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

XIV

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

XV

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]

10

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

13

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.

REFERENCES

[1] Dr.S.L Uppal, “Electrical Power”, Indian Universities, Engineering Colleges ;

Indian Institutes of Technology, State Boards and Institution of Engineers, pp.

1168 - 1198, 1988.

[2] Dr. Mohd Nor Ramdon bin Baharom, „HV Generation & Testing 1.0‟, High

Voltage Engineering, MEK10303, February 2013.

[3] K. Schon, “High Impulse Voltage and Current Measurement Techniques”,

Springer International Publishing Switzerland 2013.

[4] A Srikant & Shekhar Changra Pradhan, “Simulation Of Air Breakdown

Mechanism Using Different Electrodes”, Department of Electrical Engineering

National Institute of Technology, Rourkela Odisha 2011.

[5] Paraselli Bheema Sankar, “Measurement Of Air Breakdown Voltage And

Electric Field Using Standard Sphere Gap Method”, Department of Electrical

Engineering National Institute of Technology, Rourkela, June 2011.

[6] BSI Standard Publication, “High-voltage test techniques”, Part 1: General

definitions and test requirements, BS EN 60060-1:2010.

[7] M.S. Naidu and V. Kamaraju, „High Voltage Engineering‟, published by Tata

McGraw-Hill 3rd edition, 2004

[8] Univ.Prof. Dr. Frank Berger, 2004-2014 (online), available: https://www.tu-

ilmenau.de/en/department-of-electrical-apparatus-andswitchgear/laboratories-

and-equipments/high-voltage-laboratory/, (Accessed: 9 May 2014)

[9] TERCO “High Voltage Experiments”, manual guide at high voltage laboratory,

2014.

[10] TERCO “HV 9000 High Voltage Modular Training Set”, manual guide at high

voltage laboratory, 2014.

64

[11] IEC Publication 60052, “Voltage measurement by means of standard air

gaps”, Geneva, 2002.

[12] A. S. Pillai and R. Hackam, “Electric field and potential distributions for

unequal spheres using symmetric and asymmetric applied voltages”, IEEE

Transactions on Electrical Insulation, Vol. EI-18, No.5, October 1983.

[13] Q. Liu, Z.D. Wang and F. Perrot, “Impulse Breakdown Voltages of Ester-

based Transformer Oils Determined by Using Different Test Methods”,

Annual Report Conference on Electrical Insulation and Dielectric

Phenomena, 2009.

[14] Matthew N. 0. Sadiku “A Simple introduction finite element analysis of

electromagnetic problems”, IEEE transactions on education, vol. 32, no. 2,

may 1989.

[15] Yingyao Zhang, Zhiyuan Liu, Yingsan Geng, Lanjun Yang and Jimei Wang,

“Lightning Impulse Voltage Breakdown Characteristics of Vacuum

Interrupters with Contact Gaps 10 to 50 mm”, IEEE Transactions on

Dielectrics and Electrical Insulation Vol. 18, No. 6; December 2011

[16] David Meeker “FEMM 4.2 Electrostatics Tutorial”, January 25, 2006

[17] O.W. Andersen “Finite element solution of complex potential electric fields”

IEEE Transactions on Paver Apparatus and Systems, Vol. PAS-96, no. 4,

July / August 1977.

[18] David Meeker “Finite Element Method Magnetics Version 4.2 User‟s

Manual”, October 16, 2010

[19] Subrata Karmakar “An Experimental Study of Air Breakdown Voltage and

its Effects on Solid Insulation”, J. Electrical Systems 8-2 (2012): 209-217

[20] Emel Onal “Breakdown Characteristics of Gases in Non-Uniform Fields”

journal of electrical & electronics engineering, 2004

[21] J. B. Nah. Kang, Y. D. Chung, M. C. Ahn, D. K. Bae and T. K. Ko, “Study

on the breakdown voltage characterization of insulation gases for developing

a high voltage superconducting apparatus”, IEEE Transactions on Applied

Superconductivity, Vol.20, No.3, June 2010.

[22] Sayedsaad, September 2011 (online), available: http://www.sayedsaad.com

/High_voltage/insulating gases_4.htm (Accessed: 21 March 2014)

[23] W. Hauschild; and W. Mosch, Statistical Techniques for High-voltage

Engineering. Bristol, England: J.W. Arrowsmith Ltd., 1992.

65

[24] IEC 60060-2:1973, High-voltage test techniques - Part 2: Test procedures.

[25] Athanasios Maglaras and Frangiskos V. Topalis, “Influence of Ground and

Corona Currents on Dielectric Behavior of Small Air Gaps”, IEEE

Transactions on Dielectrics and Electrical Insulation, Vol. 16, No. 1,

February 2009.

[26] P. Ortega, R. T. Waters, A. Haddad, R. Hameed and A. J. Davies “Impulse

Breakdown Voltages of Air Gaps: A New Approach to Atmospheric

Correction Factors Applicable to International Standards”, IEEE Transactions

on Dielectrics and Electrical Insulation, Vol. 14, No. 6, December 2007.

[27] N. K. Kishore, G. S. Punekar, H. S. Y. Shastry, “Spark Over in Sphere Gaps

With Alternating voltages and Perturbed Electric Fields”, annual report

conference on „Electrical Insulation and Dielectric Phenomena‟, 2009.

[28] R. Bartnikas, “Positive Streamer Propagation and Breakdown in Air: The

Influence Of Humidity”, IEEE Transactions on Electrical Insulation, Vol. 15,

No. 2, pp. 416-425, 2008.

[29] Y. Geng, R. Zeng, J. He and B. Wang, “Pre-Breakdown Current

Measurements from High Voltage Electrode of Rod-Plane Gap Under

Lightning Impulse Voltage”, High Voltage Engineering, Vol. 37, No. 4,

2011.