AIR BREAKDOWN CHARACTERISTICS IN ROD-PLANE AND SPHERE-PLANE
ELECTRODE CONFIGURATION UNDER LIGHTNING IMPULSE
HAFIZAH BINTI NOR AZMUDDIN
A project report submitted in partial
fulfillment of the requirement for the award of the
Master of Electrical Engineering
Faculty of Electrical and Electronic Engineering
Universiti Tun Hussein Onn Malaysia
JULY 2014
VI
ABSTRACT
This project is describes the air breakdown characteristic in rod-plane and
sphere-plane electrode configuration under lightning impulse. The main problem in high
voltage power (HV) equipment is the degradation of insulation quality of high voltage
power equipment. As the high voltage power equipments are mainly subjected with
spark over voltage causes by the lightning strokes and switching action. A protective
device is used for determine the safe clearance required for proper insulation level. In
this project, the lightning impulse voltages setup is used the manual guide from TERCO
to generate the air breakdown voltage in high voltage laboratory. Two different
electrodes (rod-plane and sphere- plane configuration) will be tested to compare the U50
and electric field (Emax) between the two electrodes with different gap. Up and down
method was used to determine the U50 voltage. While to get the modeling of electrodes
and simulation for electric field (Emax) between the two electrodes is using finite element
method magnetic (FEMM) software. In the FEMM software, the gap between the
electrodes a tested is 0.5cm, 1.0cm, 1.5cm, 2.0cm and 2.5cm using the average of the
U50 voltage. In the thesis will describes the relationship between the U50 (kV), gap (cm),
electric field (Emax) and field utilization factor (η). From this project can be seen that the
voltage increased as the gap between the electrode increases. Meanwhile, when the Emax
(kV/cm) value is high, the electrode will be easier to breakdown.
VII
ABSTRAK
Projek ini menerangkan ciri-ciri pecah tebat udara (air breakdown) dalam konfigurasi
elektrod pada rod-rata dan sfera-rata di bawah denyutan kilat. Masalah utama dalam
peralatan voltan berkuasa tinggi adalah penurunan kualiti penebat peralatan voltan
berkuasa tinggi. Sebagai peralatan voltan berkuasa tinggi terutamanya adalah dengan
lebihan percikan voltan berpunca oleh panahan kilat dan tindakan pensuisan. Peranti
pelindung digunakan untuk menentukan pelepasan yang selamat diperlukan untuk tahap
penebat yang betul. Dalam projek ini, voltan denyutan kilat menggunakan panduan
manual dari TERCO untuk menjana voltan pecahan udara di makmal voltan tinggi. Dua
elektrod berbeza (konfigurasi rod-rata dan sfera-rata) akan diuji untuk membandingkan
U50 dan medan elektrik (Emax) antara kedua-dua elektrod dengan jarak yang berlainan.
Kaedah naik dan turun telah digunakan untuk menentukan voltan U50. Manakala untuk
mendapatkan pemodelan elektrod dan simulasi untuk medan elektrik (Emax) antara
kedua-dua elektrod adalah menggunakan perisian kaedah magnet elemen terhingga
(FEMM). Dalam perisian FEMM, jarak di antara elektrod yang diujia dalah 0.5cm,
1.0cm, 1.5cm, 2.0cm dan 2.5cm dengan menggunakan purata voltan U50. Dalam tesis ini
akan menerangkan hubungan antara voltan U50 (kV), jarak antara elektrod (cm), medan
elektrik (Emax) dan factor penggunaan medan (η). Daripada projek ini dapat di lihat,
voltan akan meningkat apabila jarak antara elektrod meningkat. Sementara itu, apabila
nilai Emax (kV/cm) tinggi, elektrod akan lebih mudah untuk pecah tebat.
VIII
TABLE OF CONTENTS
TITLE I
DECLARATION II
DEDICATION IV
ACKNOWLEDGEMENT V
ABSTRACT VI
CONTENTS VIII
LIST OF TABLES XII
LIST OF FIGURES XIII
LIST OF SYMBOLS AND ABBREVIATIONS XVI
CHAPTER 1 INTRODUCTION 1
1.1 Project Background 1
1.2 Problem statement 2
1.3 Objective Project 3
1.4 Project Scope 3
1.5 Organization of Thesis 4
CHAPTER 2 LIGHTNING & AIR BREAKDOWN: A REVIEW 5
2.1 Introduction 5
2.2 Lightning Impulse Voltage 5
2.3 Air Breakdown Mechanism 6
2.3.1 Townsend‟s Mechanism 7
2.3.2 Streamer Theory 9
2.3.2.1 Streamer process 9
2.4 Sparkover and Flashover 11
IX
2.5 Capacitive Divider 12
2.6 Electrode Arrangement for Measurement of Breakdown Voltage 13
2.7 Finite Element Method Magnetic 13
2.8 Previous Related Work 14
2.8.1 Summary of previous related works 17
CHAPTER 3 LIGHTNING IMPULSE TEST PROCEDURE & 19
SIMULATION MODEL
3.1 Introduction 19
3.2 Method for Generation of Lightning Impulse 19
3.3 Experimental Setup for Measurement of Lightning Impulse Voltage20
3.3.1 Equipment in the Generation of Impulse Voltages Circuit 21
3.3.1.1 Control Desk 21
3.3.1.2 Test Transformer 22
3.3.1.3 Silicon Rectifier 23
3.3.1.4 Smoothing Capacitor 23
3.3.1.5 Impulse voltmeter (digital display) 24
3.3.1.6 Low Voltage Divider 24
3.3.1.7 Load Capacitor 25
3.3.1.8 Insulating Rod 25
3.3.1.9 Charging Resistor 26
3.3.1.10 Wave-front Resistor 26
3.3.1.11 Wave-tail Resistor 26
3.3.1.12 Sphere Gap 27
3.3.1.13 Drive for Sphere Gap 27
3.3.1.14 Earthing Switch, Electrically Operated 28
3.3.1.15 Electrode 28
3.3.1.16 Earthing Rod 29
3.3.1.17 Connecting Cup, Aluminium 29
3.3.1.18 Floor Pedestal 29
3.3.1.19 Connecting Rod, Aluminium 30
X
3.3.1.20 Spacer Bar 30
3.3.1.21 Measuring Spark Gap 31
3.4 Single-stage Impulse Voltage Generator 32
3.5 50% Breakdown Voltage (U50) 33
3.6 Finite Element Method Magnetic 35
3.6.1 Create Model 36
3.6.2 Assign Boundary Condition 37
3.6.3 Mesh 38
3.6.4 Solve Setting 39
CHAPTER 4 BREAKDOWN PROPERTIES OF AIR UNDER 40
LIGHTNING IMPULSE: EFFECT OF ELECTRIC
GEOMETRY AND GAP LENGTH
4.1 Introduction 40
4.2 Lightning Impulse Voltage Waveform 40
4.3 Simulation of Electric Field, Emax using FEMM Software 42
4.3.1 Mesh 43
4.3.2 Voltage Density 44
4.3.3 Field Intensity |E| 45
4.3.4 Contour (Equipotential lines) 46
4.3.5 Vector Plot (Electric Field Intensity, |E|) 47
4.3.6 Contour & Vector 47
4.4 Graph of the Voltage, V and Magnitude of Field Intensity, |E| 49
4.4.1 Rod to Plane Configuration 50
4.4.2 Sphere to Plane Configuration 52
4.5 Result for breakdown voltage U50 (kV), electric field (kV/cm) 54
and field utilization factor (ƞ) for Rod to Plane and Sphere to Plane
4.5.1 The Relationship of U50, Emax and Field Utilization Factor 55
with the Gap between the Two Electrodes
4.5.2 U50 (kV) versus Field Utilization Factor (ƞ) 58
4.5.3 Emax(kV/cm) versus field utilization factor (ƞ) 59
XI
CHAPTER 5 GENERAL CONCLUSION & FUTURE WORK 61
5.1 Conclusion 61
5.2 Recommendation 62
REFERENCES 63
XII
LIST OF TABLES
2.1 Tolerance of standard lightning impulse voltage 6
2.2 Summary of previous related works 17
3.1 Description the button for control board 22
4.1 Emax (kV/cm) for rod to plane electrode configuration 43
4.2 Emax (kV/cm) for sphere to plane electrode configuration 43
4.3 Result for sphere to plane electrode configuration 54
4.4 Result for sphere to plane electrode configuration 55
4.5 Result of the field utilization factor for rod to plane and 57
sphere to plane
XIII
LIST OF FIGURES
2.1 Standard lightning impulse voltage waveform 6
2.2 Arrangement for Townsend‟s mechanism 7
2.3 Townsend‟s mechanism process 8
2.4 Streamer mechanism 9
2.5 Formation of secondary avalanches due to photo-ionization 10
2.6 Sparkover 11
2.7 Flashover 11
2.8 Capacitor divider connected in series 12
2.9 Types of electrodes 13
3.1 Method to obtain lightning impulse 20
3.2 Experimental setup lightning impulse voltage 21
3.3 Block diagram for lightning impulse circuit 21
3.4 HV 9103 Control desk 22
3.5 HV 9105 Test transformer 23
3.6 HV 9111 Silicon rectifier 23
3.7 HV 9112 Smoothing capacitor 24
3.8 HV 9152 Impulse voltmeter (digital display) 24
3.9 HV 9130 Low voltage divider 25
3.10 HV 9120 Load capacitor 25
3.11 HV 9124 Insulating Rod 25
3.12 HV 9121 Charging resistor 26
3.13 HV 9122 Wave-front resistor 26
3.14 HV 9123 Wave-tail resistor 26
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3.15 HV 9125 Sphere gap 27
3.16 HV 9126 Drive for sphere gap 27
3.17 HV 9114 Earthing switch, electrically operated 28
3.18 HV 9138 Electrode 28
3.19 HV 9107 Earthing rod 29
3.20 HV 9109 Connecting cup, aluminium 29
3.21 HV 9110 Floor pedestal 30
3.22 HV 9108 Connecting rod, aluminium 30
3.23 HV 9119 Spacer bar 30
3.24 HV 9133 Measuring spark gap 31
3.25 Different type of electrodes 31
3.26 Single stage impulse voltage generator block diagram 32
3.27 Single stage impulse voltage test setup 33
3.28 Method to obtained U50 by using up and down method 34
3.29 Method to obtain the electric field using FEMM 35
3.30 Example to create the model 36
3.31 Boundary condition setting 37
3.32 Mesh 38
3.33 The value of the electric field 39
3.34 The value of the voltage 39
4.1 Lightning impulse waveform for tail time, T2 41
4.2 Lightning impulse waveform for front time, T1 41
4.3 Lightning impulse voltage chopped 42
4.4 Mesh for rod to plane and sphere to plane 44
4.5 Voltage density for rod to plane and sphere to plane 45
4.6 Field intensity for rod to plane and sphere to plane 46
4.7 Contour (Equipotential lines) for rod to plane and 46
sphere to plane
4.8 Vector plot electric field intensity, |E| for rod to plane 47
4.9 Contour & vector for rod to plane and sphere to plane 47
4.10 Zoomed-in of the higher breakdown for rod to plane and 48
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sphere to plane
4.11 The legend of voltage density 49
4.12 The point a-b to get the voltage, V for rod to plane 50
4.13 The point c-d to get the field intensity, |E| for rod to plane 50
4.14 The point a-b to generate the voltage, V graph 51
4.15 The point c-d to get the field intensity, |E| graph 51
4.16 The point a-b to get the voltage, V for sphere to plane 52
4.17 The point c-d to get the field intensity, |E| for sphere to plane52
4.18 The point a-b to generate the voltage, V graph 53
4.19 The point c-d to get the field intensity, |E| graph 53
4.20 The gap (cm) versus U50 (kV) between the rod to plane 53
and sphere to plane
4.21 The gap (cm) versus Emax (kV/cm) between the rod to 56
plane and sphere to plane.
4.22 The gap (cm) versus field utilization factor (ƞ) between 57
the rod to plane and sphere to plane.
4.23 U50 (kV) versus field utilization factor (ƞ) for rod to plane 58
4.24 U50 (kV) versus field utilization factor (ƞ) for sphere 59
to plane
4.25 Emax (kV/cm) versus field utilization factor (ƞ) for rod plane 60
4.26 Emax (kV/cm) versus field utilization factor (ƞ) for sphere 60
to plane
XVI
LIST OF SYMBOLS AND ABBREVIATIONS
HV - High Voltage
T1 - Wave-front
T2 - Wave-tail
V - Voltage
FEMM - Finite Element Method Magnetic
U50 - %50 Breakdown Voltages
Emax - Electric Field
η - Field Utilization Factor
Cs - Smoothing Capacitor
Cb - Load Capacitor
Re - Wave-tail Resistor
Rd - Wave-front Resistor
CHAPTER 1
INTRODUCTION
1.1 Project Background
Lightning is one of the most serious causes of overvoltage. If the power equipment
especially at outdoor substation is not protected the overvoltage will cause burning of
insulation. The lightning also causes damage to buildings, farms, commercial houses
and other. Lightning is a huge spark caused by the electrical discharge taking place
between the clouds within the same cloud and between the clouds and the earth. In
order to prevent failure of power due to lightning, the power equipment must be
protecting [1]. Hence it is absolutely necessary to provide protection against these
travelling surges caused by lightning. Such protective devices are called as lightning
arrestors or surge diverters. They are connected between the line and earth at the
substation. The protective device have many different types which are normally used
likes rod gap arrestor, sphere gap lightning arrestor, horn gap lightning arrestor,
valve type and others.
Rapid growing this demand for use of higher voltage has given the
opportunity to power engineers to develop the insulation of high quality for sustain
high voltage (HV) over a long period. Presently, in high voltage electrical power
system, variety of materials (solid, liquid and gaseous) is used for insulation purpose
to protect the incipient failure in HV power equipment. The insulation design for
such HV power equipment is one of the important challenging tasks to the power
engineers as the HV equipments are involved with huge cost. To protect such
2
equipment different types of conducting electrodes having protective gap are used
widely throughout the world.
The purpose of this project is to protect the electric equipment‟s from the
high voltage with used the different types of gap length electrodes. This project
describes the difference electrodes are used for this purpose among those all
electrodes configuration such as rod to plane and sphere to plane. Transmission and
distribution of electrical energy involves the application of high voltage apparatus
like power transformers, switchgear, overvoltage arrestors, insulators, power cables,
transformers which are exposed to high transient voltages and currents due to
internal and external overvoltages. Before apply this project to real life, the electrode
are tested for reliability with standard impulse voltages and used the difference of the
gap length.
In this study, difference electrodes (rod-plane and sphere-plane) have been
used to generate the lightning impulse voltage experimentally in high voltage
laboratory. The single stage lightning impulse voltage circuit is used by refer the
TERCO manual guide. The standard lightning waveform used for testing is 1.2/50 us
according to the standard IEC 60060-1:2010. The first number 1.2 µs represent the
front time, and the second number, 50 µs is a tail time. Front time is determined at
about 93% just about to reach the peak voltage/current magnitude and the tail time is
measured at 50% off the peak magnitude. To determine the 50% breakdown voltages
(U50) of air, the up and down method have been used in the experiment. Finite
element method magnetic (FEMM) software is one of the most successful methods
for solving electrostatic field problem. In this study, finite element method magnetic
is used for the simulation of the electric field (Emax) between the difference
electrodes.
1.2 Problem statement
In electrical power system, high voltage power equipments are mainly damage with
spark over voltage. These over voltage which may cause by the lightning strokes,
switching action, and determine the safe clearance required for proper insulation
level. To avoid these problems in high voltage power equipment, the air breakdown
voltage with difference electrode configuration rod-plane and sphere-plane are
3
generated by using lightning impulse test. By using the difference electrodes will
determine which electrode has more easily to breakdown.
1.3 Objective Project
The main aim in this project is to investigate the air breakdown characteristic in rod-
plane and sphere-plane electrode configuration under lightning impulse test.
Objective for this project is:
i. To find the air breakdown voltage experimentally for different electrodes
(rod-plane and sphere-plane)
ii. To find the electric field for different electrodes (rod-plane and sphere-
plane) by using finite element method magnetic (FEMM)
iii. To construct relationship between U50 (kV), electric field, Emax (kV/cm),
field utilization factor, ƞ with the gap (cm)
1.4 Project Scope
In order to achieve the objectives of the project, several scopes have been outline.
The following are the scopes of the project.
i. By using difference electrodes rod-plane and sphere-plane in study the air
breakdown characteristic.
ii. Generate the air breakdown voltage by using lightning impulse setup refer to
TERCO manual guide in UTHM high voltage laboratory
iii. The simulation of electric field between the electrodes will be simulating by
using Finite Element Method Magnetic (FEMM) software.
iv. The TERCO‟s single stage voltage impulse generator capable to produced
lightning impulse at maximum 140kV.
v. Gap between electrodes 0.5cm, 1.0cm, 1.5cm, 2.0cm and 2.5cm are used for
measurement of air breakdown voltages and electric field of the high voltage
equipments
vi. Use air = gas @ atmosphere P = 1 bar
vii. Temperature and humidity effect are not considered.
4
1.5 Organization of Thesis
These thesis content five chapters:
Chapter 1: This chapter deals with the basic introduction of the lightning and project
background for this thesis. In this chapter also was describes the problem statement,
objective, and project scope that used to get the result from the experiment and
simulation.
Chapter 2: In this chapter have two parts. For the first part will describes the
lightning impulse voltage, air breakdown mechanism, capacitive divider, electrode
arrangement for measurement of breakdown voltage and finite element method
magnetic software. For the second parts, will summarize the previous related work
associated with this projects.
Chapter 3: This chapter deals with the methodology to generate lightning
impulse waveform during the experimentally. In this chapter are placed the
experiment setup diagram in the high voltage laboratory with the equipment that
used in the experiment setup. From the experiment, the 50% breakdown voltage
(U50) will be produced. After get the U50 value, the procedure to get the simulation of
electric field from finite element method magnetic will be describe in this chapter.
Chapter 4: This chapter will shown the result and analysis obtained from this
experiment and simulation. In this experiment, two different electrodes, rod to plane
and sphere to plane will be tested to compare the U50 and Emax with the change of the
gap between the electrodes. From this chapter also will describe the result for electric
field (Emax) from the simulation. Other than that, from the simulation also will be
generate the graph of voltage and field intensity, |E| for the two different electrodes.
Chapter 5: Finally, in this chapter includes the whole conclusion of the
project work and also some important discussion about the future work of the thesis.
CHAPTER 2
LIGHTNING & AIR BREAKDOWN: A REVIEW
2.1 Introduction
Literature review is a process of collecting, analyzes data and information which are
relevant to this study. The required data and information can be collected through
variable sources such as journals, articles, reference books, online database and
others. This chapter has two main reviews. The first part will focus on the theory
aspects of this project. The second part case study on previously done projects that
related to this project.
2.2 Lightning Impulse Voltage
Lightning impulse voltages is an overvoltage due to lightning are considered as an
external overvoltage and are dependent on the system voltages. An impulse voltage
is a unidirectional voltage which rises more or less rapidly to a maximum value
without appreciable oscillations and then decays, relatively, slowly to zero. The
standard waveform used for testing is 1.2/50 µs. The first number (1.2 µs) represent
the front time T1 and the second number (50 µs) is a tail time T2. In the standard
lightning waveform, T1 is determined at about 93% just about to reach the peak
voltage/current magnitude and T2 is measured at 50% off the peak magnitude [2].
6
Figure 2.1: Standard lightning impulse voltage waveform [3].
During the wave-front of an impulse voltage is the rising portion of the
voltage time characteristic (portion O-A) in the figure 2.1. The duration of the wave-
front is the total time occupied by the impulse voltage while rising from zero to the
peak value. While for the wave-tail, an impulse voltage is the falling portion of the
voltage time characteristic (portion A-B) in the figure 2.1. The time to half value of
the wave-tail of an impulse voltage is the total time occupied by the impulse voltage
in rising to peak value declining there from to half the peak value of the impulse [3].
Table 2.1 shows the tolerance of standard lightning impulse voltage:
Table 2.1: Tolerance of standard lightning impulse voltage
Tolerances Front Time (T1) Tail Time (T2)
Lightning Impulse ± 30% ± 20%
2.3 Air Breakdown Mechanism
The breakdown in air (spark breakdown) is the transition of a non-sustaining
discharge into a self-sustaining discharge. Most of the electrical equipment use air as
the insulating medium. Various phenomena occur in the air medium when a voltage
is applied. When the voltage applied is low, a small currents flow through the air and
it retains its electrical properties. On the other hand if the voltage applied is large
7
enough, then the current increases rapidly and an electrical breakdown occurs. A
strongly conducting spark is formed, creating a short circuit between the two
electrodes. The maximum voltage applied at that moment is called breakdown
voltage [4]. Normally air medium is widely used as an insulating medium in different
electrical power equipment‟s and overhead lines as its breakdown strength is
30kV/cm [5].
2.3.1 Townsend’s Mechanism
Townsend‟s mechanism is based upon:
• Ionization collision in the gas
• ionization collision on the surface of the electrodes
• Photo-ionization
Figure 2.2: Arrangement for Townsend‟s mechanism [7]
Figure 2.2 shows the arrangement for Townsend‟s mechanism. The process
of liberating an electron from a gas molecule with the simultaneous production of a
positive ion is called ionization. In the process of ionization by collision, a free
electron collides with a neutral gas molecule and gives rise to a new electron and a
positive ion. If we consider a low pressure gas column in which an electric field |E| is
applied across two plane parallel electrodes, as shown in Figure 2.2, any electron
starting at the cathode will be accelerated more and more between collisions with
other gas molecules during its travel towards the anode [7].
8
A few of the electrons produced at the cathode by some external means, say
by ultra-violet light falling on the cathode, ionize neutral gas particles producing
positive ions and additional electrons. The additional electrons, then, themselves
make `ionizing collisions' and thus the process repeats itself. This represents an
increase in the electron current, since the number of electrons reaching the anode per
unit time is greater than those liberated at the cathode. In addition, the positive ions
also reach the cathode and on bombardment on the cathode give rise to secondary
electrons. Figure 2.3 shows the Townsend‟s mechanism process and have four stages
during the process to breakdown [7].
Figure 2.3: Townsend‟s mechanism process [2]
Townsend‟s mechanism process has several stages to breakdown occur.
When the region I, at the low voltage, current increased linearly (not steady) with the
voltage up to saturation level (Io) when all electron available are conducting. This Io
can be increased by increasing the number of electrons available, such as by
illuminating the cathodes with UV light (photo-ionization).
When the region II, the current Io, through the gap effectively remains
constant between V1 and V2. For the region III, after V2, the current grows
exponentially. The exponential current to ionization of the gas by electron collision.
As the gap voltage, V increases in the gap, the electric field, E (E=V/d usually
defined in kV/cm or V/cm) increases. Thus the probability of the ionization increases
9
due to the collision of electron with uncharged particle. The rapid increases of
ionization processes in the gap region are called avalanches process.
When the region IV, anode current will be increased very sharply. The
current magnitude could reach infinity and the value is limited only by the external
resistance. Even the current behavior would not change even if the UV light source is
removed and the process is independent. Finally, the gas is to be breakdown [2].
2.3.2 Streamer Theory
Townsend mechanism when applied to breakdown at atmospheric pressure is found
to have certain drawbacks. Firstly, according to the Townsend theory, current growth
occurs as a result of ionisation processes only. But in practice, breakdown voltages
were found to depend on the gas pressure and the geometry of gap and electrodes.
Secondly, the mechanism predicts time lags of the order of 10-5
s, while in actual
practice breakdown is observed to occur at very short time of the order of 10-10
s. the
Townsend mechanism failed to explain the observed phenomena and Streamer
theory is proposed [7].
2.3.2.1 Streamer process
Figure 2.4 shows the streamer mechanism. The streamer mechanism have several
process, there were:
Figure 2.4: Streamer mechanism [7]
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a) Process 1
Ionization process by collision cause negative charges to anode and positive
charge to cathode. This process will create avalanches of electron that must lighter
and higher mobility compare to positive ion. Therefore the electron will be filled the
head and the positive ion occupied the tail.
b) Process 2
Space charges cause by ionization will distort the uniform field. The spherical
volumes concentrate at negative charges at the head and positive charge at the tail.
The field behind and a head of avalanches is increase by the space charge, εr. The
field between the electron and the cloud is reduced. Alpha d increased, field
distortion increases. Alpha is an average number ionization made by one electron per
unit drift in the direction of the field. When alpha, d at critical value, space charges
field is comparable to ε0. This condition created an intense ionization and excitation
of the gas particle in front of the avalanches head. Excited atoms return to normal
immediately. The process will release of photon, which turn generate secondary
electron by the photo ionization process. The generated secondary electrons from the
photo-ionization will generate further auxiliary avalanches as a figure 2.5. Since
photons travel with the speed of light, the process leads to rapid development of
conduction channel across the gap and develop as self-propagating streamer. The
streamer proceeds across the gap and to form a conducting filament of high ionized
gas between electrodes, the gas was breakdown [2].
Figure 2.5: Formation of secondary avalanches due to photo-ionization [7]
11
2.4 Sparkover and Flashover
Disruptive discharge is a failure of insulation under electric stress, in which the
discharge completely bridges the insulation under test, reducing the voltage between
electrodes to practically zero [5]. There two type of the disruptive discharge.
i. Sparkover that occurs in gaseous or liquid dielectric. A spark as a figure 2.6
consists of an arrangement of two conducting electrodes separated by a gap.
Figure 2.6: Spark over [8]
ii. Flashover as a figure 2.7 show that occurs over the surface of a dielectric in a
gaseous or liquid. The voltage at which an electric discharge occurs between
two electrodes that are separated by an insulator; the value depends on
whether the insulator surface is dry or wet.
Figure 2.7: Flashover [8]
12
2.5 Capacitive Divider
A capacitive divider consists of two capacitors in series. It is commonly used to
create a reference voltage, or to get a low voltage signal proportional to the voltage
to be measured, and may also be used as a signal attenuator at low frequencies. For
direct current and relatively low frequencies, a capacitive divider may be sufficiently
accurate if made only of capacitors; where frequency response over a wide range is
required, (such as in an oscilloscope probe), the voltage divider may have capacitive
elements added to allow compensation for load capacitance. In electric power
transmission, a capacitive voltage divider is used for measurement of high voltage.
Figure 2.8 shows the capacitor divider connected in series.
Figure 2.8: Capacitor divider connected in series
Capacitive dividers do not pass DC input. Any leakage current in the
capacitive elements requires use of the generalized expression with two impedances.
By selection of parallel R and C elements in the proper proportions, the same
division ratio can be maintained over a useful range of frequencies. This is the
principle applied in compensated oscilloscope probes to increase measurement
bandwidth. Formula for capacitive divider:
𝑉𝑜𝑢𝑡 =𝐶2
𝐶1+𝐶2 x 𝑉𝑖
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2.6 Electrode Arrangement for Measurement of Breakdown Voltage
There are various types of electrode arrangements and circuits for measurement of
high voltages and currents such as sphere-sphere, sphere-plane, rod-rod, rod-plane
and plane-plane. In this study two different electrodes (rod to plane and sphere to
plane) have been used for the experimental study of the short air gap. The types of
electrodes are vertically aligned as a figure 2.9. The lower plane electrode which is
above the ground plane is grounded where as the top rod and sphere electrode is
connected with HV connector. The used rod electrode has a diameter of 0.75 cm,
sphere and plane electrode same diameter of 2.5cm. The electrode is made of
aluminum material and air is acting as an insulating medium between sphere
electrodes. The upper sphere electrode is connected in the high voltage terminal and
the lower electrode is connected with the ground terminal. With the application of
the high voltage between the sphere electrodes, a non-uniform electric field is
generated as the surfaces of the sphere electrodes are not uniform. The HV electrode
is energized from the 50 Hz transformer with a power rating of 5kVA with a
transformation ratio of 220V/100kV [10].
Figure 2.9: Types of electrodes [10]
2.7 Finite Element Method Magnetic
FEMM is a suite of programs for solving low frequency electromagnetic problems
on two-dimensional planar and axisymmetric domains. The program currently
addresses linear/nonlinear magnetostatic problems, linear/nonlinear time harmonic
Rod to plane
Sphere to plane
14
magnetic problems, linear electrostatic problems, and steady-state heat flow
problems [15]. There are two type for solving using FEMM software such as
electrostatic and magnetostatic.
Finite element method magnetic (FEMM) is widely used in the numerical
solution of electric field problems. In contrast to other numerical methods, FEM is a
very general method and therefore is a versatile tool for solving wide range of
electric field problems. To start with, the whole domain is fictitiously divided into
small areas/volumes called elements. The potential, which is unknown throughout
the problem domain, is approximated in each of these elements in terms of the
potential at their vertices called nodes. As a result of this the potential function will
be unknown only at the nodes.
2.8 Previous Related Work
There have been several studies done before to develop the air breakdown
characteristic rod plane and sphere plane. This project uses a lot of projects that were
done in previous thesis, journal, and papers.
A Srikant & Shekhar Chandra Pradhan [4] has presented that “Simulation of
Air Breakdown Mechanism Using Different Electrodes”. The sphere gaps are
commonly used for measurements of peak values of high voltages and have been
adopted by IEC and IEEE as a calibration device. Generally, the standard sphere
gaps are widely used for protective device in electrical power equipment. The sphere
gaps are filled up with insulating medium such as liquid insulation (transformer oil),
and gas insulation (SF6, N2, CO2, CCl2F2 etc.) in HV power equipment. Normally, air
medium is widely used as an insulating medium in different electrical power
equipment as its breakdown strength is 30kV/cm. Therefore electrical breakdown
characteristic of small air gap under the different applied voltage has its great
significance for the design consideration of various air insulated HV equipment. To
observe the effect on insulation due to breakdown mechanism, the insulation samples
are collected both before and after breakdown voltage test and analysis has been
done with the help of scanning electron microscope (SEM). To simulate the air
breakdown voltage with and without the insulation barrier has been studied
experimentally in high voltage laboratory, a standard diameter of 25 cm spheres are
used for measurement of air breakdown voltages and electric field of the high voltage
15
equipment. The above experiment is conducted at the normal temperature and
pressure. The simulation of such air breakdown voltage has been carried out in the
COMSOL environment.
Paraselli Bheema Sankar [5] has presented that “Measurement of Air
Breakdown Voltage and Electric Field Using Standard Sphere Gap Method”. The
thesis project is to simulate the air breakdown voltage experimentally in high voltage
laboratory. The sphere gaps are filled up with insulating medium such as liquid
insulation (transformer oil), solid insulation (polyester, paper) and gas insulation
(SF6, N2, CO2, CCl2F2 etc.). Normally air medium is widely used as an insulating
medium in different electrical power equipment‟s as its breakdown strength is 30
kV/cm. Therefore electrical breakdown characteristic of small air gap under the
different applied voltage has its great significance for the design consideration of
various air insulated HV equipment. In this work to simulate the air breakdown
voltage experimentally in high voltage laboratory, standard diameter of 25 cm
spheres are used for measurement of air breakdown voltages and electric field of the
high voltage equipment‟s. The above experiment is conducted at the normal
temperature and pressure. Finite element method is also used for finding the electric
field between standard sphere electrodes. The relative air density factor and
maximum electric field are measured in MATLAB environment for different
temperature and pressure. The electric field distribution for sphere gap arrangements
is also calculated with the help of COMSOL.
Yingyao Zhang, Zhiyuan Liu, Yingsan Geng, Lanjun Yang and Jimei Wang
[15] have presented that “Lightning Impulse Voltage Breakdown Characteristics of
Vacuum Interrupters with Contact Gaps 10 to 50 mm”. The objective of this paper is
to understand the standard lightning impulse voltage breakdown characteristics of
vacuum interrupters with contact gaps 10 to 50 mm and how contact parameters
influence the breakdown characteristics. The investigated contact parameters include
contact diameter 75 and 60 mm, contact surface roughness 1.6 and 3.2 μm, and
contact radius of curvature 6 and 2 mm. Therefore we designed for high-voltage
vacuum interrupters in the experiments. The vacuum interrupters were put into a
porcelain envelope with SF6 gas as an external insulation of the vacuum interrupters.
The contact gaps can be adjusted manually up to 50mm. Positive polarity lightning
impulse voltage (1.2/50 us) was applied by an up-and-down method. Experimental
results revealed the breakdown probability distributions followed Weibull
16
distributions when the breakdown voltage saturated within the investigated contact
gaps 10 to 50mm. Within the contact gaps 10 to 50 mm, U50 of vacuum interrupter
with contact radius of curvature 2 mm was higher than that of vacuum interrupter
with contact radius of curvature 6 mm. And U50 of contact roughness 1.6 μm was
close to that of contact roughness 3.2 μm. U50 of the contact diameter 60 mm was
close to that of contact diameter 75 mm. And 50% breakdown voltage U50 depended
on the contact gap, d (10-50 mm) for four interrupters, can be expressed by an
equation U50=kdα, where α is a power exponent; k denotes a coefficient which can be
determined by experiments. And under our experimental condition, power α lay in a
range of 0.6-0.7 for the four vacuum interrupters. The breakdown phenomenon could
be due to the micro-particles.
Subrata Karmakar [19] has presented that “An experimental study of air
breakdown voltage and its effects on solid insulation”. The thesis project is to protect
such equipment‟s different types of conducting electrodes having protective gap are
used widely throughout the world. This project is a work from. The author describes
the standard sphere electrodes are commonly used for this purpose among those all
electrodes configuration. In the author study to simulate the air breakdown voltage
experimentally in high voltage laboratory, standard diameter of 25 cm spheres are
used for measurement of air breakdown voltages at NTP. In addition, the air
breakdown voltage with insulation barrier and without insulation barrier is
investigated inside the high voltage test laboratory. The effects of the breakdown
voltage on paper insulation have been investigated in this work for quality
assessment. The comparison of microstructure before and after the breakdown test
reveals the information about the effects of electrical stress on the insulating paper.
Emel Onal [20] has presented that “Breakdown Characteristics of Gases in
Non-Uniform Fields”. The present paper describes a study of the breakdown voltage-
pressure characteristics of SF6, CO2, N2 and air in rod plane gaps under alternating
voltages. All results are given for 5, 10, 15, 20, 25 mm electrode gap spacing
separately. Experiments were carried out using a rod plane electrode with a rod tip
radius of 1 mm and plane diameter of 75 mm. The experimental results have shown
that the breakdown voltages of SF6 in the practical range of pressure (100-200 kPa)
are always higher than those of other gases. Although at short gaps, the breakdown
strength of SF6 is superior at the pressure range from 100-500 kPa, at 25 mm
electrode gap spacing and 300 kPa the breakdown voltage of air is 7.8% higher than
17
that of SF6. At above pressures of 400 kPa and 15 mm electrode gap spacing, there
exists a critical field where the breakdown voltage of CO2 has a maximum value.
2.8.1 Summary of previous related works
In the process to completing this project, some thesis was used as the references.
Table 2.2 shows the summary of previous related works. From the below summary
of previous related work, this project is very useful to add some contribution air
breakdown research. From the thesis, can be used as a reference and give some idea
of this project. Many situation was tested the electrodes in the air breakdown. The
voltage is affected by the gap length between the two electrodes and if the Emax is
high, the electrode easier to breakdown.
Table 2.2: Summary of previous related works
Title Author Project Description
Simulation of Air
Breakdown Mechanism
Using Different Electrodes
A Srikant & Shekhar
Chandra Pradhan
Study about the effect of breakdown
voltage on different insulation like
lamiflex, leatherwood, plywood, craft
paper, and polyester fiber. Use a
standard diameter of 25 cm spheres for
measurement of air breakdown voltages
and electric field with the help of
COMSOL.
Measurement of air
breakdown voltage and
electric field using standard
sphere gap method
Paraselli Bheema Sankar Simulate the air breakdown voltage
experimentally in high voltage
laboratory, standard diameter of 25cm
sphere is used for measurement of air
breakdown voltage and electric field of
high voltage equipment.
Lightning Impulse Voltage
Breakdown Characteristics
of Vacuum Interrupters
with Contact Gaps 10 to 50
mm
Yingyao Zhang, Zhiyuan
Liu, YingsanGeng, Lanjun
Yang and Jimei Wang
Understand the standard lightning
impulse voltage breakdown
characteristics of vacuum interrupters
with contact gaps 10 to 50 mm and how
contact parameters influence the
breakdown characteristics.
18
An experimental study of
air breakdown voltage and
its effects on solid
insulation
Subrata Karmakar
Simulate the air breakdown voltage
experimentally in high voltage
laboratory, standard diameter of 25 cm
spheres are used for measurement of air
breakdown voltages at NTP.
Breakdown Characteristics
of Gases in Non-Uniform
Fields
Emel Onal Study of the breakdown voltage-
pressure characteristics of SF6, CO2, N2
and air in rod plane gaps under
alternating voltages. All results are
given for 5, 10, 15, 20, 25 mm electrode
gap spacing separately. Experiments
were carried out using a rod plane
electrode with a rod tip radius of 1 mm
and plane diameter of 75 mm.
CHAPTER 3
LIGHTNING IMPULSE TEST PROCEDURE & SIMULATION MODEL
3.1 Introduction
This project deals to generate the air breakdown voltage by using lightning impulse
voltage circuit by using manual guide TERCO in high voltage laboratory. Finite
element method magnetic (FEMM) software is used to simulate the electric field
(Emax) between difference electrodes. While, up and down method is used to get the
50% breakdown voltage (U50) during the experimentally.
3.2 Method for Generation of Lightning Impulse
Figure 3.1 shows the flowchart of the methodology to obtain lightning impulse.
Firstly, need to search and study about the literature review that related from journal,
relevant paper and publication. Then, from the study, plan and select the suitable
method for the projects. This experiment was to obtain the lightning impulse
waveform.
20
Start
Research and
literature review
Planning and select
suitable method for
project
Test project in
HV lab
Lightning
impulse
Save the lightning
waveform using
MS excel
End
No
Yes
Figure 3.1: Method to generate lightning impulse
3.3 Experimental Setup for Measurement of Lightning Impulse Voltage
To conduct the air breakdown test between the difference electrodes, all the
measuring instrument is voltage is refer to the TERCO experiment setup in the high
voltage laboratory. The figure 3.2 shows the experimental setup lightning impulse
voltage to be used in the lightning impulse test. From the experimental setup, are
summarized by using the block diagram. Figure 3.3 shows the block diagram for the
lightning impulse circuit.
21
Figure 3.2: Experimental setup lightning impulse voltage [9]
Figure 3.3: Block diagram for lightning impulse circuit
3.3.1 Equipment in the Generation of Impulse Voltages Circuit
3.3.1.1 HV 9103 Control Desk
The Control Desk (see Figure 3.4) is used to control and operate high voltage
AC/DC/Impulse test equipment. The table 3.1 shows the description the button for
control the board. The desk contains operating and signal elements for the control
circuit of the test equipment for warning and safety. The control desk is made to
22
house the measuring instruments (peak, impulse and DC voltmeters) and also the
trigger device. The HV 9103 is fabricated of steel and stands on four wheels.
Figure 3.4: HV 9103 Control desk [10]
Table 3.1: Description the button for control board
No Description the button
1 Control switch
2 Mains switch
3 ON – Primary
4 ON –Secondary
5 OFF – Secondary
6 OFF – Primary
7 Voltage regulation
8 Measuring sphere gap
3.3.1.2 HV 9105 Test Transformer
Figure 3.5 shows the test transformer with coupling winding for cascade connection
to produce AC high voltage. The transformer consists of three windings with
insulating shell and top and bottom corona free aluminum shielding electrodes.
1 1
5
4 3
2
8 1
7 1
6 1
23
Figure 3.5: HV 9105 Test transformer [10]
3.3.1.3 HV 9111 Silicon Rectifier
Figure 3.6 shows the silicon rectifier. The silicon rectifier is use in impulse voltage
and DC voltage generation. The value of protective resistor is 100 k Ω.
Figure 3.6: HV 9111 Silicon rectifier [10]
3.3.1.4 HV 9112 Smoothing Capacitor
Figure 3.7 shows the smoothing capacitor. Impulse capacitor is use for generation of
the impulse voltages. It can also be used as smoothing capacitor in DC voltage
generation. The value of capacitances is 25nF.
24
Figure 3.7: HV 9112Smoothing capacitor [10]
3.3.1.5 HV 9152 Impulse voltmeter (digital display)
Figure 3.8 shows the impulse voltmeter (digital display). The function of impulse
voltmeter is to measure the impulse voltage peak and can use for connection to the
load capacitor.
Figure 3.8: HV 9152 Impulse voltmeter (digital display) [10]
3.3.1.6 HV 9130 Low Voltage Divider
Figure 3.9 shows the low voltage divider. Low voltage divider is use with
incorporates the low voltage capacitors and the 50 ohm cable adapter. It is plugged in
to the UHF socket of the load capacitor and connects the impulse voltage meter by
means of co-axial cable.
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64
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