ANALYSIS OF INCIPIENT DISCHARGE ACTIVITY IN

CRYOGENIC INSULATION STRUCTURES ADOPTING UHF

TECHNIQUE

A THESIS

submitted by

LAKSHYA MITTAL

for the award of the degree

of

MASTER OF SCIENCE

(By Research)

DEPARTMENT OF ELECTRICAL ENGINEERING

INDIAN INSTITUTE OF TECHNOLOGY MADRAS

CHENNAI-600 036, INDIA

JUNE 2015

ii

THESIS CERTIFICATE

This is to certify that the thesis entitled ANALYSIS OF INCIPIENT DISCHARGE

ACTIVITY IN CRYOGENIC INSULATION STRUCTURE ADOPTING UHF

TECHNIQUE, submitted by Lakshya Mittal to the Indian Institute of Technology Madras,

Chennai for the award of the degree of Master of Science (by Research) is a bonafide record

of research work carried out by him under my supervision. The contents of this thesis, in full

or in parts have not been submitted to any other Institute or University for the award of any

degree or diploma.

Research Guide

Place: Chennai Prof. R. SARATHI

Date : Department of Electrical Engineering

Indian Institute of Technology Madras

Chennai 600036, India.

iii

ACKNOWLEDGEMENTS

Foremost, I would like to express my sincere gratitude to my guide Prof. R. Sarathi for his

continuous support at every stage of the program. I feel privileged to have been associated

with him. Productive discussions and comments received from him have shown me the right

way. I am extremely indebted to him for his constant encouragement, unbounded enthusiasm

and interest, which have helped me in all the time of research and have inspired me a lot.

I thank Prof. Harishankar Ramachandran, the Head, Department of Electrical

Engineering, IIT Madras, for all his efforts to make the department a nice place to work.

I also take this opportunity to thank my Graduate Test Committee members Prof. Bharath

Bhikkaji and Prof. M. Manivannan, who monitored this work and provided me with

valuable suggestions regarding my research work.

I would like to express my gratitude to Prof. K. Sethupathi for providing me the facility for

getting Liquid Nitrogen and for motivating and encouraging continuously me towards my

research work.

I would never forget the company, encouragement and co-operation I had from my labmates,

Dr. Aravinth, Kushal Oza, Sugunakar, Merin Sheema, Sahitya, Sri Harsha, Animesh, Rashmi,

Rajkamal, Pavan, Venkatratnam, Vignesh, John, Balaji, Arun Kumar and Satish during my

stay in the laboratory.

It would be a great mistake if I do not mention the staff of High Voltage Laboratory, Mrs.

Malarvizhi, Mr. R. Chandrasekaran, Mr. P. Murugan, Mr. B. Athinarayanan and Mr. Divagar

for extending their valuable support from time to time. I wish to thank Mr. Sasidharan and

other staff members of Low Temperature physics lab for maintaining unbroken supply of

liquid nitrogen for my experiments.

Especially, no words can express my deep sense of gratitude towards my family for their

love, moral encouragement, support and sacrifice which enabled me to complete this course.

I thank all my batch mates who have attended various courses along with me. Finally, I

would like to thank all whose direct and indirect support helped me complete my thesis.

Lakshya Mittal

iv

ABSTRACT

KEYWORDS : liquid nitrogen, treeing, XLPE, cable insulation,barrier effect, Zero span

mode.

Invention of high temperature superconductor has attracted the power equipment

manufacturers and the researchers to design and develop super-conducting power apparatus with

liquid nitrogen (LN2) as insulant as well as a coolant. Incipient discharges in cryogenic power

apparatus can cause catastrophic failure of insulation system. To identify such discharges, it

requires more sophisticated diagnostic measuring techniques. In recent times, UHF technique has

gained importance to identify the partial discharge activity in GIS and in oil filled transformer.

A methodical experimental study was carried out to characterize the UHF signal generated

due to corona/surface discharges (with and without barrier).UHF signals generated at the point of

corona inception and at near point of breakdown were characterized by using spectrum analyser.

The variation in corona inception voltage under harmonic AC voltages with different THDs was

studied. Phase resolved partial discharge studies were carried out to understand the phase at

which discharges occurs when the barrier is present in the electrode gap. Also such analysis were

extended for harmonic voltages. The results form an important contribution by the author and

forms first part of the present study.

The presence of conducting or non conducting defect in solid insulation or formation of any

void in the bulk volume of insulation, under operating stress incipient discharges can occur,

forming treeing, a pre-breakdown structure. Under highly polluted supply voltage, the impact of

tree growth especially under cryogenic temperature, the results are scanty. To identify the

formation of any defects during laying of high voltage cables, AC and DC tests were carried out.

The fundamental issue with the DC test is the formation of space charge in cable insulation and

very low frequency test is identified as an alternative. It is essential to understand the electrical

tree formation in XLPE polymer insulation at low temperature under VLF voltages as well the

rate of failure of XLPE polymer insulation with trees and its growth under VLF voltages. In

addition, it is essential to understand the phase at which the partial discharge occurs under

harmonic and VLF voltages. The frequency contents of UHF signal formed during tree growth

under VLF and harmonic voltages ought to be understood.

Having known all these factors, the author has carried out a methodical experimental study to

understand the following important aspects.

v

(i) To investigate electrical tree growth in XLPE cable insulation at low temperatures under

harmonic voltages with different THD and with VLF voltages. For the purpose of comparison,

treeing studies were carried out with 50 Hz triangular voltage wave. (ii) The shape of electrical

trees formed was categorized by measuring the UHF signal radiated during inception and

propagation. (iii) Failure analysis of cable insulation due to treeing carried out through Weibull

distribution studies (iv) Phase resolved partial discharge studies were carried out (using UHF

signal measured) to understand the phase at which discharges occurs under different voltage

profiles. The analysis of tree growth phenomena in XLPE cable insulation under harmonic AC

voltage forms second part of the study.

To understand treeing phenomena, experimental study is a time consuming and laborious

process. Thus, the author has made an attempt to understand the treeing phenomena through

modelling studies. The author has used COMSOL, to model and compute the electric field

associated with treeing phenomena manifested in the nano composite material. The results of

the study form the third part of the present study.

vi

TABLE OF CONTENTS

Title Page No.

ACKNOWLEDGEMENTS ……………………………………….. iii

ABSTRACT………………………………………………………… iv

TABLE OF CONTENTS ………………………………………….. vi

LIST OF TABLES..………………………………………………... ix

LIST OF FIGURES...……………………………………………… x

ABBREVIATIONS ………………………………………………... xii

CHAPTER 1 INTRODUCTION

1.1 General ……………………………………………………………… 1

1.2 Literature Survey ……………………………………………………. 3

1.3 Problem Formulation ……………………………………………...... 10

1.4 Organisation of The Thesis ...……..………………………………… 11

CHAPTER 2 EXPERIMENTAL AND THEORETICAL STUDIES

2.1 General ……………………………………………………………… 13

2.2 Methods Adopted For Partial Discharge Detection In Cryogenic

Power Apparatus ……………………………………………………. 13

2.3 Experimental Setup …………………………………………………. 14

2.4 Sample Preparation of Barrier Insulation ...…………………………. 18

2.5 Analysis of Discharges Using Ternary Plot ………….……………... 20

2.6 Theoretical Analysis ...………………………………….…………… 20

CHAPTER 3 RESULTS AND DISCUSSION

3.1 General ……………………………….……………………………... 23

3.2 Analysis of Corona Discharge Activity in Liquid Nitrogen (With

and Without barrier in The Electrode Gap) Under AC Voltages

Adopting UHF Technique …………..………………………………. 23

3.2.1 Corona Inception Voltage (CIV) and The Breakdown

Voltage of Electrode Gap with Barrier Insulation …………. 23

vii

Table of Contents (Contd.) Page No.

3.2.2 Analysis of Current Pulse Generated Due to Corona

Discharge ……………………..……………………………. 26

3.2.3 Characteristics of UHF Signals Generated Due to Corona

Discharges ………………..………………………………… 28

3.2.4 Corona Generated UHF Sensor Signal Analysis Using

Spectrum Analyser ……………………………...………….. 29

3.2.5 Analysis of Phase Resolved Partial Discharges

(PRPD)……………………………………………………… 30

3.3 Analysis of Corona Discharge Activity in Liquid Nitrogen Under

Harmonic AC Voltages ……………..………………………………. 32

3.3.1 Variation in Corona Inception Voltage in Liquid Nitrogen

Under Harmonic AC Voltages ……………………………... 32

3.3.2 Analysis of injected current pulse due to corona discharge

activity in presence of harmonic AC voltages ……………... 33

3.3.3 Analysis of UHF Signal Generated Due to Corona

Discharge in Presence of Harmonic AC Voltages …………. 33

3.3.4 Classification of discharges Using Ternary Diagram ……… 34

3.3.5 Analysis of Phase Resolved Partial Discharge (PRPD) …… 35

3.4 Electrical Treeing in Cable Insulation at Liquid Nitrogen

Temperature ………………..……………………………………….. 36

3.4.1 Characterization of Trees Formed Under Harmonic AC

Voltage ……………………………………………………... 36

3.4.2 Life Estimation of XLPE Cable Insulation ...………………. 39

3.4.3 Characterization of UHF Signal ……...……………………. 41

3.4.4 Phase Resolved Partial Discharge Analysis During Tree

Growth Using UHF Signals ……..…………………………. 41

3.5 Modelling of Electrical Trees …….…………………………………. 43

3.5.1 Analysis of Two Dimensional Electric Field Distributions in

insulation structure during tree growth …………………….. 44

CHAPTER 4 CONCLUSIONS

4.1 General ………………………………………...……………………. 48

4.2 Scope of Future Work ………………………………………………. 50

viii

Table of Contents (Contd.) Page No.

REFERENCES ………………………………………………………………………... 52

LIST OF PUBLICATIONS BASED ON THE THESIS ………................................ 63

ix

LIST OF TABLES

Table Title Page No.

2.1 Properties of Clay Particle ………………………………………….. 19

2.2 Electrical Conductivity of materials used in study ………………….. 22

3.1 Variation in Corona Inception Voltage (CIV), Peak to Peak Voltage,

Rise Time of Current Pulse for Positive and Negative Polarity Under

Harmonic AC Voltages With Different THDs in Liquid Nitrogen

Insulation ……………………………………………………………. 32

3.1 The Weibull Distribution Parameter Characteristics Life (α) and

Shape Factor (β) and Peak Factor of Applied AC Voltage ………… 40

x

LIST OF FIGURES

Figure Title Page No.

2.1 Different Methods For Diagnosing PD Measurement ……………… 14

2.2 Typical Photograph of The Experimental Setup ……………………. 16

2.3 Photograph of Cryostat with Test Electrode System ………….……. 16

2.4 Frequency Response of The UHF Sensor ………………..…………. 17

2.5 Geometry of The Model Used For Tree Simulation ……………...... 21

3.1 Variation In Corona Inception Voltage of Liquid Nitrogen Filled

Electrode Gap With Barrier At Different Position ………...………. 24

3.2 Variation In Breakdown Voltage of Liquid Nitrogen Filled

Electrode Gap With Barrier At Different Position .............................. 25

3.3 Typical Photograph Of Breakdown Spot In The Barrier Insulation ... 26

3.4 Typical Current Pulse Injected Due To Corona Discharge (With

Barrier) (a) In Positive Half Cycle (b) In Negative Half Cycle (c)

Typical UHF Signal Formed During Current Injection ...….....…….. 26

3.5 Typical Corona Discharge Generated UHF Sensor Output Signal In

Liquid Nitrogen Insulation (a) And Its FFT (b) (I) Without Barrier

(II) With Barrier …………………………………………………….. 27

3.6 Typical UHF Sensor Signal Measured In Sequence Mode (a) On

Corona Inception (b) At 20 kV…………………………...…………. 28

3.7 Variation In Peak To Peak Voltage Of UHF Sensor Signal

Generated By Corona Discharge (With Barrier) In Liquid Nitrogen

(A) With Barrier At 20 kV (B) With Barrier At Inception (C) Clean

Electrode Gap At 20 kV (D) Clean Electrode Gap At Inception …… 29

3.8 Typical Pulse Sequence Of UHF Sensor Signal Measured Using

Spectrum Analyzer At 1 GHz In Zero-Span Mode (a) At Inception

Measured (b) At Higher Voltage (c) Inception To Breakdown ..…… 30

3.9 Typical PRPD Pattern Formed Due To Corona Discharge (I)

Without Barrier (II) With Barrier (a) At Inception (b) At Higher

Voltage ……………………………………………………………… 31

3.10 Typical current pulse injected due to corona discharge under

harmonic AC voltages in liquid nitrogen insulation (a) In positive

half cycle (b) In negative half cycle (c) Typical UHF signal formed

during current injection ……………………………………………... 33

3.11 FFT analysis of UHF signal generated due to corona discharge

under harmonic AC voltages with different THDs in liquid nitrogen

insulation ……………………………………………………………. 34

xi

List of Figures (Contd.) Page No.

3.12 Typical Ternary diagram obtained due to corona discharge under

harmonic AC voltages with different THDs in liquid nitrogen

insulation ……………………………………………………………. 34

3.13 Typical PRPD pattern formed due to corona discharge (i) 50 Hz (ii)

H=3,THD=4% (iii) H=5,THD=4% (iv) H=7,THD=4% (v)

H=7,THD=40% (a) At inception (b) At higher voltage …………….. 35

3.14 Electrical trees under harmonic AC voltage profiles (a) 50 Hz (b) 2f

(c) 3f (d) 4f (e) 5f (f) 6f (g) 7f (h) 2f (i) 3f (j) 6f (k) 7f (l) 1 Hz (b) to

(g) with 4 % THD and (h) to (k) 40 % THD …...……………………

37

3.15 Weibull Distribution Plot For The Failure Times Of XLPE Cable

Insulation Due to Electrical Trees Under Harmonic AC Voltages (a)

4% THD (b) 40% THD ..............................………………………….

39

3.16 Typical UHF Signal Generated During Tree Growth (a) And Its

Corresponding FFT Analysis (b) (c) FFT Analysis of UHF Signal

Generated During Tree Growth Under Harmonic AC Voltage With

4%THD and (d) 40%THD ………………………..………………… 41

3.17 Phase Resolved Partial Discharge Analysis Using Spectrum

Analyser By Operating In Zero Span Mode (a) 50 Hz (b) 1 Hz …..... 42

3.18 Phase Resolved Partial Discharge Analysis Using Spectrum

Analyser By Operating In Zero Span Mode (a) 2f (b) 3f (c) 4f (d) 5f

(e) 6f (f) 7f (i) 4% THD (ii) 40% THD ............................................... 42

3.19 Distribution of electric field at the time of tree initiation with nano

particles of different conductivity (a) 101 (b) 10

3 (c) 10

5 S/m ……… 44

3.20 Distribution of electric field during tree propagation after (a) 100 (b)

300 (c) 500 and (d) 700 iterations (i) Without nanoparticles (ii) 10

nm (iii) 20 nm (iv) 30 nm (v) 40 nm (vi) 50 nm diameter of

Nanoparticles ………………………………………………………... 45

3.21 Typical Tree Generated In The Presence Of Nano Particles Of

Different Size (a) Without Particle (b) 10 nm (c) 20 nm (d) 30 nm

(e) 40 nm (f) 50 nm Diameter ............………………………………. 46

3.22 Variation In The Number Of Iterations Corresponding With The

Change In Diameter Of Nano Particles ………………………........... 46

3.23 Variation in the velocity of tree propagation with the length of tree .. 47

xii

ABBREVIATIONS

AC Alternating Current

AE Acoustic Emission

CIV Corona Inception Voltage

DC Direct Current

EPR Ethylene Propylene Rubber

FFT Fast Fourier Transform

FRP Fiber Reinforced Polymer

GIS Gas Insulated System

GTEM Gigahertz Transverse Electro Magnetic

HFCT High Frequency Current Transformer

HTS High Temperature Superconductor

HV High Voltage

IEC International Electrotechnical Commission

LN2 Liquid Nitrogen

MMT Montmorillonite

PD Partial Discharge

PF Peak Factor

PDIV Partial Discharge Inception Voltage

PRPD Phase Resolved Partial Discharge

RF Radio Frequency

RMS Root Mean Square

SA Spectrum Analyser

THD Total Harmonic Distortion

tr Rise Time

UHF Ultra High Frequency

VLF Very Low Frequency

Vpp Peak to Peak Voltage

Wt% Weight Percentage

XLPE Cross Linked Polyethylene

1

CHAPTER 1

INTRODUCTION

1.1 GENERAL

The need for bulk power transmission at higher voltages is well understood and the world-over

researchers are now trying to enhance the reliability of transmission and distribution system by

enhancing the reliability of operation of power apparatus by adopting proper maintenance and

condition monitoring strategy, minimizing the power loss in the transmission and distribution system

with optimal power flow in lines etc (M. Hanai et al., 2008). The invention of high temperature

superconductor has attracted the power equipment manufacturers and the researchers to design and

develop super-conducting power apparatus with liquid nitrogen (LN2) as insulant and as a coolant

(Garlick, 1997; Gerhold, 2002; Dai et al., 2007).

The advantages of using liquid nitrogen as insulant in cryogenic power apparatus are the

following.

1. Liquid nitrogen has a higher heat flux of 16 W/cm compared to 0.8 W/cm for liquid

helium.

2. Evaporation heat of liquid nitrogen is 160 KJ/L compared to 2.6 KJ/L for liquid helium.

Therefore, the liquid nitrogen cooling capacity is much better than that of liquid helium.

3. Liquid nitrogen could be handled easily, simpler and cheaper insulation, and low cooling

power needed for refrigeration equipment.

4. The liquid nitrogen is chemically inert and no irreversible compound/ hazardous by-

products forms within the insulation system due to any ageing/discharges.

The major issues in liquid nitrogen filled cryogenic insulation during operation, includes.

Incipient discharge activity due to corona and surface discharges in liquid nitrogen.

Aging of dielectric materials.

Low temperature breakdown of the materials.

Cracks formed in solid insulators due to cool-down and warm-up situations.

Bubbling effects due to the quench of superconductors.

One of the main forms of electrical degradation of insulating materials subjected to high

electrical fields, at normal temperatures, is incipient discharge. Similar phenomenon can occur at

2

cryogenic temperatures (Blaz et al., 2011; Koo et al., 2010). Incipient discharges can occur in

liquids, generating gas-filled bubbles at interfaces between solid and fluid insulations to produce

conducting paths forming tracking (Kraehenbuel et al., 1994). Considerable research work were

carried out to understand incipient discharge activity in liquid nitrogen (as it has low heat of

vaporization and low thermal capacity), in large gaps, under AC voltages (Frayssines et al., 2002).

During corona discharge in liquid nitrogen under AC voltage, bush type discharges occur in negative

half cycle and filamentary type discharges in the positive half cycle (Swaffield et al., 2008). Most

of the studies were carried out under AC voltages. In recent times, harmonics in supply

voltage formed due to non-linear loads in power system networks has high impact on

insulating materials in power apparatus.

Polymers have been used extensively in electrical insulation for cryogenic equipment

because of their high mechanical strength with excellent electrical insulation characteristics (F.

Krahenbuhl et al., 1994; M. Sosnowslu et al., 1985; M. Kosaki et al., 1996). This solid

insulation is used as barrier to enhance the breakdown strength of the medium. One of the

fundamental problems with solid insulating material is the formation of electrical trees from

defect site (conducting/non-conducting defect). Treeing is a carbonaceous process which forms

due to partial discharge activity. This partial discharge activity occurs due to enhancement of

local electric field near the defect site in the bulk volume of insulating material, under normal

operating voltage, which can drastically reduce the useful life of the insulating material (Len. A.

et al.,1992). Treeing studies in XLPE insulating material under AC and DC voltage are well

known in the literature (Fuji et al., 1991; A. Minoda et al., 1997). With the increase in harmonic

pollution (odd and even harmonics with different THD) in the power system network, the

database on tree inception and the dynamics is scanty.

The existing partial discharge (PD) diagnostic methods in cryogenic power equipments

include conventional PD measurement process adopting IEC 60 270, acoustic emission

technique, High Frequency Current Transformer (HFCT) and by optical emission technique.

Recently, it is observed that adoption of UHF technique is a reliable method for identification of

PD in power apparatus, as the inherent characteristics of the process is free from any external

interference. Use of UHF technique for identification of incipient discharges in cryogenic power

apparatus is at infancy stage.

Understanding treeing phenomena in XLPE cable insulation by experimental study is time

consuming and laborious process. It is essential to understand the treeing process by modelling

process through incorporation of various factors that influence the tree growth process.

3

1.2 LITERATURE SURVEY

In recent times, power transmission at high AC/DC voltages has acquired considerable

importance. Partial discharge measurement techniques were developed for the diagnosis of

incipient discharges in conventional power equipments, which operates at high voltages. In the

case of HTS power equipment, there were no evident testing techniques that have been reported

and related works are in their early stages of research and development around the world

(Grabovickic et al., 2005; Hazeyama et al., 2002; James et al., 2007).

LN2 has high dielectric strength but its performance is affected easily with bubble formation,

foreign conducting/non conducting particles and electrode surface conditions (Gerhold, 2002a;

Okubo et al., 1996).

The presence of even a minor defect in the insulation structure, conducting or non-

conducting defect, under normal operating voltages can cause local field enhancement, initiating

partial discharge activity (Hara et al., 2002; Krins et al., 1996).

Partial discharge due to contaminant is one of the major problems in cryogenic insulation.

Also no partial discharge (PD) is allowed in the HTS apparatus since organic insulation

materials are used in its insulation systems and they are easily degraded by PDs (Hara et al.,

2004). Hence it is essential to monitor the incipient discharges generated due to various factors

which include partial discharge due to particle movement, corona discharge and by surface

discharge activity.

It is well known that insertion of a thin insulating barrier in liquid insulation electrode gaps

significantly increases the dielectric strength of the medium (Zougachi et al., 1998; Hyoungku

Kang et al., 2011). However, influence of barrier on the performance of liquid nitrogen

insulation is not fully understood yet.

Y. Yamano et al. (1990) studied the suppression of the negative surface leaders at AC

voltage application in air by setting up the insulating barrier on a test insulator. They have

observed that the barrier can suppress the negative surface leaders and thus improving the AC

flashover strength, when the barrier distance is shorter than the critical value. They have

concluded that the suppression of the negative surface leader results from the interruption of the

propagation of the negative partial discharges by barrier and the reliability of the insulation will

be high when the barrier distance is shorter than the critical value.

F. Topalis et al. and M. Danikas (2005) studied breakdown in air gaps with solid insulating

barrier under impulse voltages and concluded that the best dielectric performance of the

4

arrangement is achieved when the barrier is placed near the high voltage point but never very

close to it. They have suggested that the optimum position to place barrier in electrode gap is

equal to 20% of the gap length. The performance of the insulation is degraded when barrier is

placed near ground points. Furthermore, the material of the barrier is not as important as the

position of barrier.

Jong-man Joung et al. (2003) have studied the influence of barrier and AC surface flashover

strength in liquid nitrogen insulation and have concluded that the breakdown strength of the

liquid nitrogen medium has enhanced drastically.

Paola Caracino et al. (2002) explained in detail that barrier structure would be applied in the

main gap and coil end insulation to improve the Partial Discharge inception and Breakdown

characteristics in liquid nitrogen. The barrier materials selection will be a crucial design

parameter under liquid nitrogen condition.

L.H. Truong et al. (2013a) characterized partial discharges in LN2 under AC voltages by the

means of phase resolved analysis. They have suggested that discharges in a liquid nitrogen/solid

insulation system occur primarily in the first and third quadrant of the applied voltage waveform

due to charge storage effects. Under higher electric stress, Partial Discharge (PD) pulses are also

observed before zero crossing of the waveform. They have observed different PD patterns for

different solid dielectric barriers, indicating the effects of the solid/liquid interface on the

behavior of discharges in point-plane composite systems at cryogenic temperature.

A. Denat et al. (2011) studied conduction and breakdown phenomenon in dielectric liquid

and have concluded that the streamers in liquid occur when the maximum electric field at the

needle tip reaches a certain threshold value, which allows more electrons tunneling into the

liquid and initiates discharges.

Hanaoka et al. (1993) carried out experimental study to understand the influence of ice layer

on pre breakdown current in liquid nitrogen insulation. They have concluded that the presence of

ice at the tip of needle can significantly enhance the corona inception voltage and breakdown

voltage.

Tanaka (1977) studied internal discharges in liquid nitrogen filled cavity under AC voltage.

He observed that with the increase in voltage, incipient discharge occurs. He also observed

luminescence after discharges in liquid nitrogen.

5

J Fleszynski et al. (1980) studied surface discharge characteristics of solid insulating

material in liquid nitrogen and have concluded that rate of discharge propagation is high under

positive polarity compared with that under negative polarity.

Swaffield et al. (2008) studied discharge activities in liquid nitrogen with needle-plane

configuration and have observed filamentary type and bush type discharges in the positive and

negative half cycle of the AC voltage. The multiple discharges of lower magnitudes have also

been observed which occur during negative half cycle.

Takahashi and Ohtsuka (1975) studied corona discharge activity in liquid nitrogen and have

concluded that the corona inception voltage is less with the point negative than positive ones. As

the gap spacing decreases, height and repetition rate of pulses with the point negative increases.

The corona is accompanied by bubbling similar to a spray jet from the point tip. The polarity of

applied voltage doesn’t affect the bubbling phenomenon.

Fleszynski et al. (1979) carried out discharge studies in liquid nitrogen and have observed

different phenomenon under positive and negative polarity of the AC voltage. They have

observed that the channel of pre-breakdown discharges is greatly expanded when the point

electrode is negative. They have concluded that the development of discharge depend on the

density and distribution of the space charge. The breakdown mechanism of liquid nitrogen is a

typical leader mechanism when the polarity of point electrode is positive. The highly ionized

leader carries the point electrode potential deep into the inter electrode space, thus reducing the

value of breakdown voltage.

Goshima et al. (1995) carried out experimental studies to understand the breakdown

characteristics of liquid nitrogen under positive DC and AC voltages and have concluded that the

roughness of the electrode surface has less influence on the breakdown voltage and the electron

emission from rough finished cathode affects the negative breakdown voltage.

The use of non-linear and time varying loads, which have non-linear voltage-current

characteristics, can cause harmonic pollution in the power system network causing distortion to

the AC supply voltage profile (T. H. Ortmeyer et al., 1985; V. E. Wagner et al. 1993).

IEEE 519-1992 (1992) standards indicate the limit of voltage harmonics as 1.5% THD in HV

networks and 2.5 – 5% THDs in medium voltage networks.

Dionise et al. (2010) carried out harmonic analysis in power system network connected with

a mid-frequency welder load and have observed a voltage THD as high as 60.6%.

6

M. Sali Taci et al. (2004) have observed that the non sinusoidal supply voltage to

transformers can enhance the THD of supply voltage to be as high as 44%. They have also

visualized the stress due to harmonic voltages.

Considerable research work has been carried out to understand corona formation in

liquid nitrogen insulation under AC voltages. However, the understanding of the corona

formation in liquid nitrogen under harmonic ac voltages with different THDs, are scanty.

Author has made an attempt to understand the influence of harmonic AC voltages, with different

THDs on the performance of liquid nitrogen insulation adopting the UHF technique, perhaps for

the first time and it is one of the important contributions by the author.

M. Florkowski et al. (2013) investigated the influence of harmonic on partial discharge

behavior. They have observed that compositions of various harmonic superimposed on the

fundamental sinusoidal waveform can have a significant impact on PD intensity and maximum

charge. In consequence, partial discharge pattern, phase, and amplitude distribution are distorted

and influence the derived statistical parameters as well. They have underlined the importance of

the awareness of spectral purity of the applied voltage, especially when the PD measurements

are associated with acceptability criteria and compatibility with the standards.

Polymers have been used extensively in electrical insulation for cryogenic equipments

because of their high mechanical strength with excellent electrical insulation characteristics (F.

Krahenbuhl et al., 1994; M. Kosaki et al., 1996).

M. Sosnowslu et al. (1985) described the development of crosslinked polyethylene (XLPE)

insulation structure for cryogenic high voltage cable. They have observed that XLPE structure

have good electrical and mechanical properties at cryogenic temperatures. They have concluded

that the tree propagation depends upon applied voltage magnitude, conductivity of the discharge

path, and the local electric condition at the point of inception of electrical tree.

Defect free manufacturing of polymer material is one of the major challenges. One of the

fundamental problems with solid insulating material is the formation of electrical trees from the

defect site. Electrical treeing is a carbonaceous process which forms due to partial discharge

activity. Under normal operating voltage, this partial discharge activity occur due to

enhancement of local electric field near the defect in the bulk volume of insulating material,

which can drastically reduce the useful life of the insulating material (Len. A. Dissado et al.,

1992).

7

M. Kosaki et al. (1977) studied treeing of polythene at cryogenic temperature and concluded

that tree inception voltage at low temperature is much higher and nearly equal to the tree

inception under lightning impulse voltage. It is also observed that XLPE cables at low

temperature have higher AC and impulse breakdown strength.

H. Kawamura et al. (1998) studied the influence of space charge on dc tree propagation

using many insulating polymer materials under divergent fields and have concluded that the dc

treeing breakdown phenomena are strongly affected by the space charges.

M. Fujii et al. (1991) investigated fractal dimension of dc trees in poly-methyl-metha

acrylate for both polarities of the needle electrode using the self-correlation function. They have

concluded that the fractal established range is proportional to the tree length for both positive

and negative polarities of the applied voltage. The fractal dimension increases with increasing

inception voltage and saturates at 1.5 to 1.6 for positive polarity. But for negative polarity, the

fractal dimension is nearly constant, x 1.5, as a function of voltage.

A. Minoda et al. (1997) studied DC short-circuit treeing phenomenon and space charge

effect in EPR at cryogenic temperature. They have observed that the short-circuit tree initiation

voltage of EPR at cryogenic temperature is much higher than that at room temperature.

Furthermore, space charge is less problematic at cryogenic temperature than at room temperature

because the charge injection is very limited for DC electrical insulation at cryogenic

temperature.

Treeing studies in XLPE insulating material under AC and DC voltage are well known. With

the increase in harmonic pollution (odd and even harmonics with different total harmonic

distortion) in the power system network, the database of tree inception and dynamics is scanty.

Kanao et. Al. (2009) have observed presence of even harmonics in power system network.

Thus this harmonic AC voltage is visualized by any system connected with such network and it

is essential to know its impact to life of cable insulation due to electrical treeing.

Bozzo et al. (1997) subjected insulating materials to distorted ac voltages using 3rd, 5th and

7th orders separately and studied their degradation and life performance. They have confirmed

that the voltage peak of the waveform and the likelihood of partial discharge inception may be

increased due to composite waveform component.

Montanari et al. (1999a) studied intrinsic ageing of insulating materials due to supply voltage

distortion. It is shown that the ageing is accelerated by peak factor of the voltage waveform but

rms value of voltage and waveform slope also have statistical significance.

8

Montanari et al. (1999b) studied impact of voltage distortion on ageing acceleration in

insulation system. They have concluded that the voltage peak is the main factor for accelerating

the ageing process. Increase in RMS voltage and waveform steepness provides a significant

contribution to ageing acceleration.

Fabiani et al. (2001) studied acceleration in ageing of insulation material under partial

discharge activity due to voltage distortion. They have concluded that the prevailing factor for

the accelerating degradation of insulation systems is the peak of voltage waveform.

Mazzanti et.al (2006) studied the effects of distorted voltage on life of the cable. They have

concluded that both peak factor and shape of waveform play a key role on life reduction of the

cable due to harmonic voltages.

Florkowska et al. (2007) studied the influence of harmonics on mechanism of partial

discharge and ageing processes in epoxy resin insulation. They have concluded that the number,

magnitude and phase location of individual discharges could be influenced due to distortion in

test voltage.

Bahadoorsingh and Rowland (2009) modeled partial discharges due to electrical treeing in

the presence of harmonics and concluded that the power quality can significantly influence the

phase resolved patterns produced by an electrical tree.

Bahadoorsingh and Rowland (2010) have studied partial discharge activities in epoxy resin

during tree growth and reported that the partial discharge activity is enhanced by the presence of

harmonics. Furthermore, the presence of 7th harmonic can be more detrimental to the cable

insulation, indicating that ageing process is accelerated by harmonics in supply voltage.

G. Lupo et al. (2000) carried out experimental and theoretical studies for classification of PD

in a cryogenic cable termination and have observed that under 50 Hz AC voltage, the discharge

occurs near the zero crossing as it is observed at room temperature.

After laying the high voltage cable, to identify the formation of any defects during laying AC

and DC test are carried out. Oyegoke et al. (2003) carried out experimental studies to understand

selectivity of damped AC (DAC) and VLF voltages in after-laying tests of extruded MV cable.

They have indicated that the detection of defects with DAC or VLF voltage can be done at a

lower voltage than with DC. DAC voltage is sensitive in detecting defects that cause a

breakdown due to void discharge, while VLF is sensitive in detecting defects that cause

breakdown directly led by inception of electrical trees.

9

The fundamental problem with the DC test is the formation of space charge in cable

insulation and very low frequency test is identified as an alternative (E. Ildstad et al., 2013).

J. von Neuman (1966) has carried out simulation of electrical tree growth, considering both

the barrier effect and the homocharge influence, and has suggested that both factors are vital in

preventing trees. The simulation is done with the aid of Cellular Automata.

D. Pista et al. (2010) have conducted simulation studies to understand electrical treeing

propagation in nanocomposites using cellular automata. They have concluded that the nanofillers

act as barriers to the propagation of electrical tree. Tree propagation is delayed because electrical

tree is forced to propagate through the interface between the polymer and nanofiller as the

nanofillers are more resistant to partial discharge activity than the polymer. Furthermore, during

interaction with nanofillers, electrical tree loses part of its energy thereby not propagating

further.

R.C.Smith et al. (2008) have presented the hypothesis for the mechanism leading to

improved properties of polymer nanodielectrics. They have suggested that inclusion of

nanoparticles in polymer provides countless scattering obstacles and trap sites in the charge

carriers’ paths, effectively reducing carrier mobility and thus carrier energy. As the result,

voltage required for further charge injection increases due to buildup of homocharge at the

electrodes.

M. Kozako et al. (2005) have conducted experimental study to investigate mechanical,

thermal and electrical characteristics of epoxy/alumina nanocomposites in comparison with

those of an expoxy resin without filler. They concluded that the nanocomposite specimens are

more resistant to partial discharge than the specimen without nanofillers and the electrical

breakdown time is also longer in case of nanocomposites.

G.E.Vardakis and M.G.Danikas (2004) have simulated treeing phenomenon in the case of a

small insulating spherical particle inside a solid insulating material and have concluded that the

presence of such particles inside the dielectric material may be a significant factor for

propagation of electrical tree but not for its initiation.

R. Sarathi and A. Vijaya Saradhi (1999) have modelled electrical tree in a laminated

dielectric structure, considering the influence of permittivity of material on tree growth. They

have concluded that charges accumulate on interface structure and thereby retarding the growth

of electrical tree.

10

1.3 PROBLEM FORMULATION

As discussed earlier, incipient discharges in cryogenic power apparatus can cause

catastrophic failure of insulation system. To identify such discharges, it requires more

sophisticated diagnostic measuring techniques. In recent times, UHF technique has gained

importance to identify the partial discharge activity in GIS and in oil filled transformer. The

author has made an attempt to understand the incipient discharges due to corona and surface

discharges by adopting UHF technique. The present work envisages development in this

direction. In addition, the use of polymer material as barrier insulation in an electrode gap filled

with liquid nitrogen need to be analyzed. Also the variations in corona inception voltage under

harmonic AC voltages in liquid nitrogen need to be analysed. The results of this study form

important part of the present work.

The author has carried out a methodical experimental study to characterize the UHF signal

generated due to corona/surface discharges (with and without barrier). UHF signals generated at

the point of corona inception and at near point of breakdown were characterized by using

spectrum analyser. Phase resolved partial discharge studies were carried out to understand the

phase at which discharges occurs when the barrier is present in the electrode gap. The results

form first part of the present study.

The presence of conducting or non conducting defect in solid insulation or formation of any

void in the bulk volume of insulation, under operating stress can cause incipient discharges,

forming treeing, a pre-breakdown structure. Under highly polluted supply voltage (odd and even

harmonics with different THD) in the power system network, the database on tree inception and

the dynamics is scanty. To identify the formation of defects during laying of high voltage cables,

AC and DC tests were carried out (Oyekoke et al., 2003). The fundamental issue with the DC

test is the formation of space charge in cable insulation and very low frequency test is identified

as an alternative (E. Ildstad et al., 2013). It is essential to understand the electrical tree formation

in XLPE polymer insulation at low temperature as well the rate of failure of XLPE polymer

insulation with trees and its growth under VLF voltages. In addition, it is essential to understand

the phase at which the partial discharge occurs under harmonic and VLF voltages. The

frequency contents of UHF signal formed during tree growth under VLF and harmonic voltages

need to be understood.

11

Having known all these factors, the author has carried out a methodical experimental study

to understand the following important aspects.

(i) To investigate electrical tree growth in XLPE cable insulation at low temperatures under

harmonic voltages with different THD and with VLF voltages. For the purpose of comparison,

treeing studies were carried out with 50 Hz triangular voltage wave. (ii) The shape of electrical

trees formed was categorized by measuring the UHF signal radiated during inception and

propagation. (iii) Failure analysis of cable insulation due to treeing carried out through Weibull

distribution studies. (iv) Phase resolved partial discharge studies were carried out (using UHF

signal measured) to understand the phase at which discharges occurs under different voltage

profiles. The analysis of tree growth phenomena in XLPE cable insulation under harmonic AC

voltage forms second part of the study.

To understand treeing phenomena, carrying out experimental study is time consuming and

laborious process. Thus the author has made an attempt to understand the treeing phenomena

through modelling studies. The author has used COMSOL, to solve the mathematical

formulations for calculating the electric field and to know more about the influence of nano

composite material on treeing phenomena, the results of the study forms third part of the present

study.

1.4 ORGANISATION OF THE THESIS

The first chapter introduces corona discharge activity in liquid nitrogen and the problems due

to electrical treeing in solid insulation. The need for studying corona discharge activity adopting

UHF technique is explained in detail. Upon clearly formulating the problem, the possible

method for identification of incipient discharges in liquid nitrogen due to corona activity and

treeing phenomena in solid/cable insulation has been proposed. The need for theoretical

modelling of electrical treeing is presented. A panoramic view of literature dealing the topic is

reported as completely as possible.

The second chapter deals with experimental and theoretical analysis of the study. The

different PD diagnostics used in cryogenic power apparatus were explained in detail. The details

of the experimental setup used for corona discharge study and for treeing experiments were

explained. The characteristics of UHF sensor used for PD diagnostics were explained. The

methodology adopted for modelling of electrical treeing using COMSOL is explained in detail.

12

In the third chapter, results of experimental studies on corona discharge activity in electrode

gap (with and without barrier) in liquid nitrogen are presented. The influence of harmonics on

electrical tree growth in XLPE cable insulation at liquid nitrogen temperature is presented. In

addition, the important results acquired based on theoretical modelling of trees, are presented.

In the fourth chapter, summary of author’s contribution on analysis of corona discharge

activity and on electrical treeing studies are presented. The last part of the chapter suggest for a

future work on the topic, as an extension of the present study.

13

CHAPTER 2

EXPERIMENTAL AND THEORETICAL STUDIES

2.1 GENERAL

In the present work, two sets of experimental studies were carried out.

1. Understanding corona discharge activity in liquid nitrogen in presence of epoxy

nanocomposites barrier.

2. Understanding treeing phenomenon in solid insulation/ cable insulation.

In this chapter, the details of the experimental setup established in the laboratory for

generating corona discharge activity in LN2 and for treeing studies in XLPE cable insulation at

LN2 temperature, equipments used, experimental techniques adopted for PD measurements and

analysis were detailed. Fundamental aspects on theoretical modelling of electrical trees were

detailed.

2.2 METHODS ADOPTED FOR PARTIAL DISCHARGE DETECTION IN

CRYOGENIC POWER APPARATUS

Incipient discharge is basically a discharge which will not bridge the gap between high

voltage electrode and the ground electrode. This incipient discharge is otherwise called as partial

discharge. The PD formed in the medium can inject current pulse, radiates electromagnetic

waves, ultrasonic waves, light emission, generates vibration, heat and so on. Figure 2.1 shows

the different techniques adopted for measurement of partial discharges in liquid nitrogen filled

cryogenic power apparatus.

The majority of PD detection systems that are used in cryogenic power apparatus includes

Ultra High Frequency (UHF) technique, High Frequency Current Transformer (HFCT), Optical

emission technique, Acoustic Emission (AE) sensors and by IEC 60 270 standard electrical

contact measurement technique.

In recent times, UHF technique is more popular method to identify the formation of incipient

discharges. UHF signals get generated when current pulses are injected with the rise time of a

nanosecond or less. Based on the theory of electromagnetism, signals with short rise time of

nanosecond range can radiate electromagnetic waves up to Ultra High Frequency (UHF) range

14

i.e., 300-3000 MHz. By choosing proper antenna/sensor, it is possible to identify the incipient

discharges in the insulation structure of the power equipments. Also by selecting the proper

bandwidth and having correlation between the UHF signal formed and injected current

magnitude, severity of the discharge could be identified even under online/onsite condition. UHF

technique is the least sensitive method compared to other methods of PD measurement, since

external noise interferences are completely avoided. Also the sensors have no electrical contacts

with operating high voltage system and hence the UHF sensor can be used for online monitoring.

Figure 2.1: Different methods of PD measurement

2.3 EXPERIMENTAL SETUP

The experimental setup, in general, can be sectioned into four parts such as the high voltage

source, the liquid nitrogen filled test cell (cryostat), phase resolved partial discharge analyzing

(PRPDA) system and the UHF sensor connected to spectrum analyzer/oscilloscope, respectively.

The typical photograph of the experimental setup and the test cell are shown in figure 2.2 and

figure 2.3.

The experimental setup for corona discharge study and was segmented into four. They are (i)

High AC voltage source, (ii) Test cell for corona studies, (iii) UHF signal measurement unit and

(iv) Partial discharge measurement system.

(i) Generation of high AC voltage

The discharge free test transformer (100 kV, 5kVA ) was used for generating high AC

voltage. Corona inception studies and breakdown studies were carried out independently to

15

avoid impact of any space charge formed due to long duration corona studies. For corona

inception and breakdown voltage studies, the AC voltage was applied at the rate of 300 V/s. The

capacitance divider was used for applied AC voltage measurement.

(ii) Test cell for corona studies

The cryostat used for the study is a double wall bell jar made of Quartz. The gap between the

inner and outer wall is 3mm and was evacuated to medium vacuum level, for thermal stability.

The inner vessel is filled with liquid nitrogen.

In the present work, the cryogenic test cell containing Needle-plane electrode with barrier is

fitted in a cylindrical container, immersed in the liquid nitrogen filled cryostat. Needle-plane

electrode with a gap distance of 10mm was used for carrying out corona discharge studies. The

thin sharp stainless needle electrode with tip radius of 15 µm is a high voltage electrode. The

bottom plane stainless steel electrode with its diameter of 5cm is connected to the ground.

To understand the influence of barrier on corona inception, fiber reinforced epoxy

nanocomposite insulating material is used as barrier material. Fiber reinforced epoxy

nanocomposite is generally used for enhancing the mechanical strength of the material (W.

Dauda et al., 2009). Sarathi et al. (2006) have studied electrical, thermal and mechanical

properties of epoxy nanocomposites and have concluded that 1 wt% Montmorillonite (MMT)

nano clay loaded epoxy resin has good electrical and hydrophobic properties. Thus, in the

present study, the fiber reinforced epoxy nanocomposite is prepared with 1 wt% MMT clay. The

barrier insulation is nearly 1mm thick and diameter of 5cm. The barrier position is always

referenced with respect to the top electrode. The edge grooved fiber glass reinforced epoxy

nanocomposite barrier is positioned in the electrode gap by fitting tight to the cylindrical

container. The liquid nitrogen gets filled between the barrier and the ground electrode through

the edge grooved in the barrier insulation.

The cryogenic test cell with electrode gap immersed into the liquid nitrogen filled in the

inner vessel of the cryostat and the nitrogen gas is circulated gently in the gap between the liquid

nitrogen and the top plate of the cryostat to avoid any moisture formation. On immersion of

cryogenic test cell in liquid nitrogen, thermal bubbles of large and smaller diameters were

observed. Resting time of several minutes was allowed before start of experiments to obtain

stabilized bubble free medium. Corona inception studies and breakdown studies were carried

out independently to avoid impact of any space charge formed due to long duration corona

studies.

16

Figure 2.2: Typical photograph of the experimental setup

Figure 2.3: Photograph of Cryostat with Test electrode system

(iii) UHF Sensor

The functional requirements of the sensor are as follows:

1. The sensor must be able to sense and record the intensity of PD signals and their relative

spectral magnitudes for a set of predefined frequency bands.

2. The detector must also be able to differentiate between PD and background RF noise

such as mobile phone signals and other impulsive events such as corona discharge.

Transformer

Coupling Capacitor

Cryostat

N2 Gas

Vacuum Pump

Spectrum Analyzer

Digital Oscilloscope

DAU & Zm

UHF Sensor

Z

HVAC

High

Voltage Ground

17

3. As the detector is ultimately to be directly attached to plant, it must be relatively small

although size constraints are not paramount as the plant under observation will be orders

of magnitude larger than the monitoring device.

4. UHF sensors used for partial discharge detection must have broadband response, the

reason being the frequency content of signals from a PD varies depending on its

discharge mechanism and the signal path.

The UHF sensor used in the present study is a broadband type sensor, which is placed at a

distance of 20 cm away from the test cell. Judd et al. (1998) reported in detail the use of

broadband sensor for measurement of UHF signal radiated due to partial discharges. The

sensitivity response of the broad band sensor is shown in figure 2.4. The output of the UHF

sensor is connected to the spectrum analyzer/high bandwidth digital storage oscilloscope. The

UHF signals were captured using a digital storage oscilloscope (LeCroy, 4 channel, 3 GHz

bandwidth, operated at 10 GSa/s) with an input impedance of 50 ohms. The spectrum analyzer

Hewlett Packard E4402B ESA-E-Series was used to measure the signal in zero span mode with

1GHz centre frequency.

100

10

1.0

0.1

Se

nsitiv

ity (

mV

/Vm

-1)

Frequency (MHz)

200 400 600 800 1000 1200 1400 1600 20001800

Figure 2.4: Frequency response of the UHF sensor

The important precautions considered during measurement of UHF signal are:

1. The noise level in the laboratory has to be measured at the operating voltage magnitude

using Spectrum analyser.

2. During UHF measurement no interference from external sources has to be confirmed.

3. No applied voltage fluctuation is allowed.

4. The length of the cable from UHF sensor to oscilloscope should be same for all

measurement, if any comparison has to be made.

5. Also position of the sensor should be kept same if any comparison has to be made at the

level of magnitude of UHF signal formed.

18

6. The input of the oscilloscope has to be operated at 50 ohms matching impedance so that

no reflection occurs and change in the characteristics of measured signal.

7. During measurement of PD under lightning impulse, the connecting cable between the

source and test chamber should free from any discharges.

8. The contact between the source lead and the test cell is properly connected to avoid any

contact discharges.

9. The noise level of the laboratory has to be measured.

(iv) Partial Discharge Measurement System

The phase resolved partial discharge studies were carried out by using an LDIC PD detector.

This circuit was calibrated using a commercial PD calibrator (LDC5/S3) in the range 1 to 5000

pC, to inject known amount of charge. -Q analysis was carried out to understand the occurrence

of discharge in the phase of applied AC voltage. In the present work, the PD magnitude is

measured by measuring the first pulse which is appearing, irrespective of polarity of the applied

voltage, with sensitivity level of 10 pC. The PD currents are measured by connecting a series

impedance of 50 Ω and the potential drop across it is measured through the digital oscilloscope.

The important precautions considered during PRPD measurement in cryogenic power

equipment are:

1. As a first step, it is essential to identify every electrical component in the electrical circuit

should be free from any discharges.

2. PD measurement should be carried out on achieving the preset voltage magnitude.

3. The PD magnitude has to be calibrated before by injecting known magnitude of charge

signal for reference.

4. The leads connecting the circuit should be free from corona/discharges

5. Narrow/wide band PD measurement should be identified.

2.4 SAMPLE PREPARATION OF BARRIER INSULATION

Barrier insulation used in the present work is an epoxy nanocomposite insulating material. In

the present work, mechanical shear mixing technique is adopted to disperse the nano fillers in

epoxy resin. The nano clay filler used in this study was organophillic Montmorillonite clay

(MMT) procured from Southern clay products Inc. (Gonzales, Texas) under the trade name of

Garamite 1958. The Table 2.1 shows the physical properties of Garamite 1958 (Southern clay

products). The shear mixing was carried out for half an hour and then mixed with high frequency

sonicator for one hour. The temperature rise was controlled by carrying out the mixing process

19

in a water bath. After adding required amount of curing agent, the glass fiber was impregnated

with epoxy clay mixture and the laminates were cured using 10 wt% of Tri Ethylene Tetra

Amine (TETA) hardener and kept in compression molding machine at room temperature for 24

hours for complete curing. The thickness and diameter of the barrier material is 1mm and 5cm

respectively.

Table 2.1: Properties of Clay Particle

Colour Off White

Form Fine Powder

Moisture Content <6 %

Bulk Density g/cc 0.128

Specific Gravity 1.5 – 1.7

Weight loss at 1000°C 37%

The barrier fits exactly to the inner diameter of the cylindrical container and the barrier could

be positioned at the required level in the electrode gap easily. The position of the barrier is

always represented with reference to bottom ground electrode.The selection of barrier insulation

to use in cryogenic power apparatus should be based on the following important characteristics.

1. Low dissipation factor (tan δ)

2. High breakdown strength

3. Partial discharge free

4. Extremely high treeing resistivity

5. Thermal or chemical deterioration free

6. No heat cycle during operation

Incipient discharges due to corona were initiated with the needle plane configuration, where

the top electrode is a needle electrode with its tip radius of curvature of 40μm and the bottom is

a plane electrode. The gap distance between the needle tip to the ground electrode is maintained

at 3mm.

The surface discharges were simulated by placing a thin epoxy nanocomposite material of 2

mm thick in the needle plane electrode gap. The needle electrode was allowed to touch the

surface of the insulating material. On increasing the applied voltage magnitude, the discharge

gets initiated and propagates over the surface of the thin barrier insulating material simulating

surface discharges.

20

2.5 ANALYSIS OF DISCHARGES USING TERNARY PLOT

In the present work, to understand the characteristics of the UHF signal formed due to corona

discharge under harmonic AC voltages with different THDs, the partial power analysis was

carried out. The FFT output of the UHF signal, which is the input data for generating the

Ternary plot. Partial Power is calculated by summing the power spectrum in a user specified

range of frequencies, dividing it by the total power. The power spectrum was calculated up to 3

GHz. The partial powers were calculated in three zones (x= (100 kHz –1 GHz), y= (1-2 GHz)

and z= (2-3 GHz)). The triangle coordinates corresponding to normalized energy content were

calculated as follows.

Normalized energy content of x = x/(x+y+z); Normalized energy content of y= y/(x+y+z)

and the normalized energy content of z =z/(x+y+z). This process is identical to the process used

to generate the gas in oil ratio used to plot the Duval’s triangle (Duval et al., 2002).

2.6 THEORETICAL ANALYSIS

There are two important parameter which are needed to be considered in the insulation

design of power apparatus

1. Electrical stress distribution in the bulk volume of insulation

2. Dielectric strength of the material

Recently, polymer nanocomposites have drawn considerable attention because of their

potential to improve electrical, mechanical and thermal properties as compared to neat polymer.

Several physical, chemical and electrical factors can severely influence the insulating capability

of an insulating material. Electrical treeing is one of the main causes for degradation and

breakdown of insulating materials. Credible experimental data base is available on the process of

electrical treeing and subsequent breakdown phenomenon in neat polymer. Studying the

behaviour of electrical trees experimentally is an expensive and time consuming process. Thus a

need of reliable computer simulation study has been felt to produce tree patterns similar to the

experimentally generated tree patterns, keeping in mind the physical model for the initiation and

propagation of dendrite structure.

Significant work has been carried out to understand treeing phenomenon in neat polymers

but the research in the domain of nanocomposites is still in its infancy stage. Thus the author has

attempted to understand electrical treeing phenomenon in nanocomposites, through computer

simulation studies.

21

FEM based software, COMSOL V5, has been used to carry out the electrical treeing studies.

In the present work 2D modelling of electrical trees was attempted. A monolithic dielectric is

considered with a high voltage electrode inserted into it. The bottom surface of the dielectric is

connected to the ground (i.e. zero potential). The two side edges follow Neumann boundary

conditions under steady state condition.

Under steady state condition, the governing equations can be written in classical form as

0. J and 0 E … (2.1)

With electric scalar potential reduced to

0.. 2 … (2.2)

With the homogeneous conductivity the above equation reduces to Laplace equation. The

nano particles are distributed uniformly in the bulk volume of the dielectric as shown in Figure

2.5. In the present work, the nano particles are represented with definite conductivity and the

ratio between conductivity of bulk volume insulation and the nano particle conductivity forced

to be of large number. It has to be metioned that with such large conductivity of nano particle,

the field distribution is not altered in the bulk volume of insulation.

Figure 2.5: Geometry of the model used for tree simulation

A needle plane electrode configuration is used to simulate the local electric field in the

insulating material. In all the simulations, top electrode which is at high potential (30 kV) and

the bottom most layer of dielectric is assigned zero potential. The tip radius of the electrode used

in study is 50nm. The numerical values of electrical conductivity of materials used in simulation

can be found in Table 2.2.

22

Table 2.2: Electrical conductivity of materials used in study

Geometry Electrical Conductivity (S/m)

Top electrode 5.9 × 107

Dielectric 1×10-4

Nanoparticles 1×103

In the tree structure the location and direction of tree propagation is chosen at random such

that the probability of propagation is basically a power law P E where is an exponent. In

the present work the value of η is chosen as 1.0. By introducing the probability concepts, one

could equate the actual treeing phenomenon with the simulated tree structure.

The probability of growth of electrical tree can be written as

1

( , )( , ', ')

( , )n

i

E i jP i j i j

E i j

… (2.3)

Where i, j are the co-ordinates.

Once the pre-breakdown occurs, further propagation can be explained by the addition of a

link to the previous structure. The potential at each and every point has to be calculated with the

altered boundary conditions. A very important assumption made, as a first part of study, is that

the potential of the connecting links are the same as that of the top electrode, which means that

there is no potential drop along the discharge path. The tree propagation is terminated as it

reaches the required length.

In actual practice, the tip potential of the tree is not at the same as that of the applied voltage.

The tip potential could be reduced due to discontinuity in the conducting path of the tree or due

to local conditions (pressure and temperature) of tree-incepted zone. Hence, to understand the

influence of potential drop on the tree shape, a separate methodology is formulated and the rate

of tree propagation was analyzed.

23

CHAPTER 3

RESULTS AND DISCUSSION

3.1 GENERAL

This chapter presents the results obtained based on the methodical experimental study and

the theoretical analysis carried out to understand the partial discharge/ incipient discharge

activity in liquid nitrogen medium. Critical assessment of the obtained results has been made and

compared with the available literature in order to accrue important conclusions based on the

study. In general, the results of the study sectionalized in to four major parts.

1. Analysis of corona discharge activity in liquid nitrogen (with and without barrier in

the electrode gap) under AC/harmonic AC voltages adopting UHF technique.

2. Analysis of treeing phenomena in cable insulation at cryogenic temperature under

harmonic AC voltages.

3. Modelling of electrical treeing in composite insulation through COMSOL.

3.2 ANALYSIS OF CORONA DISCHARGE ACTIVITY IN LIQUID NITROGEN (WITH

AND WITHOUT BARRIER IN THE ELECTRODE GAP) UNDER AC VOLTAGES

ADOPTING UHF TECHNIQUE

With the advent of HTSC material, development of superconducting power apparatus, with

liquid nitrogen as insulant, has gained considerable importance. One of the major problems in

liquid nitrogen insulation is the formation of incipient discharges. Especially the corona

discharge activity is a major concern to insulation engineers. The most conventional way of

identifying incipient discharges is by conventional method adopting IEC 60270. The technique

has fundamental limitation for online monitoring. Thus the author has adopted UHF technique

and characterized corona discharge formation in liquid nitrogen (with and without barrier in the

electrode gap) under AC voltage.

3.2.1 Corona inception voltage (CIV) and the breakdown voltage of electrode gap with

barrier insulation

In liquid nitrogen, the breakdown occurs due to different methods and especially through

bubbles, which occur in liquid nitrogen due to any hot spot formation or due to any local

temperature variation or by due to instantaneous injection of high current, which can initiate

24

incipient discharges thereby lowering the breakdown strength. Figure 3.1 shows variation in

corona inception voltage of liquid nitrogen filled electrode gap (with barrier), under AC

voltages. The corona inception voltage was obtained based on the measurement of the first UHF

signal generated by the sensor, due to corona formation. The corona inception voltage is almost

the same (in presence of barrier) and its value is much higher than the clean electrode gap.

Assuming the needle profile is parabolic, the local electric field at the needle tip can be

calculated as (Coelho et al. 1971),

rr

p

p

d

UdE

41ln

2max

… (3.1)

Where, U is the applied voltage, rp is the tip radius, and d is the gap distance between the two

electrodes. Takahashi et al. (1975) have observed that 11 MV/cm is required for discharge

inception in vapour bubble formed in liquid nitrogen. Truong et al. (2013a) observed corona

discharge occurs in liquid nitrogen at 1.5 MV/mm. In the present work, the local electric field at

the needle tip (based on equation 3) is calculated as 0.45 MV/mm, which is much lower than the

corona discharge in liquid nitrogen. The cause for it could be due to the formation of micro

bubbles in the electrode gap. In liquid nitrogen, the bubbles originate from the hot spot zone or

due to any local temperature variation or due to, instantaneous injection of high magnitude of

current in the volume of liquid nitrogen (Kraehenbuel et al., 1994).

Figure 3.1: Variation in corona inception voltage of liquid nitrogen filled electrode gap

with barrier at different position

On application of voltage to the Needle-plane electrode gap, the micro bubbles in the

electrode gap come in contact with the needle electrode during voltage rise. The discharges get

25

initiated in the vapor bubble due to increase in electric field within the bubble. In this process,

the free electrons get injected into the vapour bubble and to the liquid nitrogen due to first

discharge, which thereby enhances the field intensity near the edges of the vapour bubble

forming sustained corona discharge activity. This discharge process cause cold plasma in the

vapour bubble raising local temperature (Denat et al., 2005). The temperature variation near the

electrode tip causes ice formation suppressing continuous corona discharge activity. By

increasing the cryostat pressure, the discharge initiated due to bubbles could be reduced

(Kraehenbuel et al., 1994). Badent et al. (1996) have reported that when hydrostatic pressure of

the liquid is increased the electrical breakdown strength increases but the number and amplitude

of injected current pulses reduces thereby reducing the streamer velocity.

Figure 3.2: Variation in Breakdown Voltage of liquid nitrogen filled electrode gap with

barrier at different position

Figure 3.2 shows breakdown characteristics of the liquid nitrogen filled electrode gap (with

barrier), under AC voltages. The breakdown voltage has increased when the barrier is positioned

near to the high voltage electrode. Topalis et al. (2005) studied breakdown in air gaps with solid

insulating barrier under impulse voltages and have observed highest breakdown strength when

the barrier is placed near 20% away from high voltage electrode. Similar characteristics were

observed in liquid nitrogen with barrier, under AC voltages.

In general, the author opines that, the breakdown characteristics (Figure 3.2) can be

sectionalized as decreasing zone (Zone 1), stabilized zone (Zone 2) and further reduction in

breakdown voltage (zone 3). Jong-man-Joung et al. (2003) have observed similar trend in their

studies. It is also noticed that the breakdown occurs through the barrier when they are near

ground electrode. Typical photograph of breakdown spot in the barrier insulation is shown in

26

Figure 3.3. If the barrier is in proximity to the needle electrode, the discharge glides over the

barrier insulation and from the edge of the barrier insulation to the ground through liquid

nitrogen, the breakdown occurs, causing increased breakdown voltage. No physical surface

damage to the barrier insulation is observed. In author’s opinion, the cause for it could be due to

liquid nitrogen which quenches the heat generated due to discharges thereby no damage occurs

to the solid barrier insulation.

Figure 3.3: Typical photograph of breakdown spot in the barrier insulation

3.2.2 Analysis of current pulse generated due to corona discharge

Figure 3.4 shows typical corona generated current pulse, with barrier in the electrode gap,

under AC voltage (Figure 3.4a and b). Also the typical UHF signal formed during current

injection is shown (Figure 3.4c).

Figure 3.4: Typical current pulse injected due to corona discharge (with barrier) (a) In

positive half cycle (b) In negative half cycle (c) Typical UHF signal formed

during current injection

The current signal injected due to corona discharge in liquid nitrogen insulation, at the point

of its inception and during streamer formation, have rise time, in the negative and positive half

27

cycle of the AC voltage are about 2.9 ns and 2.5 ns respectively and the characteristics has not

altered due to barrier insulation. The rise time was calculated as the time required for the current

pulse to rise from 10% of its peak magnitude to reach to 90% of its peak. Okubo et al. (2005)

analysed the nature of current pulse in polypropylene -liquid nitrogen composite insulation and

have measured the rise time of the current pulse formed, at different voltages, due to surface

discharges is about 18ns. Suzuki et al. (2002) measured PD current signal generated due to

discharges in liquid nitrogen impregnated butt gap, and they found rise time to be few nano

seconds and the amplitude is of tens of micro amperes. Y. Tanaka et al. (2008) studied injected

current pulses due to surface discharges in liquid nitrogen medium. They have measured current

pulses to be few nanoseconds and also showed that current pulses of shorter and longer width

formed due to surface discharges. The short rise time of current pulses in nano second duration

confirms that it can radiate electromagnetic signals. When the current pulses involve rise time of

about 1 ns or less, signals in the UHF range (300– 3000 MHz) are excited (Judd et al., 1996;

Pearson et al., 1995).

Figure 3.5: Typical corona discharge generated UHF sensor output signal in liquid nitrogen

insulation (a) And its FFT (b) (I) Without barrier (II) With barrier

28

In general, it is observed that the results obtained by the author correlates well with the

results of other researchers and could conclude that the PD/ incipient discharge current pulses

generated due to corona discharge in liquid nitrogen have rise time of few ns.

3.2.3 Characteristics of UHF sensor signal generated due to corona discharges

Figure 3.5a shows corona discharge generated UHF sensor output signal and its FFT output

is shown Figure 3.5b, under AC voltages. The UHF signals generated at corona inception

voltage and at higher voltages, in the liquid nitrogen electrode gap (with barrier) have frequency

contents between 0.5 -1.5 GHz.

Figure 3.6: Typical UHF sensor signal measured in sequence mode (a) On corona

inception (b) At 20 kV

Figure 3.6 shows typical UHF signal generated due to corona discharge activity, in sequence

mode. It is clear that low and high magnitude of UHF signal is formed during corona discharge

process. Denat (2005) studied discharge process in liquid nitrogen and reported that discharge

path is a vapor column and by avalanche ionization within the vapor column energy is imparted

to the liquid molecules adjacent to it causing further growth of streamers. He concluded that

filamentary channels propagate in the direction of high electric field and have observed higher

magnitude of injected current during propagation. Beroual et al. (1998) have observed in liquid

insulation that the irregular discharges are the indication of growth of streamer in steps. Hebner

(1988) concluded that increasing the electric field leads to the formation of streamers which are

filamentary in structure. Truong et al. (2013b) used high speed camera images to understand the

discharge process and could conclude that low magnitude discharges occurs when bush type

discharge occurs and on its transition to streamer, high magnitude UHF signal occurs.

Figure 3.7 shows UHF signal magnitude variation on corona inception (with barrier at

different levels in electrode gap) and at and at higher voltages. The magnitude of UHF signal

29

generated on corona inception is high when the barrier is in middle of the electrode gap and at

the same time, its magnitude gets reduced when the applied voltage magnitude is increased. It

could be due to predomination of surface discharge activity with the barrier insulation, where

low magnitude discharges occurs.

Figure 3.7: Variation in peak to peak voltage of UHF sensor signal generated by corona

discharge (with barrier) in liquid nitrogen (A) With barrier at 20 kV (B) With

barrier at inception (C) Clean electrode gap at 20 kV (D) Clean electrode gap at

inception

3.2.4 Corona generated UHF Sensor signal analysis using spectrum analyzer

In recent times, condition monitoring of power apparatus adopting UHF technique has

gained considerable importance to use in GIS and transformers for identification of incipient

discharges. The analysis of UHF signals through spectrum analyser has added advantage to

operate it in full span and zero span mode. The full span analysis can provide the frequency

contents present in the signal and zero span mode can provide signal with particular frequency

content. A spectrum analyzer (SA) performs a Fourier analysis of a (continuous) input signal.

Like a radio receiver it tunes at consecutive frequencies and could measure the amplitude of the

spectral content of the input signal at the tuned frequency. Compared with the broadband

approach, the narrow band UHF method has the advantage that incipient discharge signals can

be distinguished from external signals in a densely occupied UHF frequency spectrum so that

much better signal to noise ratio can be achieved (Feger et al., 1999). Thus a single dominant

frequency measurement is helpful for recording the general dynamics of discharges.

30

The UHF sensor output formed due to corona discharge (with barrier), under AC voltages,

on inception and at higher voltages have dominant frequency at 1 GHz. Figure 3.8 shows typical

pulses generated due to corona discharge (with barrier), under ac voltages. The generated signals

were recorded by operating spectrum analyzer in zero-span mode, for required sweep time with

centre frequency of 1 GHz.

Figure 3.8: Typical pulse sequence of UHF sensor signal measured using spectrum analyzer

at 1 GHz in zero-span mode (a) At inception measured (b) At higher voltage

(c) Inception to breakdown

The magnitude of signal formed has direct relationship with applied voltage (fig 3.8a and b).

The number of discharges that occurs, at the point of corona inception (with barrier), in liquid

nitrogen insulation, is nearly 48pulses/sec. To understand the breakdown process, the voltage

was constantly increased up to the point of breakdown (Figure 3.8c). During this period, it is

realized that, just before breakdown high intensity discharges occur.

3.2.5 Analysis of Phase Resolved Partial Discharges (PRPD)

Figure 3.9 shows typical PRPD pattern formed due to corona discharge (with barrier) under

AC voltages, on corona inception and at higher AC voltages. The corona activity is predominant

at the peak of the sinusoidal AC voltage. The corona discharge inception in liquid nitrogen

occurs in the range 54-90º and between 235- 284º of the AC voltage. In the presence of barrier,

the discharge occurs in the range 63-99º and 244-287º. When the applied voltage is increased

above corona inception voltage, with clean electrode gap, discharges occurs in the phase range

of 40-103º and from 235-284º. Similarly, in the presence of barrier, the discharge occurs from

31

50-130º and for 217-288º. This clearly indicates the phase window at which the corona discharge

occurs (with or without barrier insulation) at the point of inception is less compared with

discharges at higher voltage. Swaffield et al. (2008) concluded that number of discharges that

occurs from cathode top is high but magnitude of discharges is low and the converse with anode

initiated streamers. Takahashi and Ohtsuka (1975) carried out corona study in liquid nitrogen

and have concluded that the corona inception voltage is smaller with the point negative than with

the point positive, and increases as the gap spacing increases. Negative corona pulses are more

frequent but smaller than positive ones. The height and repetition rate of the pulse with the point

negative increase as the gap spacing decreases. The corona is accompanied by bubbling similar

to a spray jet from the point tip. The bubbling formed is independent of the polarity of the

applied voltage. Kurihara et al. (2004) carried out PD study in artificial air filled void in solid

insulator at liquid nitrogen temperature and have concluded that PD magnitude and number of

pulses formed are different at low temperature and at room temperature.

Figure 3.9: Typical PRPD pattern formed due to corona discharge (I) Without barrier (II) With

barrier (a) At Inception (b) At higher voltage

It is well known that formation of ice in liquid nitrogen is one of the major problems and if

any ice forms at the tip of the sharp protrusion in the electrode gap, it can alters the discharge

characteristics. Hanaoka et al. (1993) carried out influence of ice layer on pre breakdown current

in liquid nitrogen. They have concluded that ice present at tip of needle in the needle plane

configuration increases the discharge inception voltage and the breakdown voltage. In the

present work, corona discharge activity predominates at the first stage (Figure 3.9(ii) a) and at

slightly higher voltages surface discharge along with corona activity is observed, causing

32

discharge in the rising portion of AC voltage (Figure 3.9(ii) b). It is noticed that the magnitude

of PD occurrence is high in the positive half cycle compared to negative half cycle.

3.3 ANALYSIS OF CORONA DISCHARGE ACTIVITY IN LIQUID NITROGEN

UNDER HARMONIC AC VOLTAGES

The use of non-linear and time varying loads, which have non-linear voltage-current

characteristics, can cause harmonic pollution in the power system network causing distortion to

the AC supply voltage profile (Ortmeyer et al., 1985; Wagner et al., 1993; Florkowski et al.,

2013; Cavallini et al., 2010). Considerable research work has been carried out to understand

corona formation in liquid nitrogen insulation under AC voltages. Understanding of the corona

formation in liquid nitrogen under harmonic ac voltages with different THDs, the database is

scanty. Thus the author has characterized corona discharge formation in liquid nitrogen under

harmonic AC voltages adopting UHF technique.

3.3.1 Variation in corona inception voltage in liquid nitrogen under harmonic AC voltages

Table 3.1 shows variation in corona inception voltage in the electrode gap filled liquid

nitrogen, under harmonic AC voltages, with different THDs. The corona inception voltages were

obtained based on the first UHF pulse captured by the oscilloscope using the UHF sensor output.

In liquid nitrogen insulation, a marginal reduction in corona inception voltage is observed with

increase in order of harmonics and with different THDs.

Table 3.1: Variation in Corona inception voltage (CIV), peak to peak voltage, rise time of

current pulse for positive and negative polarity under harmonic ac voltages with

different THDs in liquid nitrogen insulation

H THD K

CIV

(kV)

Vpp

(mV)

tr (ns)

+

Pulse

-

Pulse 1 0 1.41 9.2 18.5 2.87 2.9

3 4 1.36 9 16.2 2.23 2

3 10 1.27 8.4 14.3 2.23 1.9 3 40 1.30 8.4 13.6 2 2

5 4 1.47 8 13.5 2.067 1.85 5 10 1.55 8.13 16.5 1.97 1.77 5 40 1.84 7.2 14.8 2.7 2.67

7 4 1.38 8.2 12.9 2.9 2.25 7 10 1.43 8.64 19.5 1.975 1.83

7 40 1.71 7.4 15 3.3 2.25

The variation in the discharge magnitude could be realised with the measured UHF signals

by operating the oscilloscope in sequential mode (Table 3.1). It is observed that burst type or

33

impulsive type discharge occurs at the point of inception and during propagation. The peak to

peak amplitude of the UHF signal is the magnitude between the highest peak in positive and the

negative half cycle of the UHF signal generated. It is observed (Table 3.1) that UHF signal

magnitude formed at the point of corona inception, is almost constant irrespective of harmonic

AC voltage with different THDs. It is observed that with increase in applied AC voltage, above

corona inception voltage, irrespective of harmonics with different THDs, shows an increase in

UHF signal magnitude.

3.3.2 Analysis of injected current pulse due to corona discharge activity in presence of

harmonic AC voltages

Figure 3.10 shows typical wave shape of the injected current pulse generated due to corona

discharge, in the positive and negative half cycle of the applied AC voltage (Figure 3.10a and b).

Also the typical UHF signal formed during current injection is shown (Figure 3.10c). The shape

of the current signal generated due to corona discharge in liquid nitrogen under harmonic AC

voltages with different THDs is nearly the same. In general, the rise time of the current signal

generated in the negative half cycle is always less than in the positive half cycle. The rise time

was calculated as the time required for the current pulse to rise from 10% to 90% of its peak

magnitude.

Figure 3.10: Typical current pulse injected due to corona discharge under harmonic AC voltages

in liquid nitrogen insulation (a) In positive half cycle (b) In negative half cycle

(c) Typical UHF signal formed during current injection

3.3.3 Analysis of UHF signal generated due to corona discharge in presence of harmonic

AC voltages

Figure 3.11 shows FFTs of the UHF signals generated due to corona discharge activity in

liquid nitrogen under harmonic AC voltages with different THDs. It demonstrates that UHF

signals are generated by corona discharges, under AC voltage, in the liquid nitrogen. Generated

0 10 20 30 40 50Time (ns)

Am

plitu

de (

a.u

)

(c)

(b)

(c)

(b)

(c)

(b)

(a)

(c)

34

signals lie in the frequency range of 0.7- 1.5 GHz. Also the bandwidth of the UHF signals

generated due to corona discharge under harmonic AC voltages with different THDs at the point

of its inception and at higher voltages remains the same.

Figure 3.11: FFT analysis of UHF signal generated due to corona discharge under harmonic AC

voltages with different THDs in liquid nitrogen insulation

3.3.4 Classification of discharges using ternary diagram

Figure 3.12 shows typical ternary diagram obtained for the discharges initiated due to corona

activity under harmonic AC voltages with different THDs. It is observed that there is no change

in location in the ternary plot with UHF signals obtained due to corona discharges. This clearly

indicates that irrespective of level of harmonics with different THDs, the characteristics of UHF

signal formed due to corona activity under harmonic AC voltages with different THDs are the

same.

Figure 3.12: Typical Ternary diagram obtained due to corona discharge under harmonic AC

voltages with different THDs in liquid nitrogen insulation

35

3.3.5 Analysis of Phase Resolved Partial Discharge (PRPD)

Figure 3.13 shows typical PRPD pattern formed due to corona discharge under harmonic AC

voltages with different THDs, at the point of inception and at higher AC voltages. The corona

activity is predominant at the peak of the AC voltage.

Figure 3.13: Typical PRPD pattern formed due to corona discharge (i) 50 Hz (ii) H=3,THD=4%

(iii) H=5,THD=4% (iv) H=7,THD=4% (v) H=7,THD=40% (a) At inception (b) At

higher voltage

36

It is observed that corona discharge inception in liquid nitrogen occurs in the range 37-55º

and between 229- 266º in the positive and negative half cycle of the AC voltage respectively.

When the applied voltage is increased, corona discharge occurs in the phase range of 73-100º

and from 219-270º, of the applied AC voltage. In the presence of harmonics, the phase at which

the corona inception occurs depends on the rate of rise of voltage than the peak of the applied

AC voltage. It was observed that when the applied voltage magnitude is increased much above

the corona inception voltage, low magnitude discharges occurs at the rising portion and high

magnitude discharges occurs at the peak of the applied voltage.

3.4 ELECTRICAL TREEING IN CABLE INSULATION AT LIQUID NITROGEN

TEMPERATURE

One of the mechanisms by which the solid insulating material can fail, is due to electrical

treeing. The influence of ambient temperature on tree inception and propagation is known to

some extend under AC voltages. In recent times, with the increase in non-linear load the supply

voltage gets polluted and the cryogenic equipment connected with the system gets subjected to

the polluted supply voltage. Thus it is essential to understand the fundamental aspects of treeing

phenomena in solid insulation (XLPE material) under harmonic AC voltage, which forms third

part of the present study.

3.4.1 Characterization of Trees formed under harmonic AC voltage

Figure 3.14 shows typical electrical tree structure formed in XLPE cable insulation

immersed in liquid nitrogen under harmonic AC voltages. The shape of electrical trees formed

under different voltage profiles depends basically on the local electric field near the tree

inception zone/defect site. Mason (1955) suggested the local electric field (Emax) at the needle tip

electrode as

r

dr

UdE

41ln

2max … (3.2)

Where, r is the radius of curvature of the high voltage needle electrode, d is the gap distance

between the needle tip and the ground electrode and U is the magnitude of the applied AC

voltage. Thus, on application of high voltage to the needle electrode, the local electric field at

the tip of the needle/defect site enhances and if the local electric field is sufficiently higher than

breakdown strength of the material, incipient discharge occurs. It is observed that an intrinsic

type or a fibrillar type tree form under AC / harmonic AC voltage. Kosaki et al. (1996) observed

that tree inception voltage in XLPE cable insulation, at low temperature, is much higher than the

37

tree inception voltage at normal room temperature. The tree pattern formed under 50 Hz have a

thick main branch with fibrils at the edges. Under Very low frequency of 1 Hz, bush type

damage occurs near to the tree inception zone and as and when it propagates more of fibrillar

type damage occurs in the bulk volume of insulation. The shape of the tree is much similar to the

DC tree (Minoda et al., 1997).

The shape of damage gets altered depending on the local electric field, conductivity of the

discharge path and the trapped charges in the discharge path which can alter the shape of

electrical tree formed to a fibrillar type, bush type tree or a tree-like-tree (Sosnowslu et al.,

1985).

Figure 3.14: Electrical trees under harmonic AC voltage profiles (a) 50 Hz (b) 2f (c) 3f

(d) 4f (e) 5f (f) 6f (g) 7f (h) 2f (i) 3f (j) 6f (k) 7f (l) 1 Hz (b) to (g) With 4 %

THD and (h) to (k) 40 % THD

The cause for fibrillar type damage could be due to high packing density of the material (due

to shrinkage of material) at low temperature and fast quenching of local temperature rise

occurred due to partial discharge activity, the amount of damage to the insulating material gets

38

reduced thereby forming thin fibrillar type damage than voluminous damage that occurs in same

material at room temperature (Sarathi et al., 2012). It is also observed with higher order

harmonics with increased THD, the diameter of the main branch is reduced and more of fibrillar

type structure forms.

Since the level of carbonization is less, the tree growth mostly depends on the local electric

field at the edges of the tree than the treed zone. Densley (1979) studied the tree growth in XLPE

insulation at room temperature and has concluded that under AC voltage, mostly bush type of

trees are formed and the trapped charges alters the shape of the trees. Sometimes the charges are

also injected into the insulation structure through the defect formed zone-enhancing field at one

point causing further enlargement of the channel resulting in “Tree-like” tree structure.

Otherwise local discharges will occur causing increased diameter of the damaged zone forming

“Bush-type” of electrical tree. Some author (Olyphant, 1963) suggested that channel propagation

will continue provided there is a critical energy stored at the tip of the channel allowing

degradation of the material by discharges. If there is no sufficient energy, the channel will stop

growing and discharge will recur in the main branches forming a Bush-type tree structure.

The author’s opinion based on the experimental study is that, stressing the insulation ( at

room temperature) at higher voltages will result in formation of Tree-like tree structures and in

turn at lower voltages, local damage will cause formation of Bush-type of tree structures. Also it

is observed from the experimental study that, under low voltages, the unfailed samples were

found to have tree structures. Whereas, in the specimens stressed at high voltage magnitudes, no

clear cut tree structures were noticed. This is due to the injected charges which cause local

reaction at some point surrounding the pin electrode, initiating the tree to form quickly and reach

the ground electrode causing breakdown. Whereas at the low voltage magnitude, the injected

electrons will not attain sufficient energy to cause cleavage of material and only local discharge

will occur increasing the diameter of the defect. This allows us to conclude that even the damage

is voluminous, Bush-type tree structures is less dangerous compared to Tree-like tree structures,

where the rate of propagation is aided by the applied voltage. At low temperature, the inception

voltage is high and even the rate of propagation is very much reduced under AC voltage. More

than that, the shape of electrical tree formed is almost bush type of tree under AC voltages,

irrespective of applied AC voltage magnitude, which is much different from room temperature,

generated electrical trees.

39

3.4.2 Life Estimation of XLPE cable Insulation

The life of insulation material could be assessed by certain statistical techniques. Normally,

the life of insulation is estimated by obtaining failure time from a specified number of samples.

Repeating the test several times with identical specimens usually yields greatly varying values of

failure times. It has been identified earlier that life of insulation structure could be estimated

using Weibull distribution. The experimental data are used to estimate the parameters of the

distribution.

Failure analysis of XLPE cable insulation due to electrical treeing was studied through

Weibull probability distribution. Treeing studies were carried out with 20 specimens and their

failure times were observed for the first 10 failures and the remaining samples were used for

observing the shape of the tree formed.

It is well established that the two parameter (scale parameter (α) and shape parameter (β))

Weibull probability distribution function is used for understanding the failure analysis of

insulating material and the function is written as (Nelson et al., 1982)

ttF exp1

… (3.3)

Where F(t) is the ratio of specimens which fail in time ‘t’. The scale parameter (α) indicates the

time required for 63.2% of specimen fails.

Figure 3.15: Weibull distribution plot for the failure times of XLPE cable insulation due to

electrical trees under harmonic AC voltages (a) 4% THD (b) 40% THD

Figure 3.15 shows Weibull plot of failure times of XLPE cable specimen failure due to

electrical tree under harmonic AC voltages. Table 3.1 shows the Weibull distribution parameters

obtained from the failure times of XLPE cable insulation due to electrical treeing. A reduction in

40

characteristic life (α) of cable insulation failure due to electrical trees is observed with increase

in order of harmonics and THDs. Nguyen et al., studied breakdown characteristics of kapton

layers at low temperatures and have observed that Weibull shape parameter reduces with

increase in number of layers of the film. The cause for it is due to formation of bubbles in the

layers and contaminants in bulk insulation [22]. Bahadoorsingh et al., have analysed failure

characteristics of XLPE cable insulation due to treeing under AC voltages through Weibull

studies. They have reported that an increase in value of β is an indication of increase in failure

time of the insulating material due to treeing. Similar characteristics were observed with XLPE

cable insulation failure due to treeing under harmonic AC voltage, with cable insulation at liquid

nitrogen temperature.

Table 3.2: Shows the Weibull distribution parameter characteristics life (α) and shape

factor (β) and peak factor of applied AC voltage

K %

THD

(α) (β) Peak

Factor

f (50 Hz) - 95.46 1.523 1.41

2f 4 287.34 1.758 1.41

3f 4 81.39 2.925 1.35

4f 4 33.31 2.065 1.42

5f 4 35.72 2.471 1.46

6f 4 67.80 2.478 1.44

7f 4 154.61 1.569 1.37

2f 40 48.37 2.630 1.59

3f 40 89.10 1.407 1.30

4f 40 32.18 3.013 1.75

5f 40 29.24 1.495 1.83

6f 40 36.59 2.105 1.79

7f 40 17.79 3.148 1.71

1 Hz* - 1462.6 2.829 1.41

* applied voltage 14 kV

Failure time due to treeing is high under 2nd and 7th harmonic AC voltage with 4% THD. It

is also noticed that failure times is reduced to one third with fourth and fifth harmonic voltage

compared with 50 Hz AC voltage. For harmonics with higher THD’s, the failure times are nearly

the same. Florkowski et al. (1997) have clearly indicated that partial discharge activity has high

41

dependency on applied voltage wave shape. Thus, the results of the study indicate that on

inception of electrical tree, the rate of change of voltage, and the peak factor of the applied AC

voltage alter the number of partial discharge occurrences near the treed zone; thereby varying

time to failure of cable insulation due to treeing. It is also observed (Table 3.2) that higher the

peak-factor of the harmonic AC voltage, shorter the life of insulating material due to treeing.

3.4.3 Characterization of UHF signal

Figure 3.16a and b shows typical time domain and frequency domain analysis of UHF signal

generated during tree growth in XLPE cable insulation at low temperature. Fig. 3.16c and d

shows the typical FFT analysis of the UHF signal formed during the tree growth under harmonic

AC voltages with 4% and 40% THDs respectively. The frequency content of the UHF signal

formed during tree propagation in XLPE cable insulation at low temperature lies in the range 0.5

– 1.5 GHz. Sarathi et al. (2012) studied treeing phenomena in XLPE cable insulation under odd

harmonic voltages and have observed frequency content of UHF signal in the same range but the

shape of tree formed are different. The characteristics of the UHF signal formed during tree

growth in XLPE cable insulation at low temperature, under harmonic voltages and at VLF

voltage are the same.

Figure 3.16: Typical UHF signal generated during tree growth (a) and its corresponding FFT

analysis (b) (c) FFT analysis of UHF signal generated during tree growth under

harmonic AC voltage with 4%THD and (d) 40%THD

3.4.4 Phase analysis of discharges during tree growth using UHF signals

UHF signal formed during tree growth have dominant frequency of 1 GHz and hence the

spectrum analyser was operated in zero span mode with 1 GHz component. The typical spectrum

42

obtained during tree growth along with indicating phase of the supply voltage at which discharge

occurs, is shown in Figure 3.17.

Figure 3.17: Phase resolved partial discharge analysis using spectrum analyser by operating

in Zero Span mode (a) 50 Hz (b) 1 Hz

(i) (ii)

Figure 3.18: Phase resolved partial discharge analysis using spectrum analyser by operating

in Zero Span mode (a) 2f (b) 3f (c) 4f (d) 5f (e) 6f (f) 7f (i) 4% THD (ii) 40%

THD

It is shown that, under 50 Hz AC voltage, the discharges occur near the zero crossing (Figure

3.17a) as it is observed at room temperature (Suwarno et al., 1996; G. Lupo et al., 2000). Under

very low frequency voltages, especially under 1.0 Hz, discharges occur at the peak of the AC

voltage (Fig. 3.17b).Under harmonic AC voltages, with lower THDs, the discharges occur at the

raising portion of the applied voltage than at the peak of the AC voltage (Figure 3.18). It can also

be realized that irrespective of harmonics voltage with different THDs, the discharges occur

when the rate of rise of voltage is high. The characteristics are much similar to electrical trees

43

generated under harmonic voltages at room temperature (Sarathi et al., 2012). Florkowska et al.

(2006) studied partial discharge activity with stator bar thermoset insulation under standard and

non standard voltage profiles. Partial discharge (PD) pulse phase density shows good correlation

with applied voltage slew-rate as reported in their work.

Suwarno et al. (1995) have showed that the magnitude of PD number increases with applied

voltage. It is observed that, the number of discharges that occur with harmonic voltages (with

higher THDs) is much high than under 50 Hz AC voltages.

3.5 MODELLING OF ELECTRICAL TREES

In recent times, with the increase in operating voltage magnitude, it has become essential to

design and develop insulating material with higher operating stress. There are various factors

which can influence the formation of electrical trees. Simulating the experimental condition is a

major drawback and carrying out experiments requires longer time and more than that it requires

sophisticated equipments to visualise the treed zone. Hence modelling is important and it can

help us to incorporate mathematical formalism for the physical phenomena to understand the

dynamics of the treed structure. The literature on modeling of treeing phenomenon in

nanocomposites material is scanty. Hence the author has made an attempt to model the tree

growth with nano particle dispersion in base insulating material. The author has used COMSOL

has been used to calculate the local electric fields.

In the present work, the influence of the diameter of nanofillers on electrical tree propagation

in nanocomposites is studied. The diameter of nanofillers is varied from 10nm to 50 nm and the

total number of steps required for tree to reach the ground electrode is calculated to understand

propagation rate of electrical tree.

First Niemeyer et al., (1984) modeled the gaseous discharge patterns by adopting statistical

techniques (NPW model). Following NPW model, Wiesmann and Zeller (1986), introduced the

WZ model for the study of growth dynamics of electrical tree in solid insulation. The WZ model

indicates that the local electric field determines the shape of the tree. Later by adopting WZ

model, Barclay et al. (1990) studied the chaotic behavior of electrical trees using fractal

geometry studies. They have concluded that the field exponent η decide the size and shape of

tree. Increasing the value of exponent η in the model simulates intrinsic type breakdown pattern

with few side branches. Kudo (1998) simulated the electrical tree patterns and classified the tree

patterns based on fractal dimension.

44

Sweeney et al. (1992) studied the growth dynamics of electrical trees in composite insulation

and concluded that the number density of links included in the barrier material is high, if the

dielectric strength of barrier is less. Also they observed that the tree grows along the interface of

the barrier material in the composite insulation. Farr et al. (2001) studied the tree dynamics in a

composite insulation under AC voltages. They identified that in a composite insulation, the tree

propagates along the interface forming a Bush-type of tree.

The present work takes into account of the insight rendered by the aforementioned authors. It

could be realized based on simulation study, i.e. under ideal conditions, only Tree-like tree is

formed. In composite insulation structure, it could be realized that the Tree-like tree structure or

Bush-type tree structure can form depending on zone of inception and the presence of nano

particles. It means that the dielectric constant of material plays an important role in the growth

dynamics of electrical trees.

3.5.1 Analysis of Two Dimensional electric field distributions in insulation structure

during Tree growth

Figure 3.19 shows the distribution of electric field at the time of tree initiation with the nano

particles having different conductivity. It is observed that the electrical field is enhanced around

nanoparticles but the electric field intensity due to variation in conductivity has not altered. In

the present study, irrespective of the conductivity of nano particles, the maximum electric field

intensity is observed to be 1.4 × 1011 V/m.

Figure 3.19: Distribution of electric field at the time of tree initiation with nano

particles of different conductivity (a) 101 (b) 10

3 (c) 10

5 S/m

Figure 3.20 shows the distribution of electrical field at different instances, during the

propagation of electrical tree. It is observed that electric field is higher near the needle tip at the

time of propagation of tree. Danikas et al. (1996) have studied electrical field configuration

variation in insulation structure and have also observed that the electrical field variation during

step growth is high near the tree tip.

45

Figure 3.20: Distribution of electric field during tree propagation after (a) 100 (b) 300

(c) 500 and (d) 700 iterations (i) Without nanoparticles (ii) 10 nm (iii) 20 nm

(iv) 30 nm (v) 40 nm (vi) 50 nm diameter of Nanoparticles

Figure 3.21 shows patterns of electrical tree generated under the change of the diameter of

nano particle. It is clear from Figure 3.19 that in case of neat polymer, direct tree channel

propagates towards the ground electrode but in the case of nanocomposites the degree of

branching is comparatively high, clearly indicating the difference in the mechanism of tree

propagation in case of nanocomposites. Also the degree of branching increase with the increase

46

in the diameter of the nano particles and tree propagates through multiple channels when

diameter of nanoparticles is higher or equal to 30nm.

Figure 3.21: Typical tree generated in the presence of nano particles of different size

(a) Without nanoparticle (b) 10 nm (c) 20 nm (d) 30 nm (e) 40 nm (f) 50

nm diameter of nanoparticle

The comparison of treeing propagation rate with nano particles of various diameters is done

on the basis of number of iterations required for tree to reach the ground electrode. Figure 3.22

shows the variation of number of iterations with change in diameter of nano particles.

Figure 3.22: Variation in the number of iterations corresponding with the change in

diameter of nano particles

It is clear from Figure 3.22 that the number of iterations is high when fillers are present in

the dielectric, which clearly indicates that electrical trees have lower propagation rate in case of

nanocomposites as compared to neat polymer. The slow propagation rate in nanocomposites

10 20 30 40 50600

700

800

900

1000

1100

Nu

mb

er

of

Ite

rati

on

s

Diameter of Nano Particles

With fillers

Without fillers

47

may be because of the enhanced local conductivity in the region around each nanoparticle,

which provides mechanism for redistribution of charge around the nanoparticle and thus slowing

the propagation of electrical tree.

Figure 3.23: Variation in the velocity of tree propagation with the length of tree

Figure 3.23 shows the variation in the velocity of tree propagation. Irrespective of size and

the presence of nano particles in polymers, a sharp rise in the velocity of tree propagation is

observed near the point of breakdown. It is also observed that in case of neat polymer, the

velocity of tree propagation is constant till the tree grows to 416 nm in length, while in case of

nanocomposites, a reduction in the velocity of tree propagation is observed for 416 nm of tree

growth. It is also evident from the Figure 3.22 and Figure 2.23 that the tree propagation is

significantly delayed when diameter of nanofillers is greater than or equal to 30 nm. The

possible reason for this delay may be the reduction in inter particle distance. As the diameter of

nanoparticle increases, the inter particle distance decreases, which makes the propagation of tree

more difficult and delays the process as a result. M.G. Danikas and T. Tanaka (2009) have

discussed in detail about electrical treeing and breakdown phenomenon in nanocomposites. They

have shown experimental evidence of the obstruction created by the nanoparticles to electrical

tree propagation and have observed zig-zag tree path because of this obstruction, delaying the

tree growth. They have also stated that the decrease in inter particle distance has positive effect

on tree delay to reach to the ground electrode. Also it is noticed that when the tree reaches near

to ground electrode, the rate of propagation is high.

48

CHAPTER 4

CONCLUSIONS

4.1 GENERAL

This chapter presents the results obtained based on the methodical experimental study and

the theoretical analysis carried out to understand incipient discharges in cryogenic insulation

structure. The results obtained are thoroughly analyzed and the interpretation is made on

comparison with the earlier work carried out by other researchers. The important conclusions

arrived at based on the present study are narrated in this chapter.

In general, the conclusions were broadly classified under following sections

(i) Analysis of corona discharge activity in liquid nitrogen (with and without barrier in the

electrode gap) under AC/harmonics AC voltages adopting UHF technique.

(ii) Analysis of corona discharge activity in liquid nitrogen under harmonic AC voltages.

(iii) Analysis of treeing phenomena in cable insulation at cryogenic temperature under

harmonic AC voltages.

(iv) Modelling of electrical treeing in composite insulation through Comsol.

It is well known that most of the studies on corona activity in liquid nitrogen are based on

conventional partial discharge measurement studies. In the present work, the author has made an

attempt to understand corona activity in liquid nitrogen in presence of barrier in the electrode

gap and the important conclusions are narrated below.

1. In short electrode gap with liquid nitrogen as insulant, irrespective of position of barrier,

the corona inception voltage is almost the same and its value is much higher than the

clean electrode gap. Also, the breakdown voltage has increased when the barrier is

positioned near to the high voltage electrode. It is also noticed that the breakdown occurs

through the barrier when they are placed near ground electrode.

2. No variation in corona inception voltage is observed with increase in harmonic voltages

with different THD’s. The magnitude of UHF signal formed at the point of corona

inception, irrespective of harmonics AC/harmonics AC voltages, is nearly the same.

3. The current signal injected due to corona discharge in liquid nitrogen insulation, at the

point of its inception and during streamer formation, have rise time, in the negative and

49

positive cycle of the AC voltage are about 2.9 ns and 2.5ns respectively and the

characteristics has not altered due to barrier insulation.

4. The UHF signals generated by corona discharges under AC/harmonics AC voltages, on

corona inception and at higher voltages, in the liquid nitrogen electrode gap (with barrier)

have frequency contents between 0.5 -1.5 GHz.

5. The magnitude of UHF signal generated on corona inception and at higher voltages (not

at point of near breakdown) shows inverse relationship.

6. Zero span analysis indicates number of discharges that occur per sec in electrode gap

(with barrier), at the point of corona inception is almost the same.

7. PRPD pattern formed due to corona discharges indicates that phase window at which

corona discharge occurs became narrow with barrier in the electrode gap. Also, the phase

window the corona discharge occurs (with or without barrier insulation) at the point of

inception is less compared with discharges at higher voltage.

8. Ternary diagram clearly indicates that UHF signal generated under harmonic AC

voltages, due to corona discharges in liquid nitrogen, its location has not altered.

9. Phase resolved partial discharge studies clearly shows that corona activity in liquid

nitrogen is predominant when the rate of rise of supply voltage is high.

Electrical treeing is one of the major problems in solid insulating material. The tree growth

in XLPE cable insulation at liquid nitrogen temperature especially under harmonic AC voltage,

is scanty. The author has carried out a methodical experimental study and the important

conclusions arrived at based on the present study are the following.

10. Fibrillar type electrical tree forms in XLPE cable immersed in liquid nitrogen specimen

under 50 Hz AC and harmonic AC voltages.

11. Presence of fourth and fifth harmonics in supply voltage can reduce the life of cable

insulation due to treeing.

12. The applied voltage wave shape and its peak factor impart high influence on life of cable

insulation due to treeing.

13. The frequency content of the UHF signals formed during tree growth in liquid nitrogen

immersed XLPE cable lies in the range of 0.5-1.5 GHz.

50

14. PRPD pattern obtained using spectrum analyser indicates that during tree growth, under

50 Hz AC voltage, discharges occur near zero crossing. Similarly under harmonic AC

voltages, discharges predominate when the rate of rise of voltage is high. Under very low

frequency voltages (1 Hz), the discharges occur at the peak of AC voltage.

It is well known that studying the characteristics of electrical trees in an insulation structure

in laboratory is a cumbersome and time consuming process. Hence, it is essential to model the

electrical trees and the important parameters governing the growth process ought to be identified

and analyzed. Based on the present study, the following important conclusions could be arrived

at.

15. It is observed that simulated electrical trees are much similar to the experimentally

generated tree structures.

16. In homogeneous insulation structure, it is observed, tree-like trees and bush-type trees are

formed by simulation.

17. The rate of tree propagation of the electrical trees is high at the time of inception and

then the growth is limited

18. The presence of nano particles in polymers can significantly delay tree propagation and the

breakdown.

19. In case of neat polymer the tree channel propagates towards the ground electrode. While in

case of nanocomposites with large diameter of nanoparticles, multiple tree channels

propagate towards ground electrode thereby enhancing the breakdown process.

20. Volume of damage and branching from the main tree channel is higher in case of

nanocomposites.

21. Inter particle distance can significantly influence the treeing phenomenon and process of

breakdown. Breakdown process is delayed when the inter particle distance decreases.

4.2 SCOPE OF THE FUTURE WORK

Identification of incipient discharges is one of the major challenges in liquid nitrogen. Most

of the studies carried out by researchers have restricted with PD identification with one type of

defect, under AC voltage. An attempt has to be made to understand the PD activity with multiple

defect and localization and classification need to be carried out using non-intrusive type sensor

signals.

51

To enhance the reliability of insulation in cryogenic power apparatus, it is essential to adopt

modern sensor and communication techniques and monitor the condition of the insulating

material during operation.

52

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LIST OF PUBLICATIONS BASED ON THE THESIS

I. REFEREED JOURNALS

1. Mittal, Lakshya, Sarathi, R., and Sethupathi, K., Electrical Treeing in XLPE cable insulation

at cryogenic temperature under harmonic AC voltages. Cryogenics (Accepted for publication).

II. PRESENTATIONS IN CONFERENCES

1. Sarathi, R. and Mittal, L., Understanding corona discharge activity in liquid nitrogen under

harmonic AC voltages adopting UHF technique. Proceedings of the 2014 International

Symposium on Electrical Insulation Materials (ISEIM 2014), Niigata, Japan, June 1-5, pp. 164-

167.