.S. Zaengl, J.Kuffel, ‘High voltage Engineering
fundamentals’, Newnes Second Edition Elsevier , New Delhi, 2005.
3. C.L. Wadhwa, ‘High voltage Engineering’, New Age International
Publishers, Third Edition, 2010.
REFERENCES
1.L.L. Alston, ‘High Voltage Technology’, Oxford University Press, First
Indian Edition, 2011.
2. Mazen Abdel – Salam, Hussein Anis, Ahdab A-Morshedy, Roshday Radwan, High
Voltage Engineering – Theory &Practice, Second Edition Marcel Dekker, Inc., 2010.
3. Subir Ray,’ An Introduction to High Voltage Engineering’ PHI Learning Private
Limited, New Delhi, Second Edition, 2013 Explain the causes of over voltage
and protection methods in power system.

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