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2018 UHVnet Colloquium

PROGRAMME

15th - 16th January 2018 Winchester Guildhall, UK

Dielectrics and Electric Insulation Society

The Institute of Electrical and Electronics Engineers, Inc.

Sponsored by:

Contents

Section Pages

Welcome 3

Schedule 4-5

Oral Presentation Abstracts 6-21

Poster Presentation Abstracts 22-44

Welcome Welcome to the 11th UHVnet colloquium hosted by the University of Southampton on the 15th and

16th of January 2018. This is the second time the UHVnet is visiting Winchester Guildhall and this

meeting will consist of registration and poster session on the Monday evening to encourage relaxed

discussion of current work between early career researchers. The second day of the event will start

with a keynote lecture by Iliana Portugues, Head of Innovation at National Grid, followed by four

sessions with oral presentations, covering diverse topics from High Voltage Plant, Condition

Monitoring, Dielectric Materials and Theories, to Methods and Models.

UHVnet is an informal grouping of universities and was set up in 2005 to further interests of HV

research within the United Kingdom. The university members are Cardiff University, Glasgow

Caledonian University, University of Liverpool, University of Leicester, University of Manchester,

University of Southampton, University of Strathclyde and the University of Surrey. These institutions

are supported by a steering group which includes industrial representation from HFDE, GE Grid

Solutions, National Grid, Narec and Ricardo.

Objectives of the group include raising awareness of the researcher capabilities of group members

to UK HV related industry, particularly manufacturers and electricity supply companies and lobbying

research funding organisations for ear-marked high-voltage related programs.

We would be delighted to receive any feedback about this event as we are keen to further improve

our communication with both UK and overseas stakeholders. Please visit uhvnet.org.uk for updates

and the most recent information.

3

Schedule

Monday 15th January, Keats Room 18:00 – 19:30 Registration and Poster session

Tuesday 16th January, Bapsy Hall 8:30

Welcome

9:00

Keynote Address

10:00

Oral session 1. Electrical Treeing in HVDC Cable Insulation - M. Azizian Fard et al. 2. Electrical Tree Growth by Partial Discharges within a Spherical Cavity

under Accelerated Life Tests - T. Tanmaneeprasert et al. 3. Ageing and Failure Mechanisms of Electrical Materials for High Voltage

Systems in Aerospace Applications - H. Haghighi et al. 4. Advanced On-line Condition Monitoring and Inter-turn Short Circuit

Detection of Power Transformers - E. Aburaghiega et al.

11:20

Break

11:40

Oral session 1. Review of the accuracy of the IEC method for calculating the induced

losses in HVAC Submarine Cables in the absence of armour - D. Chatzipetros et al.

2. Offshore Cable Optimization by Probabilistic Thermal Risk Analysis - M.A. Hernandez Colin et al.

3. Finite Element Modelling of Gas Insulated Bus for Very Fast Transients - J. James et al.

4. Characterisation of a Corona-stabilised Switch Filled with Environmentally-friendly Gas Mixtures - R. MacPherson et al.

13:00

Lunch

14:00

Oral session 1. The effect of thermal history on the electrical properties of

polypropylene based nanocomposites - A. V. Shaw et al. 2. The Effect of Temperature on DC current of Silica Based Nanocomposites

- Y. Wang et al. 3. Schlieren Study of Streamer Initiation and Propagation in Insulating

Liquids - S. Shen et al.

15:00

Break

4

15:15

Oral session 1. Electric Field Effects on Boiling Heat Transfer of Thermosyphon Coolant -

S. Wu et al. 2. Chemometrics in the Study of Liquid Dielectrics - H. Herman et al. 3. Self-healing dielectric fluids for autonomous repair of fluid filled cables -

S. Basu et al.

16:15

Close

5

Oral Presentation

Abstracts

UHVnet 2018 Winchester, UK

Electrical Treeing in HVDC Cable Insulation

M. Azizian Fard*1, Emad Farrag1, Scott McMeekin1 and Alistair Reid2 1Glasgow Caledonian University 2Cardiff University

*E-mail: [email protected]

Power cables are exposed to various stresses in their service lifetime due to loading and operational conditions,

which can degrade the insulation and can even result in premature failure depending on the severity of the

deterioration. Treeing phenomenon is one of the main deleterious processes that can result in early failure of

polymeric cables. This study presents an investigation of the effects of continuous, non-continuous and polarity

reversal scenarios of DC voltages on electrical tree behaviour within test samples of HVDC XLPE insulation.

Double needle electrode systems were embedded within the insulation samples to produce highly non-uniform

fields acting as the cause of electrical trees. The treeing processes were optically monitored and the

accompanying emanating partial discharge pulses were detected simultaneously. Figure 1 illustrates ET

development under polarity reversal condition. The results show reasonable correlations between ET

formations and the test electric field intensity, type, and dynamics. Accordingly, it is found that polarity

reversal of the applied test voltages has considerable influence on the development of ETs compared to the

unipolar DC voltages. Owing to the effects of such highly divergent electric field sources within the insulation

and the subsequent behaviour of the originating ETs, consideration of this phenomenon is of critical

importance in the design and diagnostics of insulation systems operating under such HVDC conditions for the

purpose of asset management.

Figure 1: Electrical tree growth under polarity reversal.

[1] Dissado, L. A., Fothergill, Electrical Degradation and Breakdown in Polymers, 1st ed., Peter Peregrinus Ltd., London,

1992.

[2] L. A. Dissado, "Understanding electrical trees in solids: from experiment to theory," in IEEE Transactions on

Dielectrics and Electrical Insulation, vol. 9, no. 4, pp. 483-497, Aug 2002.

[3] P. Morshuis, A. Cavallini, D. Fabiani, G. C. Montanari and C. Azcarraga, "Stress conditions in HVDC equipment

and routes to in service failure," in IEEE Transactions on Dielectrics and Electrical Insulation, vol. 22, no. 1, pp. 81-

91, Feb. 2015

7


Electrical Tree Growth by Partial Discharges within

a Spherical Cavity under Accelerated Life Tests

T. Tanmaneeprasert*, P.L. Lewin and G. Callender

School of Electronics and Computer Science

University of Southampton

Southampton, United Kingdom


This paper considers partial discharge (PD) activity in an air-filled cavity due to accelerated ageing under

electrical stress. In the ageing process, partial discharges in the cavity can bombard the inner cavity surface to

form chemical by-products, i.e. liquid droplets and protrusions that might lead to the propagation of electrical

trees in the solid dielectric material.

For the experiment methods, a test sample containing a spherical cavity was made using silicone rubber. The

sample was placed between parallel plate electrodes with the upper electrode connected to a 50 Hz HV AC

supply and the lower electrode connected to ground. Both the sample and the electrode arrangement were

immersed in a silicone oil bath in order to eliminate any surface discharges. The voltage level for applying the

ageing test was obtained from the measurement of PD inception voltage of 35 silicone samples of different

cavity diameters between 0.5 mm and 2.42 mm. The measured inception voltage was then increased by 80%

for the purposes of long-term ageing.

The measurement results show that phase-resolved partial discharge (PRPD) patterns can be associated with

evidence of electrical tree growth at the cavity surface as shown in Figure 1. The chemical by-products at the

location of electrical tree generation were closely examined using scanning electron microscope (SEM) and

energy dispersive x-ray (EDX) microanalysis. The physical tree can be modelled to establish the local electric

field using a Finite Element Analysis (FEA) model in order to better understand electrical treeing phenomena.

Figure 1: Electrical tree growth at the cavity surface

after long-term ageing 1070 h.

8


Ageing and Failure Mechanisms of Electrical Materials for High

Voltage Systems in Aerospace Applications

H.Haghighi*1, I.Cotton1 1School of Electrical & Electronic Engineering, University of Manchester, Manchester, M13 9PL,

United Kingdom


Aerospace electrical systems are continuing to increase their voltage levels to meet the on-board power

demands of more-electric aircrafts (MEAs) where hydraulic and pneumatic systems are replaced with electrical

equivalents [1]. Higher power demands are leading to the use of higher voltages and as such it is essential to

explore the behaviour of the insulation system in the aerospace environment. This degradation takes place

when exposed to a varying environment consisting of operating temperatures exceeding 250 °C or as low as -

50 °C, varying air pressure, humidity and ozone. Understanding the impact of these variables on high voltage

insulation systems is crucial in predicting their behaviour over the lifetime of the aircraft.

This project explores the effect of chemical, electrical, mechanical stressors on the failure mechanisms of

commonly used aircraft insulation materials such as polyimide (Kapton), PTFE (Teflon) and PEEK, by varying

environmental parameters of thermally aged thin polymer sheets and twisted wire samples and observing the

degree of degradation. The test conditions to simulate a typical aircraft flight profile include: temperatures of

up to 95 °C, pressures from 1 – 0.1 bar, and humidity between 0 – 100 %RH. Following ageing under oxidising

and inert atmospheres, structural changes are observed through chemical functional group characterisation by

Fourier-transform infrared spectroscopy (FTIR), mechanical tensile strength and elongation measurements,

and permittivity and breakdown strength tests; to indicate the change in the dielectric properties of the

polymeric sheets following multi-stressor ageing. This data will be used to consider the relative life of

insulation systems in an aerospace application against similar ground based systems.

[1] J. A. Rosero, J. A. Ortega, E. Aldabas, et. al., "Moving towards a more electric aircraft", IEEE Aerosp.

Electron. Syst. Mag. no. 22, pp. 3–9, 2007

9

Advanced On-line Condition Monitoring and Inter-turn Short Circuit Detection

of Power Transformers

Ehnaish Aburaghiega∗1, Dr. Mohamed Emad Farrag∗2, Dr. Donald M Hepburn∗3 Dr. Belen

Garcia∗4

Glasgow Caledonian University123, Universidad Carlos III de Madrid, Spain.4.

[email protected]

Abstract

An on-line monitoring system is particularly suitable for utilization with power transformers, with the aim of guaranteeing a reliable electrical power supply in connection with reduced maintenance expenditure and an optimum exploitation of the active part [1]. Furthermore the remaining operating life of the transformer can be estimated by recording important operating data. Understanding the transformer behavior/ internal conditions when a short circuit exists is very important and recently has massive attention from researchers [2]. In the study reported in this paper, experimental work has been conducted for on-line condition monitoring of power transformer based on understanding the internal and external conditions of the transformer when an internal short circuit is occurred. A single phase, two winding transformer: each winding contains 144 turns, subdivided into sections, as shown in Figure 1, is used, The individual sections of the transformer are connected through wires to terminals fitted on the upper panel on the transformer housing, which can be used to make healthy coils and generate different number of shorted turns in different locations. Voltages / currents in both sides of the transformer are continually measured. The transformer has been run as healthy with constant load, afterword, different number of shorted turns has been tested in order to classify and understand the healthy and faulty conditions of the transformer. The transformer parameters such as resistances and inductances are measured using appropriate devices; the measured parameters are used to simulate the transformer using PSPICE software for the results validation. Experimental and simulation results, relating to a number of short circuit winding faults, are analysed and discussed. The results confirm that it is possible to identify a variety of short circuit conditions in primary and secondary windings from externally measurable parameters, i.e. measurement of voltages and currents in a transformer’s windings can, based on available winding data, identify changes inside the transformer. The effect of load variation on the magnitude of the measured voltages and currents is also considered in this analysis.

10

Figure 1: Winding construction, showing discs and number of turns

Reference

[1] S. Tenbohlen and F. Figel , “On-line Condition Montoring of Power Trnaformers,” Power Engineering Society Winter Meeting, IEEE, vol. 3, no. 2, pp. 2211–2216, 2000.

[2] R. Schwarz and M. Muhr, “Diagnostic methods for transformers,” Cond Monit and Diag (CMD). Int Conf on IEEE, pp. 974–977. 2008

11


Review of the accuracy of the IEC method for calculating the induced

losses in HVAC Submarine Cables in the absence of armour

D. Chatzipetros*1, J. A. Pilgrim1

1The School of Electronics and Computer Science (ECS), University of Southampton, SO17 1BJ, UK *E-mail: [email protected]

The number and size of installed Offshore Wind Farms (OWFs) have grown rapidly in recent years.

Submarine, HVAC cables are required to transmit the power generated offshore to the mainland and they are

of particular importance in terms of both the reliability and sustainability of the entire project. The temperature

limit of 90oC set for most XLPE insulated cables imposes the relevant losses being as low as possible. IEC

60287 [1] is to date used to calculate the induced losses generated inside the cable. Although the Standard

method has been strongly criticised as overestimating the armour loss [2], [3], less work has been published in

terms of its accuracy around the unarmoured cable. The present study employs 2-D Finite Element (FE) models

to evaluate how accurate the Standard method is, and to identify reasons for any deviations. As it is shown,

IEC 60287 seems to overestimate the losses induced in the metallic sheaths of unarmoured cables. This is

primarily due to the significance of the proximity effects in such cables, which are not fully accounted for by

the standard. This overestimation becomes larger when increasing the conductor size, a fact particularly

significant for the forthcoming OWF projects involving increasingly larger cable sizes.

Figure 1: The proximity effect in phase conductors affects the

losses induced in the metallic sheaths and is not currently

considered by [1].

[1] IEC 60287, “Electric cables - calculation of the current rating - part 1-1: Current rating equations (100%

load factor) and calculation of losses - general.”, Technical report, International Electrotechnical

Commission, 2014-11.

[2] J. J. Bremnes, G. Evenset, R. Stølan. “Power Loss And Inductance Of Steel Armoured Multi-Core Cables:

Comparison Of IEC Values With “2,5D” FEA Results And Measurements.” B1-116, CIGRE 2010.

[3] Kevin F. Goddard, James A. Pilgrim, Richard Chippendale, and Paul L. Lewin. “Induced Losses in Three-

Core SL-Type High-Voltage Cables.” IEEE Transactions on Power Delivery, Vol. 30, No. 3, June 2015.

12


Offshore Cable Optimization by Probabilistic Thermal Risk Analysis.

M.A. Hernandez Colin*1, J.A. Pilgrim1 1 University of Southampton, UK


The offshore wind industry in the UK reported a total of 5GW of installed capacity by the end of 2016, which

represented 5% of the total energy generated during that year. An additional 5.3GW of new capacity is under

construction and could represent 10% of the energy produced by 2020 [1]. Given the fast development of the

offshore industry as well as the challenging energy prices set in the Contracts for Difference (CfD) auction

during this year [2], the optimization of the transmission capacity in the export cables becomes an important

strategy for the reduction of the overall cost of the energy generated offshore.

Traditionally, power cables are sized using continuous ratings which assume the worst-case weather conditions

that generate underestimated ratings that limit the real capacity of the cable. The existing alternative is the use

of dynamic rating methodologies which consider more realistic weather conditions that can give room for

increasing the amount of power transferred in the cables [3]. However, in the case of an offshore wind farm

the energy generated is a product of the wind speed variations which produce a dynamic load current that

changes in short periods of time. In order to increment the cable ratings without exceeding the cable

temperature limit, it is necessary to estimate the load current and conductor temperature several hours ahead

and analyse the likely risk of exceeding the cable maximum temperature.

This study uses probabilistic methodologies and historical data from an offshore location to generate an

algorithm for the estimation of cable temperature exceedance 6, 12 and 24 hours ahead considering an overload

of 7%, 9.9% and 13.7% in the export cable system. A probabilistic conductor temperature risk is calculated

based on the likely load that the system could experience according to the historical data using a Monte Carlo

Analysis. The methodology generates hours ahead information that can be used as a decision tool to avoid

curtailment of wind power when the energy generated is higher than the continuous rating but the risk of

exceeding the conductor temperature limit is non-existent.

[1] The Crown State, “Offshore wind operational report January-December 2016”, Offshore Wind Operational

Report, 2017.

[2] Department for Business, Energy and Industrial Strategy, “Contracts for Difference Second Allocation Round

Results”, www.gov.uk/government/publications/contracts-for-difference-cfd-second-allocation-round-results,

2017.

[3] Walldorf, S.P., J.S. Engelhardt, and F.J. Hoppe, “The use of real-time monitoring and dynamic ratings for power

delivery systems and the implications for dielectric materials”. IEEE Electrical Insulation Magazine, 1999.

15(5): p. 28-33

13


Finite Element Modelling of Gas Insulated Bus for Very Fast

Transients 1Jonathan James 2 Dr Maurizio Albano

3Prof Manu Haddad


Very Fast Transients (VFT) are a complex phenomenon, most commonly associated with disconnector

operations within Gas Insulated Substations (GIS). They are a consequence of the rapid breakdown speed of

Sulphur Hexafluoride (SF6). During disconnector operation, multiple breakdowns of the gas will occur

between contacts, exciting electromagnetic waves that propagate throughout the system, reflecting and

refracting at discontinuities. The superposition of VFTs can result in Very Fast Transient Overvoltage (VFTO),

reaching magnitudes of up to 2.5pu at frequencies up to 100MHz [1]. VFTOs are often associated with

insulation degradation and even flashover of insulating spacers. In addition to the issues associated with

internal transients, VFTs that refract externally, continue to propagate via enclosure modes. These external

transients are known as Transient Enclosure Voltage (TEV), presenting both an EMC and shock hazard [1].

Modelling for this class of transients is traditionally carried out using circuit based methods such as that

employed by EMTP. These models can be very detailed but will often include several assumptions, for

example, a capacitance between the HV conductor and enclosure is used to represent the coupling and

generation of TEV [1]. Finite Element Modelling (FEM) of GIS components provides a clearer insight into

the propagation and effects of these transients. However, at higher frequencies, absolute voltages and currents

are not well-defined and it is more suitable to consider these transients in terms of the propagation of

electromagnetic waves by solving Maxwell’s equations. This can be elucidated through FEM for

electrodynamic fields by application of the curl-curl equations [2]. The results demonstrated in Figure 1 were

produced using the RF Module of COMSOL Multiphysics® [3].

In addition to the propagation of internal transients, through a similar process, it is possible to compute the

coupling of internal and external transients. Limited guidance exists for the representation of coupling for

circuit based models, however using FEM, a more accurate representation can be realised; results of which can

be used to enhance circuit models. Finite element based models can also be used for accurate computation of

field interactions for actual geometries and subsequently for the development of mitigation or measurement

devices.

Figure 1: 2D plot showing normalised electric field

propagation through a section of GIS.

[1] J. A. Martinez, P. Chowdhuri, R. Iravani, A. Keri, and D. Povh, “Modeling Guidelines for Very Fast

Transients in Gas Insulated Substations,” Chapter 6-Modeling and Analysis of System Transients

Using Digital Programs Part 2, 1998.

[2] J. Smajic, W. Holaus, J. Kostovic, and U. Riechert, “3D full-Maxwell simulations of very fast

transients in GIS,” in IEEE Transactions on Magnetics, vol. 47, no. 5, pp. 1514–1517, 2011.

[3] COMSOL Multiphysics® v. 5.3. www.comsol.com. COMSOL AB, Stockholm, Sweden.

14


Characterisation of a Corona-stabilised Switch Filled with

Environmentally-friendly Gas Mixtures

R.W. Macpherson *, M.P. Wilson, S.J. MacGregor, I.V. Timoshkin, M.J. Given, T. Wang

Dept. Electronic and Electrical Engineering

Royal college building

204 George Street, Glasgow

G1 1XW

*[email protected]

Sulphur hexafluoride (SF6) has traditionally been used as a switching medium within corona-stabilised

switches (CSS) [1] due to its high breakdown strength and strong electronegative properties. However, SF6 has

high global warming potential (GWP). Therefore, other gases with a lower environmental impact are being

considered in order to find a suitable alternative that can be used within CSS, without compromising on

switching performance. This paper reports results obtained using a CSS filled with the refrigerant 1,3,3,3-

tetrafluoropropene, known as HFO-1234ze, as the basis of the operating gas. The global warming potential

(GWP) of this gas is 6 in a 100-year time horizon, compared to SF6 with a value of 23900. The electronegativity

of HFO-1234ze can make it an attractive alternative to SF6 for switching applications.

The performance of the triggered CSS has been characterised in terms of triggering range, delay time and jitter,

with a HVDC applied voltage when filled with nitrogen (N2) as a reference fluid. Tests were then performed

with HFO-1234ze in various mixtures with N2 as a buffer gas: 5%/95%, 10%/90% and 20%/80%

HFO-1234ze/N2. The presented results provide data on the feasibility of the approach of using HFO-1234ze

as the operating dielectric gas in corona-stabilised plasma closing switches. Examples of negative self-

breakdown voltage and triggering range data are shown in Figure 1 for the three different gases mixtures tested,

as well as 100% N2 for reference. Under certain conditions, the breakdown voltages achieved were seen to

increase by a factor of 3 compared to those for 100% N2.

[1] J M Koutsoubis, S J Macgregor - Effect of gas type on high repetition rate performance of a triggered, corona-stabilised

switch [Journal]. - Glasgow : IEEE Transactions on dielectrics and electrical insulation, April, 2003. - 2 : Vol. 10.

[2] M P Wilson, I V Timoshkin, S J Macgregor, M J Given, J M Lehr - Characterisation of a triggered corona-stabilised

switch in dry air [Journal] // Proceedings of the 18th international conference on gas discharges and their applications,

Grieswald, Germany. - Grieswald, Germany : Proceeding of the 18th int conference on gas discharges and their

applications, 2010. - pp. 474 - 477.

Figure 1 - Negative polarity self-breakdown (SB) voltage and triggering

threshold voltage (TV) data for 100% N2, as well as 5%/95%, 10%/90%

and 20%/80% HFO-1234ze/N2 mixtures from 0-3 bar gauge.

0 0.5 1 1.5 2 2.5 35

10

15

20

25

30

35

40

Pressure (Bar)

Voltage (

kV

)

SB N2

TV N2

SB 5%

TV 5%

SB 10%

TV 10%

SB 20%

TV 20%

15


The effect of thermal history on the electrical properties of

polypropylene based composites

A. V. Shaw*1, S. T. H. Virtanen1, T. Andritsch1 and A. S. Vaughan1 1 The Tony Davis High Voltage Laboratory, University of Southampton, SO17 1BJ, UK

*E-mail: Allison Shaw, [email protected]

An organically modified MMT clay (OMC) has been incorporated into blends of isotactic polypropylene (iPP)

with ethylene vinyl acetate (EVA). The resulting materials are highly sensitive to thermal treatment upon

cooling from the melt phase. Two thermal conditions were investigated: quenched or isothermally cooled at

120 °C.

In particular, iPP is known to form large spherulites which increase rigidity and reduce electrical strength. [1]

When the material is quenched, the spherulites are prevented from forming extensively which changes the

electrical properties of the films compared with isothermally cooled samples – a condition more conducive to

the formation of spherulite structures.

In this investigation, two clay loadings were studied alongside an equivalent but unfilled blend. The solution

blending synthesis was conducted to achieve clay loadings of 2.5 wt % and 5 wt%. Thermal gravimetric

analysis (TGA) was used to confirm the clay loading in the films to be accurate to that desired from the

synthesis. TGA also revealed that the presence of clay increases the thermal degradation temperature of the

blend. Thin film samples were investigated using techniques to establish the crystallinity, namely X-ray

diffraction and differential scanning calorimetry. The electrical properties of the thin films was investigated

using AC dielectric breakdown strength and DC conductivity. The quenched samples were found to have a

higher breakdown strength and lower DC conductivity than samples that were isothermally cooled at 120 °C

irrespective of the presence of clay. The different crystalline structures obtained from the cooling methods are

responsible for such differences in the electrical properties and this work seeks to quantify this.

[1] I. L. Hosier, A. S. Vaughan, S. G. Swingler, “An investigation of the potential of polypropylene and its blends for

use in recyclable high voltage cable insulation systems”, Journal of Material Science, vol. 46, no. 11, pp. 4058-4070,

2011

16


The Effect of Temperature on DC current of Silica Based

Nanocomposites

Yan Wang *1, Darren Qiang1, George Chen1 and Alun Vaughan 1 1The Tony Davies High Voltage Laboratory, University of Southampton


During last decade of the research about nanocomposites, many convinced results have shown that some

electrical properties, for example, space charge, treeing lifetime and PD resistance can be improved by loading

nanofillers appropriately. However, other electrical properties such as DC current is still full of uncertainty

which is one of limitation to the practical application of nanocomposites. Apart from this, DC current

measurement is also important for further understanding the space charge accumulation and movement

mechanism in the nanocomposites. Recent results suggest that the presence of deep traps is the key to reveal

how the nanoparticle loading ratio influence the conduction in the nanocomposites. In the present paper, DC

current of polyethylene nanocomposites containing different nanosilica loading ratios including 0.5wt%, 2wt%

and 5wt%, either untreated or surface treated using the trimethoxy(propyl)silane coupling agent, has been

investigated as a function of temperature. Basing on the different temperature during the DC current

measurement, the behaviour of nanocomposites measured under higher temperature 313K and 333K is

obviously different compared with the one measured under room temperature 293K. In addition, the

nanocomposites filled with the higher loading ratio or any specimen measured under high temperature

possesses relative higher conductivity initially and followed with rapidly decreasing. Both of these two results

suggested that the amount of charge trapping sites and temperature which can change charge dynamics will

influence the conductivity of nanocomposites and neat polyethylene. Moreover, comparing with the effect of

loading ratios, the temperature which is higher than the room temperature 293K has more significant impact

on dominating the conduction of nanocomposites.

Figure 1: The DC current of 0.5wt% C3 treated

nanocomposites as a function of temperature

[1] W. Yan, X. Zhiqiang, C. George, and A. Vaughan, "DC current in nanosilica-based polyethylene nanocomposites,"

in 2015 IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), 2015, pp. 515-518.

[2] G. Chen, L. Shengtao, and Z. Lisheng, "Space charge in nanodielectrics and its impact on electrical performance," in

Properties and Applications of Dielectric Materials (ICPADM), 2015 IEEE 11th International Conference on the,

2015, pp. 36-39.

[3] K. Lau, A. Vaughan, G. Chen, I. Hosier, and A. Holt, "Absorption current behaviour of polyethylene/silica

nanocomposites," in Journal of Physics: Conference Series, 2013, p. 012003.

17


Schlieren Study of Streamer Initiation and Propagation in Insulating

Liquids

Shuhang Shen, Qiang Liu* and Zhongdong Wang

School of Electrical & Electronic Engineering, The University of Manchester

Manchester, M13 9PL, United Kingdom


Abstract – Transformers are key links in power system to provide stable and high-quality power supply. Most

power and distribution transformers in UK and worldwide are oil-filled type, and transformer oil plays a

prominent role in transformer insulation integrity [1]. Electrical pre-breakdown phenomenon in liquids,

commonly termed as streamer, is normally acknowledged as being initiated on a pre-formed gaseous cavity

(in sub-microsecond range) [2]. A comprehensive understanding of streamer formation and development

mechanism will be helpful to better understand the failure process of transformers, and to improve its insulation

design. Most streamer studies have been made under non-uniform or quasi-uniform field. Nevertheless, a semi-

uniform field study could be helpful to understand streamer initiation conditions by a protrusion on a plane

electrode [3].

In this study, Schlieren imaging technique is adopted as the optical approach to visualise streamer, due to its

higher sensitivity compared to conventional shadowgraph method [4]. The experimental setup utilising

Schlieren technique is demonstrated first. A fast solid-state switch together with a 90kV DC voltage source

operates as the impulse generator. The standard 1.2/50µs impulse voltage is then applied to a stainless-steel

test cell accommodating point-plane electrodes. A 500J flash light serves as the light source and streamer

images are captured by a SIM16® high-speed camera; current and voltage signals are recorded at the same

time. To synchronize the system, the triggering signal for camera and impulse generator is delayed 150µs from

triggering signal for flashlight. Preliminary test results of streamer images are presented here. A comparison

between streamer images obtained by Schlieren and shadowgraph techniques respectively is demonstrated.

Furthermore, the characteristics of positive streamer propagation at 10mm gap distance under needle-plate and

plate-needle-plate electrode configurations are investigated. The influence of needle protrusion over paralleled

plate electrodes on streamer characteristics is illustrated and discussed.

DC

Rc

Rs1

Cc

A

B

Rs2

Rf

Rt

Current

Shunt

Flash head

SIM16Cam

OscilloscopeControl

box

Aperture

Impulse

Generator

High Voltage

DeviderSchlieren Optical

System

Cubic test cell

with point-plane

electrodes inside

5V Signal

Generator

Pulse

Generator

#1

Trigger

Module

Record & Analysis

System

Pulse

Generator

#2

150µs delay

Figure 1: Schematic diagram of experimental setup of

Schlieren streamer imaging system

[1] M. J. Heathcote, "The J & P transformer book," Elsevier Ltd, Printed in Great Britain, 2007.

[2] B. Peter and L. Christophe, "Non-thermal plasmas in and in contact with liquids," Journal of Physics D: Applied

Physics, vol. 42, p. 053001, 2009.

[3] T. V. Top and A. Lesaint, "Streamer initiation in mineral oil. Part II: influence of a metallic protrusion on a flat

electrode," in IEEE Transactions on Dielectrics and Electrical Insulation, vol. 9, no. 1, pp. 92-96, Feb 2002.

[4] G. S. Settles, Schlieren and Shadowgraph Techniques: Springer-Verlag Berlin Heidelberg, 2001.

18


Electric Field Effects on Boiling Heat Transfer of Thermosyphon

Coolant

S. Wu*1, R. D. Chippendale1, P. L. Lewin1, J. Hemrle2 and L. Kaufmann2 1University of Southampton, Southampton, United Kingdom

2ABB Switzerland Ltd., Baden-Dattwil, Switzerland


In order to improve overall HV plant efficiency, there is increased use of thermosyphon technology to provide

temperature control, replacing conventional pumps, fans and radiators [1]. Thermosyphon operates on a closed

two-phase cycle that utilises latent heat of boiling to dissipate thermal energy. Hence, thermosyphon

performance is according to coolant boiling behaviour.

Literature has shown electrohydrodynamic (EHD) is an effective strategy for increasing two-phase system heat

transfer substantially [2]. This study presents a series investigations on the EHD heat transfer mechanisms,

which coexist in a d.c. electric field.

Furthermore, an experimental platform has been constructed. It aims to investigate the influence of a d.c.

uniform electric field on boiling heat transfer characteristics of potential coolants. In this approach, an HV

mesh plane and a novel grounded electrode were applied to create a uniform electric field. A high-speed image

system was selected to observe the coolant boiling stages. A temperature control system was designed to

maintain the coolant saturate temperature. The essential component of this experiment is a novel grounded

electrode, which is shown in Figure 1. It can be seen that this electrode contains a temperature sensor array

and a ‘vacuum jacket’.

The unique grounded electrode has been designed and manufactured with the aid of a thermal Finite Element

Analysis (FEA) model. Both FEA simulation and initial experimental results indicate there is a potential to

use this unique experimental system to investigate the uniform d.c. electric field effect on boiling heat transfer

of both coolants using in the thermosyphon.

Figure 1 Cross-section of novel grounded electrode with three temperature array

[1] F. Agostini, M. Habert, F. Molitor, R. Flueckiger, L. Kaufmann, A. Bergamini, M. Rossi, and S. Besana,

“Double-loop thermosyphon for electric components cooling,” IEEE Trans. Components, Packag. Manuf.

Technol., vol. 4, no. 2, pp. 223–231, 2014.

[2] P. Wang, P. L. Lewin, D. J. Swaffield, and G. Chen, “Electric field effects on boiling heat transfer of liquid

nitrogen,” Cryogenics (Guildf)., vol. 49, no. 8, pp. 379–389, 2009.

19


Chemometrics in the Study of Liquid

Dielectrics

H. Herman1, N. Freebody1, I. L. Hosier2, G.C. Stevens1 and A. S. Vaughan2 1Gnosys Global Ltd 2Tony Davis HV Laboratory


Chemometrics is the science of applying multivariate mathematical and statistical analysis methods to the

extraction of maximal information from complex data sets and the determination of inter-relationships between

variables controlling outcomes and properties.

In this paper we illustrate the application of these methods to a detailed dataset obtained on the oxidative and

inert pyrolytic thermal aging of dodecylbenzene (DDB) liquid dielectrics. This data set formed part of

extensive work by the University of Southampton on the ageing of DDB containing insulation systems. This

dataset covers isothermal ageing from 90 to 135 °C with ageing time extending up to 3000 hours in air and

nitrogen in the presence of aluminium, copper, paper or copper and paper which relate to the interest in these

materials on ageing effects in power transformers and fluid filled cables. The DDB studied was largely

comprised of common mixed isomer fluids but data was also examined for single isomer DDB.

The chemometrics application operates on the infrared spectrum and relates the spectral information to the

change in fluid chemistry on ageing and on resulting fluid metrics such as dielectric permittivity and loss, acid

number and water content. We show how chemometric measurement models can be established and used to

determine the precise condition of thermally aged fluids.

The paper will conclude with a view of the future potential application of these methods in dielectric fluids

research, manufacturing support and asset condition assessment linked to improved asset management.

Figure 1: Principal Component Ageing Trajectory Plot,

showing both 90C (blue) and 105, 135C (red) pathways

categorized by ageing times in hours.

[1] B Lavine and J Workman, “Chemometrics”, Anal. Chem. 2010, 82(1), pp 4699-4711

[2] R L Huynh, F J Davis, D Patel and A S Vaughan, “Degradation of dodecylbenzene under conditions of high electric

field”, J8th Int. Conf. on Dielectric Materials, Measurements and Applications, 2000)

[3] [3] I L Hosier, J E A Koiraj and A S Vaughan, “Effect of Aging on the Physical, Chemical and Dielectric Properties

of Dodecylbenzene Cable Oil”, ICD 2016: 1st Int. Conf. on Dielectrics, Montpellier, FR, 03 - 07 Jul 2016. IEEE,

812-815

20


Self-healing dielectric fluids for autonomous repair

of fluid filled cables

S.Basu, I. German, R. Rhodes, and G. C. Stevens*

Gnosys Global Ltd.

*Email: [email protected]

Fluid filled cables (FFCs) are present throughout UK electricity networks as legacy systems. FFCs are

insulated with a layer of tightly lapped cellulosic paper, impregnated with a low viscosity mineral oil that

performs a range of functions. Historically, these cables could sustain a higher operating voltage than polymer-

insulated cables until the 1980s, at which point most were replaced. Currently, there are ~7,860km of FFCs

remaining in the UK[2] operating substantially beyond their intended service life, and the fragility of these

cables is of growing concern.

If an FFC is damaged, oil will leak from the insulation layer into the surrounding environment. If a leak

continues unchecked, the loss of oil pressure will lead to the formation of voids in the insulation and the

eventual failure of the cable. Although lost oil can be easily replenished, the operator may face fines from the

Environment Agency and (if the leak meets certain threshold values) may be required to undertake expensive

unplanned maintenance.

In response, Gnosys has developed a range of self-healing fluids (SHFs) that are capable of providing a number

of protective functions. In the case of low-level leaks, the SHF will solidify within the breach itself, thereby

preventing the further loss of oil. However, should the leak be too severe for sheath healing to occur then the

SHF will then solidify in the surrounding backfill, resulting in the formation of an oil proof barrier that can

substantially reduce environmental contamination. As SHFs must also function as insulation oils, work has

been undertaken to understand the rheological and electrical properties of these fluids, and the results so far

are highly promising.

As a result of the initial investigative work, Gnosys is now undertaking a large scale project that will assess

the performance of the SHFs under cable operating conditions. This will take place firstly in the laboratory,

using bespoke rigs capable of holding ex-service cables, before proceeding to trials on decommissioned circuits

in preparation for commercial roll-out across the UK.

Figure 1. (Left) Leak rate against time for samples of backfill treated with SHF and T3788 (Right) Sample of backfill

treated with SHF and subsequently challenged with T3788. The formation of a permanent reservoir demonstrates that the

SHF has formed an oil proof barrier.

[1] N. Hampton, R. Hartlein, H. Lennartsson, H. Orton, R. Ramachandran, 2007, “Long-life XLPE insulated power cable”, Jicable07.

[2] Ofgem FOI request; response received 16-October-2017

21

Poster Presentation

Abstracts


The Effect of Bridging Formation under Standard Lightning

Impulse Voltage.

S.Saaidon1, G. Chen, I.O. Golosnoy

The Tony Davies High Voltage Laboratory

School of Electronics and Computer Science


SO17 1BJ United Kingdom * [email protected]

This paper described the breakdown strength of transformer oil impurity when subjected to bridging formation

under the influence of standard lightning impulse stress 1.2/50µs. The cylindrical test vessel fitted with

spherical-spherical electrodes system configuration with 2mm gap distances has been developed in order to

generate a quasi-uniform field as well as to obverse a bridging formation across the gap. Therefore, as a well-

controlled processes is necessary to avoid external impurities, commercial cellulose microcrystalline powder

of 20µm size used as a bridging particle to contaminate the mineral oil with 0.004%, concentration level of

fibres dust by weight. The rising voltage method of IEC60897 is applied to measure the Lightning Impulse

Breakdown Voltage (LIBV). To subsequent analysis, the Weibull Probability Distribution Function is applied

to determine the 50% breakdown probability under the oil condition of clean, contaminated and bridging

formation. Based on the experimental results, the cellulosic powder particle reduced the breakdown strength

of the mineral oil up to 10% reduction. The particle influence on breakdown strength is also more prominent,

with a further 14% breakdown when bridging is formed. It is clear that the presence of bridge structures has

detrimental effects on liquid strength, reducing the strength of insulating oil up to 24% under lightning impulse

stress. Therefore, as a conclusion the fiber immersed in mineral oil when subjected to a bridge and non-bridge

formation clearly affected the electrical performance of the mineral oil by lower its dielectric strength.

Figure 1: Weibull Distribution Plot of the Bridging Influence to

the Breakdown Strength

[1] W. Lu and Q. Liu, “Effect of cellulose particles on impulse breakdown in ester transformer liquids in

uniform electric fields,” IEEE Trans. Dielectr. Electr. Insul., vol. 22, no. 5, pp. 2554–2564, 2015.

[2] Q. Liu and Z. D. Wang, “Streamer characteristic and breakdown in synthetic and natural ester transformer

liquids under lightning impulse voltage,” IEEE Trans. Dielectr. Electr. Insul., vol. 18, no. 6, pp. 1908–

1917, 2011.

[3] Q. Liu and Z. D. Wang, “Breakdown and withstand strengths of ester transformer liquids in a quasi-

uniform field under impulse voltages,” IEEE Trans. Dielectr. Electr. Insul., vol. 20, no. 2, pp. 571–579,

2013.

23


High Voltage Ignition Unit for Electric Space Propulsion C. Dobranszki*1, I. O. Golosnoy1 and S. B. Gabriel1

1University of Southampton *E-mail: [email protected]

Pulsed plasma thrusters (PPTs) are one of the most versatile EP devices built, since their pioneering in 1962 [1], due to the technology scalability, high power-to-mass ratio and high accuracy operational throttling [1], however, they suffer from relatively low efficiency and high contamination issues. Liquid-fed pulsed plasma thrusters (LPPTs) are considered in overcoming these issues; LPPTs require redesign of the ignition and feeding sub-systems in order to make LPPTs more competitive with other types of propulsion units, especially for small satellites.

The use of traditional semiconductor spark plugs operating in the range of 6 – 12 kV [2] are a major limitation in terms of design complexity for PPTs since it often requires a separate power supply [1]. The potential decrease in the ignition voltage for PFPE-propelled LPPTs to the values of main power unit of ~2kV can be based on two crucial phenomena which have been experimentally observed, Maxwell stress effects and enhancement of local electric field. It is recommended to reduce ignition voltage by changing the geometry of the ignition unit such that enhancement of electric field is produced due to creepage of PFPE in the small gaps between the dielectric cover of the active electrode and the active electrode itself. The drawback of the Electrohydrodynamic-Tip Ignition (ETI) is the time delay due to creepage velocity (see figure 1). From an application perspective this would lower the operational frequency of the LPPT, however it would decrease the required ignition voltage resulting in increased efficiency. In the preliminary experiments, the lowest breakdown voltage (ignition voltage) achieved is –7.5 kV with a delay time of 12.16 μs, which is associated with the creepage velocity. It is believed that optimisation of the cover-liquid-electrode geometry will further reduce the ignition voltage to ~2kV.

Figure 1: Creepage delay time at various applied voltages, with tungsten (2% Th) active electrode across 0.2 mm gap.

1. Burton, R.L. and P.J. Turchi, Pulsed plasma thruster. Journal of Propulsion and Power, 1998. 14(5): p. 716-735. 2. Ciaralli, S., M. Coletti, and S.B. Gabriel, Performance and lifetime testing of a pulsed plasma thruster for

Cubesat applications. Aerospace Science and Technology, 2015. 47: p. 291-298.

24


The Effect of Surface Modification on the Dielectric Properties of

Polypropylene/Aluminum Nitride Nanocomposites

Xinyu Wang*1, Thomas Andritsch1 and George Chen1 1School of Electronics and Computer Science


Due to the excellent dielectric properties and potential high operation temperature, polypropylene (PP) is

considered as a great potential for renewable High Voltage Direct Current (HVDC) cable insulation material.

However, the development and application of PP are limited by its relatively poor thermal conductivity [1].

Nano-fillers with high thermal conductivity, such as aluminum nitride (AlN) have been used to improve the

thermal conductivity and maintain the dielectric properties at the same time. Based on recent studies, the

polymer-particle interphase region is believed to have a significant influence on the dielectric properties of

nanocomposites. Thus, the proposed work is to investigate the influence of polymer-particle interphase by

obtaining diverse particle surface states via different surface treatments.

The experimental work start with sample preparation. Two different Silane Coupling Agents (SCA) were

selected to obtain distinct surface states of Nano-AlN. The PP/AlN nanocomposite samples were made by

solvent blend method with different AlN weight percent (5 wt.% and 10 wt.%). For AC breakdown strength,

samples were placed between two ball electrodes with an increasing voltage of 500 V/s until samples

breakdown. Dielectric spectroscopy test was used to analyze the polymer-particle interphase properties, the

applied voltage is 3 V with the frequency range of 10-1~105 Hz, and the sample thickness is 210 μm.

In present work, two different SCA treated Nano-AlN with PP nanocomposites show a distinct difference on

both AC breakdown and dielectric response results, which indicate the dielectric properties of PP/AlN

nanocomposites are highly related to the polymer-particle interphase state. In order to obtain a more

comprehensively understanding of interphase properties, more work in regard of the dielectric properties of

Nano-AlN/PP need to be investigated in the future.

[1] X. Huang, P. Jiang and T. Tanaka, "A review of dielectric polymer composites with high thermal conductivity", IEEE

Electrical Insulation Magazine, vol. 27, no. 4, pp. 8-16, 2011.

25


Resistance Changes of Thin Film Copper Strip due to Sulphur

Corrosion

M.S. Ahmad Khiar*1, R.C.D. Brown 2 and P. L. Lewin1 1The Tony Davies High Voltage Laboratory, 2Chemistry,

University of Southampton, SO171BJ, UK University of Southampton, SO171BJ, UK


One of the critical properties of mineral oils is its level of corrosiveness because a highly corrosive oil may

lead to catastrophic transformer failure. Hence, in recent years, a large number of studies have been carried

out to monitor the degradation of mineral oils due to sulphur corrosion. Dibenzyl disulfide (DBDS) has been

identified as the main contributing factor that leads to the progression of sulphur corrosion [1]. The chemical

reaction between DBDS and copper results in the formation of semi-conductive copper sulphide on the

insulation paper [2] which leads to inter-turn breakdowns.

On the other hand, oil passivation and metal passivators have been introduced to address degradation of mineral

oils due to sulphur corrosion. However, the duration of these passivators in preventing the progression of

sulphur corrosion is still vague. For this reason, it is crucial to monitor the progression of sulphur corrosion in

mineral oils before implementing any preventive measures.

At present, all of the standard tests including BS EN 62535 standard test method [3] are based on comparing

the discolouration of the copper surface with a standard reference colour scale (ASTM D130-12 standard test

method). This technique will likely lead to misinterpretation of the results since the analysis of sulphur

corrosion depends on visual observations, which may vary from one operator to another. Hence, the objective

of this study is to develop an alternative technique that provides direct indication on the level of oil

corrosiveness, based on thin film technology. An electronic beam evaporator is used to deposit the copper thin

film onto a highly tempered glass sheet. The preliminary results indicate that there is a potential relationship

between corrosive sulphur and changes in resistance of the thin film copper strip.

Figure 1: Shadow mask used for the deposition of the thin film

copper.

[1] F. Scatiggio, V. Tumiatti, R. Maina, M. Tumiatti, M. Pompili, and R. Bartnikas, “Corrosive Sulfur in Insulating

Oils: Its Detection and Correlated Power Apparatus Failures,” IEEE Trans. Power Deliv., vol. 23, no. 1, pp. 508–

509, 2008.

[2] P. S. Amaro, “Corrosive Sulphur in Large Transformers: Impact, Quantification and Detection,” PhD Thesis,

University of Southampton, 2015.

[3] “BS EN 62535:2009: Insulating liquids — Test method for detection of potentially corrosive sulphur in used and

unused insulating oil.” pp. 1–22, 2011.

26


Thermo-Mechanical Stress in Underground Cable

Joints

M. A. Hamdan*, J. A. Pilgrim and P. L. Lewin

Tony Davies High Voltage Laboratory


Southampton, United Kingdom

*[email protected]

The dielectric strength of cable insulation is highly affected by the degrading stresses such as electrical,

thermal, mechanical and environmental stresses. One of the main sources of problems in bulk insulation or at

insulation interfaces is the formation of cavities (voids) [1]. The cavities will be subjected to electric field due

to applied voltage by the system. If the electric field within any void surpasses a critical field, a discharge will

be ignited [2, 3]. Cavities can be developed as bubbles at the interface (insulation/insulation/conductor) or

within the dielectric because of different factors [2, 3]. Movements in the interface, reduction of the interface

pressure, electrical aging, and stress caused by different thermal expansion between the conductor and

insulating material are examples of such factors. It is acknowledged that mechanical stresses have some effect

on the electric behavior and degradation rate of insulation under high electric stress. Smoothness of the surface,

contact pressure, electrical field distribution and temperature change are all parameters that affect the electrical

withstand strength of interfaces.

Using FEM, a 2-D thermal-mechanical coupling analysis model of a cable joint was proposed in order to

investigate the mechanical forces generated in joint under different load currents.

Figure 1: Axial forces generated by the conductor in the joint and cable (in green, dashed blue) with cable joint conductor

temperature and cable conductor (in red), due to a typical wind load current.

[1] A. C. Cjerde, “Multifactor Ageing Models-Origin and Similarities,” IEEE Electrical Insulation Magazine,

1997.

[2] S. A. Boggs, “Partial Discharge –Part III: Cavity-Induced PD in Solid Dielectrics,” IEEE Electrical

Insulation Magazine, 1990.

[3] J. C. Chan, P. Duffy, L. J. Hiivala and J. Wasik, “Partial Discharge-Part VIII: PD Testing of Solid Dielectric

Cable,” IEEE Electrical Insulation Magazine, 1991.

27


Analysis of through-thickness thermal conductivity of wind turbine

blade CFRP materials

E.C. Senis*1, I.O. Golosnoy2, J. Dulieu-Barton1, O.T. Thomsen1 1Faculty of Engineering and the Environment, University of Southampton, United Kingdom 2Faculty of Physical Sciences and Engineering, University of Southampton, United Kingdom


The thermal response of CFRP (Carbon Fibre Reinforced Polymers) materials has received significant

scientific interest during recent years due to the increasing use of these materials in aerospace and automotive

applications, which in turn has created new scientific and technical. The use of CFRP materials in wind turbine

blades causes problems with susceptibility to lightning strike, which are more severe than for wind turbine

blades made of glass fibre reinforced composites (GFRP) [1-2]. Thus, the thermal response of CFRP materials

is of significant importance for the understanding of the effect of lightning strike, and the temperature

dissipation is a parameter of crucial importance. For the research presented in this paper, CFRP samples of

three different thicknesses were manufactured using Vacuum Assisted Liquid Resin Infusion. The fibre volume

content was approximately 57%. Disk shaped samples with a diameter of 50mm were made from the

manufactured plates using waterjet cutting. The through-thickness thermal conductivity was measured by

means of a steady-state technique based on a Guarded Hot Plate method [3]. To obtain accurate heat flux

readings a thin film flux sensor was utilized, Figure 1. Polymethacrylimide (PMI) foam was used as insulation

to minimize lateral heat losses. Measurements conducted in samples with three different thickness to estimate

the interface conductance and characterize the thermal interfaces. The validation of the experimental technique

was achieved using Fused Silica (quartz-glass) and PTFE samples, with a diameter of 50 mm and different

thicknesses. The obtained values showed good agreement with the literature.

Figure 1: Schematic representation of the apparatus without the insulation

[1] A. Candela Garolera, S. F. Madsen, M. Nissim, J. D. Myers, and J. Holboell, "Lightning Damage to Wind

TurbineBlades From Wind Farms in the U.S," IEEE Transactions on Power Delivery, vol. 31, pp. 1043-1049,

2016.

[2] F. Rachidi, M. Rubinstein, J. Montanya, J.-L. Bermudez, R. R. Sola, G. Sola, et al., "A review of current issues

in lightning protection of new-generation wind-turbine blades," IEEE Transactions on Industrial Electronics,

vol. 55, pp. 2489-2496, 2008.

[3] R. Rolfes and U. Hammerschmidt, "Transverse thermal conductivity of CFRP laminates: a numerical and

experimental validation of approximation formulae," Composites Science and Technology, vol. 54, pp. 45-54,

1995.

28


Modelling the Characteristics of Electrical Streamers

in Dielectric Liquids

Donglin Liu*1, Dr.Qiang Liu1, Prof.Zhongdong Wang1 1 School of Electrical & Electronic Engineering, The University of Manchester



Abstract – Dielectric liquids, such as mineral oil and ester oil, are commonly used in the power system due to

their intrinsic advantages over solid dielectrics and gaseous dielectrics such as high thermal conductivity, great

ease of use for different geometries and self-heating ability. However, the mechanism of discharge within

dielectrics liquids is still not in a consistent conclusion. In order to make it clearer, lots of experimental works

related to streamer, acted as a further stage of partial discharge and an earlier stage of breakdown, have been

conducted by researchers over the past years. The character of the initiation and propagation of streamer is

related to the following factors, (1) the discharge gap and the shape of geometry of electrodes, (2) the excitation

of applied voltage such as polarity, rise and down-time, (3) hydrostatic pressure, etc. Streamer propagation

under extremely short voltage pulse occurs because of ionization process in liquid phase. Compared with it,

the discharge development usually relates to the formation and propagation of filamentary gaseous channels.

This work will use numerically calculating methods, known as simulation, to analyse the initiation and

propagation of streamers in order to elucidate the mechanism behind pre-breakdown in mineral oil and ester

oil. A 2-phase, 2-D model including liquid and gaseous phase is used based on poison equation and charge

continuity equation, which are also coupled by energy mapping equation. Parameters in the equations, such as

charge generation rate and recombination rate, are carefully selected either by experiments or calculated.

Different lengths of gaps under various voltage excitations are adopted to analyse the characteristics of the 4

modes of streamers. The modelling results will also be examined and verified by experimental results.

29


Dissolved Gas Analysis (DGA) in Transformer Oil-Paper Insulation

System under Thermal Stress

Xiaohan. Li*1, Qiang. Liu1 and Zhongdong Wang1 1School of Electrical & Electronic Engineering, The University of Manchester



Abstract

As indispensable and expensive electrical equipment, power transformers act an important role in the power

grid all over the world. Electrical faults and thermal faults are the main causes of the failure of the transformers.

In order to avoid the failure of the transformers and provide a stable operation of the power grid, the evaluation

of the health condition of the power transformers is significant. Dissolved gas analysis (DGA) is a rapidly

developed method that investigates the gases which are generated from the insulation material due to the

transformer faults. Typical gases analyzed by DGA are hydrogen (H2), methane (CH4), ethane (C2H6), ethylene

(C2H4), acetylene (C2H2), carbon monoxide (CO) and carbon dioxide (CO2).

This PhD study mainly focuses on thermal faults, where two experimental setups including immersed heating

setup and tube heating setup are designed to create thermal stresses. In addition, SERVERON TM8 and GE

Transfix are used to directly conduct DGA measurement. The overall aim of the project is to investigate the

fault gas generation among different oil-paper insulation systems. Both traditional mineral oil and

environmentally friendly ester liquids are considered. In this poster, it mainly focuses on the introduction of

the operating principles of the experimental setups. Some preliminary experiments and their results will be

introduced.

30


An Investigation into the Next Generation of Ultra High Voltage DC

Power Supplies

R. E. P. Frost*, J. A. Pilgrim and P. L. Lewin

Electronic and Electrical Engineering, Electronics and Computer Science



There has recently been growing interest, in industry, for a power supply that runs at more than a megavolt

and is capable of outputting more than a megawatt. Considerations for prospective development of this

technology should also be made so that in the future it may be used to produce tens of megawatts at a potential

of five, or more, megavolts. For safety reasons, it is imperative that the final design stores as little energy as

possible. The device should also be portable. This places significant restrictions on the overall size of the

design and limits the range of potential inputs to power sources that are ubiquitous in the UK, such as 415V

three phase power outlets.

High Voltage DC power is currently used in a number of applications. These include: medical imaging; lasers;

plasma and particle physics; material and component testing; and power transmission.

Different designs have been considered as part of this project, including: Cockroft-Walton Generators;

Conventional and Electronic Transformers; Resonant Transformers; Insulated Core Transformers; Nested

High Voltage Generators; and Kinetic Energy Based Systems. Of these, the Insulated Core Transformer shows

the most promise for future work as it has a high power density and stores relatively little energy. It is possible

that multiple designs will have to be combined, such as ICTs and Voltage Doublers, in order to achieve a

suitable specification. As well as these, additional technologies such as the use of superconductors or flux

diverters has been examined.

Work so far has focussed on reducing magnetic flux loss in Insulated Core transformers (as seen in Fig 1.), in

turn reducing the drop in voltage generated in windings further away from the primary coil. Previous designs,

such as the ones developed by Van De Graaf [1] and Cross [2] in the Twentieth Century, have attempted to

solve this problem by varying the number of secondary windings and using capacitors, respectively. The focus

of this project, however, is to reduce the flux loss using flux diverters.

Figure 1: A series of cores, insulated from each other, are

stacked up with a primary winding wrapped around the bottom

core. It can be seen that the flux density decreases significantly

in cores that are further away from the primary winding.

[1] R. J. Van de Graaff, “High Voltage Electromagnetic Apparatus Having an Insulating Magnetic Core,” 3289066,

1952.

[2] J. D. Cross, “Modular High Voltage Power Supply with Integral Flux Leakage Compensation,” US6026004A, 1998.

31


MV cable accumulative electro-thermal lifetime analysis through

voltage changes

B. Sheng*1, F. P. Mohamed1, W. H. Siew1, B. G. Stewart1 1University of Strathclyde


Majority medium voltage (MV) cables in the UK are approaching, or have exceeded, their expected operational

life [1]. In order to minimise operating costs, it is important to develop methods to extend the cable lifetime.

Cable life depends mainly on electrical and thermal stress [2], which relates to the voltage and current applied

on the cable. As system load current varies at different seasons, cable temperature changes seasonally. Based

on an accumulative degradation model, the accumulated cable lifetime can be estimated during seasonal load

cycles.

Currently, industry voltage statutory requirements are ±6% of the nominal voltage which can be achieved by

changing the transformer taps. Assuming the load in low voltage systems is constant at certain time duration,

voltage changes in MV cables due to tap changes will also change the current flowing through the cable, which

will change the cable temperature [3]. Therefore, this paper aims to simulate and analyse the accumulated

electro-thermal lifetime improvement of cables through voltage changes within the statutory levels.

The statutory voltage requirements only apply to customer connections, which means a greater scope for

varying the voltage at certain points on the network. Therefore cable lifetime will be analyzed for voltage

changes between ±10% of the nominal voltage. The IEC standard (60287) method for rating and modelling

cables is applied to evaluate the cable temperature under different voltages and relative currents. Different

cable configurations will also be considered in simulations as temperature is dependent on the cable

dimensions. A typical electro-thermal lifetime analytical expression will be used to evaluate the influence of

voltage changes on theoretical life-time evaluations.

The obtained accumulated electro-thermal lifetime improvement under different voltage changes will provide

a potential understanding for industrial implementation in relation to anticipated cable lifetime changes.

[1] Renforth, L., Mackinlay, R., Michel, M., “MV cable diagnostics-applying online PD testing monitoring”,

MNCCIRED Asia Pacific Conference on MV Power Cable Technologies, paper number 14. 2005.

[2] M. Marzinotto, G. Mazzanti, “Merging the electro-thermal life model for power cables with the statistical volume

enlargement law”, 2014 IEEE conference on electrical insulation and dielectric phenomena, pp. 502-505, 2014.

[3] B. Sheng, F. P. Mohamed, W.H. Siew, B. G. Stewart, “MV cable lifetime improvement analysis through transformer

tap changes”, 2017 IEEE Conference on Electrical Insulation & Dielectric Phenomenon (CEIDP 2017), pp. 1-4,

USA, 2017.

I (A)

Season

U2

U1

32


Spectroscopic And Chemometric Analysis Of Thermally Stressed Liquid

Dielectrics

G.Smith, A.S.Holmes-Smith and S.McMeekin

School of Engineering and Built Environment, Glasgow Caledonian University, Cowcaddens Road,

G4 0BA


Mineral and ester oils are employed as liquid dielectric insulators in many high voltage transformers. It is

recognised that the dielectric properties of these oils degrade over time with continued exposure to high

voltages and thermal stress. The chemical breakdown of the oils can be monitored by measuring changes in

the UV-vis and FT-IR spectra [1]. Here mineral and ester oils have been thermally stressed by exposure to

120oC over a prolonged time period with changes in the FT-IR and UV-Vis spectra measured weekly. Different

samples of oil were prepared with some exposed to Kraft paper, copper or a combination of the two. The

ratio of oil: copper: Kraft paper was consistent with that commonly found in transformers. Control samples

of the same constituents were also prepared and kept at ambient temperature. The FT-IR spectra were

collected in transmission mode and a consistent path length was obtained by using a calcium fluoride cell

(path length 100 m) and the PearlTM Accessory from Specac. The major change in the FT-IR spectrum for

the mineral oil was the observed increase in the intensity of the peak at 1740 cm-1 which is associated with

the growth of C=O. This was observed for all thermally stressed samples [1-3]. In order to extract useful

information from these spectra the spectral region between 1650cm-1 – 1800cm-1 was analysed using

principal component analysis. The score plot of the first two components is shown in the figure. Here it can

be clearly seen that as the oils age then the value of PC1 increases with little change observed for PC2 in all

samples. This shows that PC1 is associated with the growth of carboxylate acid groups within the mineral oil

over time. Interestingly PC2 separates out the oils which have been exposed to different components. This

demonstrates, for the first time, a statistical method of assessing the degradation of mineral oil. Further

results will be presented showing the statistical analysis of the UV-Vis absorbance data and the spectroscopic

degradation of the Ester oil.

Figure 1: PCA of C=O region of Eden Transformer Oil

[1] D. M. Hepbum, F. Waite, and I. J. Kemp, “Effect of metal and paper inclusions on mineral oil degradation,” Conference Record of the 2004 IEEE Intemational Symposium on Electrical Insulation, Indianapolis. IN USA, 19-22 September 2004 Effect, no. September, pp. 19–22, 2004. [2] R. Blue and D. Uttamchandani, “Infrared Detection of Transformer Insulation Degradation Due to Accelerated Thermal Aging,” IEEE Transactions on Dielectrics and Electrical Insulation Vol. 5 No. 2, April 1998, vol. 5, no. 2, pp. 165–168, 1998. [3] P. Verma, M. Roy, R. K. Tiwari, and S. Chandra, “Generation of furanic compounds in transformer oil under accelerated thermal and electrical stress,” Proceedings - Electrical Insulation Conference and Electrical Manufacturing Expo, 2005, vol. 2005, pp. 112–116, 2005

33


Study of Re-ignition Prediction in Low Voltage Switching Devices

Dongkyu Shin*1, Igor O. Golosnoy1 and John W. McBride2 1 School of Electronics and Computer Science, University of Southampton 2 Faculty of Engineering and the Environment, University of Southampton


The re-ignition of the arc during the interruption process deteriorates the switching performance of low voltage

switching devices (LVSDs). Avoiding re-ignition is thus a key goal in the effective design of the quenching

chamber. The re-ignition prediction provides the opportunity to improve the switching performance of a LVSD

during the design process and to refine the product prior to manufacture and empirical device testing. It has

been suggested that if the breakdown voltage is greater than the recovery voltage (applied voltage across the

breaker at zero current point), there will be a successful interruption without re-ignition [1], [2]; however, the

problem of determining the breakdown voltage of the arc plasma after the current zero point has not been fully

addressed due to the changes at the breakdown characteristics of the gas-plasma mixture by complex

recombination and cooling processes in the breakers.

In this paper, re-ignition evaluators are investigated through the analysis of interruption test data for several

types of LVSDs under the single-phase and three-phase circuit conditions. In addition, we present the results

of arc modelling, based on the magnetohydrodynamics, which can predict the switching performance of a

LVSD as shown in Figure 1 [3].

Figure 1: The simulated and measured electrical waveforms.

[1] J. Slepian, “Extinction of an A-C. arc,” Trans. AIEE, vol. 47, no. 4, pp. 1398–1407, 1928.

[2] P. G. Slade, Electrical Contacts Principles and Applications. New York: CRC Press, 2014, pp. 553–616.

[3] D. Shin, I. O. Golosnoy, and J. W. McBride, “Development of switching performance evaluator and arc modelling

tool for low-voltage switching devices,” COMPEL-Int. J. Comput. Math. Elect. Electron. Eng., to be published.

34


Investigating the Potential of Retro-filling C3F7CN for Application in

Gas Insulated Lines

L. Loizou*, L. Chen and Q. Liu

The University of Manchester


The increasing utilisation of renewable energy sources, such as wind, hydropower and solar have resulted in

energy production far away from the load centres. Concerns associated with environmental, aesthetic and

footprint in populated areas have resulted in growing interest to move the conventional overhead lines

transmission to underground. Gas insulated lines (GIL) are considered to be an attractive alternative technology

to overhead lines for transmission and distribution of high power and over long distances [1]. However, the

main dielectric medium used in gas insulated lines is sulphur hexafluoride (SF6), a gas that is identified as a

highly potent greenhouse gas with a global warming potential that is 22,800 times higher than CO2 and an

estimated atmospheric lifetime of 3,200 years [2].

There is a growing interest, from both industry and academic, in finding a suitable replacement of SF6 that is

more environmentally friendly. Several groups of gases like perfluoronitriles, perfluorocarbons,

perfluoroketones and many others have been investigated in the literature. The SF6 replacement research has

proved to be a challenging task as it is hard to find a gas that combines all the excellent properties that the SF6

has, such as: high dielectric strength, arc-quenching capabilities, low boiling point, chemical stability, non-

toxicity and at the same time has low environmental impact. So far, there are two emerging candidates

developed by 3M: Novec 4710 (C3F7CN) and Novec 5110 (C5F10O) [3]. The two candidates are being trialled

in pilot projects and are showing promising potential of replacing SF6.

At the University of Manchester, the current research is focused on C3F7CN, a gas which in its pure form has

a dielectric strength almost double of SF6 and shares similar physical properties to SF6. Like other alternative

gases, the high boiling point of C3F7CN means it has to be used in low concentration and as part of a binary

mixture. At Manchester, a 400 kV GIL demonstrator has been assembled in the high voltage laboratory and is

subjected to standard type testing. Extensive tests have been conducted to evaluate the performance of C3F7CN

in comparison to the SF6. The initial results have shown that a mixture of 20%/80% C3F7CN/CO2 has

comparable electrical performance to pure SF6. The aim of this project is to experimentally investigate the

potential of retro-filling a C3F7CN gas mixture in existing SF6 filled gas insulated equipment. The results could

provide utilities worldwide with a technically viable retro-fill solution to aid the process of phasing out the use

of SF6 in the electrical industry.

[1] H. Koch, GAS-INSULATED TRANSMISSION LINES (GIL). A John Wiley & Sons, 2012.

[2] IPCC, “Direct Global Warming Potentials,” IPCC Fourth Assessment Report: Climate Change 2007.

[Online]. Available: https://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-10-2.html.

[3] M. Hyrenbach, T. Hintzen, P. Müller, and J. Owens, “Alternative gas insulation in medium-voltage

switchgear,” Cired, no. 587, pp. 1–5, 2015.

35


An Examination of the Breakdown Strength of Modified Epoxy

Resin

Istebreq A. Saeedi*, Alun. S. Vaughan and Thomas Andritsch

The Tony Davies High Voltage Laboratory, University of Southampton

Southampton, UK

[email protected]

Over the past decade, epoxy resins have been used in several commercial applications including coating,

cast resin transformers and cable terminations [1]. The properties of these epoxy resins are affected by

the formulation of the epoxy network. Therefore, knowledge of the molecular structure would help in

the construction of an efficient system with optimized properties. Changing the resin/hardener

stoichiometry has been reported to alter the thermal and electrical properties of the epoxy system [2].

The existence of an excess of hardener in the epoxy system resulted in reduced breakdown strength of

the tested samples. In this work, we aim to study the effect of different functional groups on the thermal

and electrical properties of modified epoxy resin systems. Therefore, the amount of epoxy/hardener and

diluent were calculated in order always to conform to the theoretical stoichiometry. Hence, a complete

reaction of all amine and epoxide groups (supplied by both the resin and the modifier) is ensured.

Modifying epoxy resin systems using a reactive diluent featuring a long chain alkyl group was studied

in [3]. The work reported a reduction in the glass transition temperature (Tg) of the modified epoxy

network, while the addition of up to 4% of the diluent resulted in enhanced breakdown strength [3].

Here, different percentages of epoxy based reactive diluents featuring either a cyclic functional groups

or a short chain alkyl groups with branched multi terminal epoxide rings were used. The modifiers were

1,2-epoxycyclododecane ECD and trimethylolpropane triglycidyl ether TTE obtained from sigma

aldrich. The Tg and the AC breakdown strength of the modified samples were measured. The results

indicated that, as the added percentage of diluent increases, Tg decreases; the breakdown strength varies

depending on the percentages and the types of the functional groups attached to the diluent.

In conclusion, modifying epoxy resin networks using different diluents affects the thermal and electrical

properties of the bulk material. This would allow tailoring of these properties to suit particular

applications.

References:

[1] H. Lee and K. Neville, Handbook of Epoxy Resins. 1967.

[2] V. T. Nguyen, A. S. Vaughan, P. L. Lewin, and A. Krivda, “The effect of resin stoichiometry

and nanoparticle addition on epoxy/silica nanodielectrics,” IEEE Trans. Dielectr. Electr.

Insul., vol. 22, no. 2, pp. 895–905, 2015.

[3] I. A. Saeedi, A. S. Vaughan, and T. Andritsch, “On the dielectric performance of modified

epoxy networks,” Proc. 2016 IEEE Int. Conf. Dielectr. ICD 2016, vol. 2, pp. 1044–1047,

2016.

36


Butterworth zero-phase filter for Multiple PD Analysis of High

Voltage Transformer Windings

A. S. Nasorudin*1, N. H. Nik Ali 1, P. Rapisarda2 and P. L. Lewin1 1The Tony Davies High Voltage Laboratory, 2Comunications, Signal Processing and Control,

University of Southampton, Southampton, SO171BJ, UK University of Southampton, Southampton, SO171BJ, UK


Measurements of PD signals can be distorted by noise sources, which reduces the ability to identify PD sources

in high voltage transformer windings. De-noising techniques in PD analysis are therefore very important.

There is several noise reduction techniques have been proposed for application in PD analysis such as fast

Fourier Transforms, hard thresholding and low pass filter [1]. A Butterworth filter is a type of signal processing

filter that ensures maximally flat (i.e. has no ripples) magnitude response in the passband and adequate roll-

off [2]. A major advantage of the filter is the smooth and monotonically decreasing frequency response.

However, the drawback is that the filter causes phase distortion to the original signal. A reasonable approach

to tackle this problem is by applying forward-backward filtering.

The main project is concerned with the feasibility of locating two PD sources simultaneously in a transformer

winding based on measurement data from wideband radio frequency current transformers (RFCTs) placed at

the neutral to earth point and the bushing tap-point to earth. The proposed processing technique relies on the

assumption that the PD pulses generated from different sources exhibit unique waveform characteristics. Due

to the termination and path taken characteristics, the PD signals will suffer attenuation and distortion during

the propagation of the PD signals along transformer windings. Therefore, it will cause changes in the energy

characteristics of the PD pulses at both measurement points [3]. The techniques applied will generates two

clusters in 3D space representing detected signals from each sensors. This paper is concerned with the effect

of the de-noising on the separation distance between the clusters.

It is concluded that a combination of Butterworth and backward-forward filter, with a certain number of order

and cut-off frequency, will maintain the original PD pulse frequencies and at the same time, eliminate

frequencies of possible additive noise and external signal interference. Figure 1 shows the frequencies above

cut-off frequency decrease and the rate of roll-off depends on the order. It is hypothesized that using a

de-noising algorithm gives an improvement in terms of separation distance when analysing multiple PD

sources in high voltage transformer windings.

Figure 1:Frequency domain of a PD pulse before and after filtered with n=2, Wc = 25 MHz.

[1] S. Sriram, S. Nitin, K. M. M. Prabhu, and M. J. Bastiaans, “Signal denoising techniques for partial discharge

measurements,” IEEE Trans. Dielectr. Electr. Insul., vol. 12, no. 6, pp. 1182–1191, 2005.

[2] R. S. and J. B. A. Oppenheim, “Discrete-time signal processing,” 1st Ed. Up. Saddle River, N.J. Prentice-Hall

Int., 1999.

[3] N. H. Nik Ali, M. Giannakou, R. D. Nimmo, P. Rapisarda, and P. L. Lewin, “Classification and Localisation of

Multiple Partial Discharge Sources Within a High Voltage Transformer Winding,” in IEEE Electrical

Insulation Conference (EIC), 2016, pp. 519–522.

37


Intensity Measurement of Propylene Carbonate using The Parallel-

Plane and Sphere-Plane Electrodes under DC Electric Field

Z. N. Zakaria*, T. Andritsch and P. L. Lewin

The Tony Davies High Voltage Laboratory


Southampton, UK, SO17 1BJ


Research has shown that electric field between two electrodes in a dielectric liquid under high electric field

stress is distorted by the presence of space charge [1], [2]. Furthermore, this phenomenon will deteriorate the

performance of insulation material. This study describes the light intensity distribution of the electric field

using the parallel-plane and sphere-plane electrodes using Kerr effect. Kerr effect is an optical method which

is widely used in measuring the electric field in dielectric fluid [3]. In the study, the linear crossed polarisers

has been setup to measure the electric field distribution in propylene carbonate under high DC electric field.

The optical measurement configuration consists of the 7mW, 633nm He-Ne laser, an expander and two

polarisers were attached on an optical bench. The first polariser was set at +45o, while the analyser was set at

-45o relatively to the applied electric field axis. The linear polarised light from the polariser will pass through

the test cell becomes elliptically polarised light. Then, the elliptically polarised light is converted back to the

linearly polarised light as it passes through the analyser. In the experiment, the high resolution camera was

used to capture the intensity of the transmitted light after the analyser. Figure 1 shows the measurement system

setup with camera as a detector to record the light intensity change. The Kerr experiment was based on the

light propagation in the liquid medium under high electric field. Thus, the intensity change was calculated

between the transmitted light incident, Ii and the incident light, Io from the recorded images by using the image

processing technique. From the results obtained, the intensity ratio between the incident and the transmitted

light of both electrodes configurations increased as the applied electric field increased. The contribution will

also describe the comparison of the intensity to the applied electric field between both electrodes geometries.

Figure 1: Linear polariscope with crossed polariser

experimental setup

[1] X. Zhang and M. Zahn, “Kerr electro-optic field mapping study of the effect of charge injection on the impulse

breakdown strength of transformer oil,” Appl. Phys. Lett., vol. 103, no. 16, p. 162906, 2013.

[2] T. Takada, Y. Zhu, and T. Maeno, “Progress in Japan of electrical field measurement technology for liquid

dielectrics,” IEEE Electr. Insul. Mag., vol. 12, no. 2, pp. 8–20, 1996.

[3] T. Takada, “Acoustic and optical methods for measuring electric charge distributions in dielectrics,” IEEE

Trans. Dielectr. Electr. Insul., vol. 6, no. 5, pp. 519–547, 1999.

38


Epoxy Resin/Graphene Oxide nanocomposites:

Optimising exfoliation parameters

O. Vryonis, T. Andritsch, A. S. Vaughan, P. L. Lewin

The Tony Davies High Voltage Laboratory Electronics and Computer Science

University of Southampton Southampton, SO17 1BJ, UK

Email: [email protected]

The utilization of carbon-based fillers, such as carbon nanotubes, graphite or carbon black has attracted the

scientific interest in order to improve the mechanical, electrical and thermal response of polymers like epoxy

resins [1]. Carbon-based fillers offer all three geometry configurations with a wide variety of aspect ratios, and

functionalization potential. Graphite-based fillers offer high aspect ratio, compared with no exposure

restrictions or health hazards, and relatively low cost. Thus, they are considered as a fine filler material for

polymer composites. The nanometric equivalent of graphite, graphene, can be retrieved under the appropriate

treatment as graphene oxide (GO). Various chemical procedures can be followed in order to achieve different

levels of functionalization of the graphite [2]. This will allow enhanced compatibility with the epoxy matrix

and subsequently, improved mechanical, electrical and thermal properties at low filler contents, ensuring that

the processibility of the nanocomposite system is maintained. Recently, graphene-based fillers have been used

to improve the performance of CFRPs (carbon fibre reinforced polymers) [3]. A potential application for these

systems are wind turbine components, as well as aerospace applications. In this study, different

functionalization, as well as, preparation methods are being used in order to determine the optimum dispersion

level of GO and its effect on mechanical, electrical and thermal properties of the respective epoxy resin

nanocomposites.

Figure 1: Schematic representation of the exfoliation, while

suspended in solvent, of the graphite after it was functionalized

by oxidation [4].

[1] N. Jović, et al. "Temperature dependence of the electrical conductivity of epoxy/expanded graphite

nanosheet composites." Scripta Materialia, Vol. 58, pp. 846-849, 2008

[2] S. Miller, “Physical properties of exfoliated graphite nanocomposites by variation of graphite surface

functionality” 52nd International SAMPE symposium June 3-7, 2007

[3] W. Qin, et al. "Mechanical and electrical properties of carbon fiber composites with incorporation of

graphene nanoplatelets at the fiber–matrix interphase." Composites Part B: Engineering Vol. 69, pp. 335-

341, 2015.

[4] J. R. Potts, et al. "Graphene-based polymer nanocomposites." Polymer Vol. 52, pp.5-25, 2011.

39


Complex Insulators in Gas/Solid Interfaces: Flashover Properties and

Topological Optimisation

Z. Wang, I. Timoshkin, S. MacGregor, M. Wilson, M. Given, T. Wang

Department of Electronic and Electrical Engineering

University of Strathclyde

204 George Street, Glasgow, G1 1XW


High-voltage systems rely on solid/gas interfaces for electrical insulation. The main aim of this project is to

develop optimal dielectric topologies which will help to improve the flashover properties of such solid/gas

interfaces in highly-divergent and virtually-uniform electric fields. Carbon dioxide, dry and ambient air will

be used as insulating gases; complex solid dielectrics will be designed and developed which will allow grading

of the electrical field across the solid/gas interface to be achieved via dielectric material permittivity. The

effects of dielectric permittivity, surface charge and surface conductivity on the flashover characteristics will

be investigated. The transient electric field governed by the space charge will be modelled, and an algorithm

for optimisation of dielectric topologies based on the field grading approach will be developed.

The present paper discusses initial results of modelling of streamer propagation across solid/gas interfaces with

different solid dielectrics. Based on the Toepler’s phenomenological approach to determining the breakdown

voltage across solid/gas interfaces, [1], the proposed model allows calculation of the flashover voltage for

interfaces comprised of solid materials with different permittivities. The effect of surface charge accumulated

on the solid surface will be investigated, as it was shown that the surface charge introduced by previous

discharges can influence the flashover characteristics, [2]. The proposed model and the obtained results will

help in further optimisation of solid/gas interfaces, and in development of complex solid dielectric materials

for practical power and pulsed power applications.

[1] M. Toepler, Ueber die physikalische Grundgesetze der in der Isolatorentechnik auftretenden elektrischen

Gleiterscheinungen, Archiv fuer Elektrotechnik, Heft 5/6, S. 157-185, 1921

[2] Y. Huang, D. Min, D. Xie, S. Li, X. Wang and S. Lin, "Surface flashover performance of epoxy resin

microcomposites infulenced by ozone treatment," 2017 International Symposium on Electrical Insulating Materials

(ISEIM), Toyohashi, Japan, pp. 235-238, 2017,

40


Failure Mode and Effects Analysis for HVDC

C. J. Mackinnon*1, B. G. Stewart1 1University of Strathclyde


The number and overall global capacity of HVDC projects is set to increase significantly, partly since HVDC

technologies offer important functionality and can make “greater than the sum of their parts” contributions to

electrical networks, as well as being key enablers for long-distance and high-capacity transmission [1], [2].

Individual HVDC projects often need high availability to be cost-effective, implying not only that adequate

network conditions prevail, but also that individual components and the system as a whole are reliable.

In an effort to aid improvements in reliability for such systems, this poster presents work toward a failure

modes and effects analysis for HVDC converter stations.

The analysis aims to reveal causes and effects of failures, to: provide industry stakeholders with a basis for

discussion; highlight potential future research directions; and to prompt an awareness of the effects of different

operating conditions on asset condition.

FMEAs share similarities with frameworks like the smart grid architecture methodology, insofar as both are

designed to highlight potential problems as early in the lifecycle of a project or system as possible [3].

Remaining work includes the need to populate an analysis with probability data, to validate dependencies, and

to allow for variance in project specific conditions. Challenges arise where the contribution of protection and

control systems are considered as interactions with failure modes, disturbances, and faults.

While extensible in principle, this analysis could be made available online in future, such that new failure

modes can be added as they are discovered, and data provided as it becomes available.

[1] D. Jovcic, K. Ahmed, “High Voltage Direct Current Transmission: Converters, Systems and DC Grids”, First

Edition, 2015 John Wiley and Sons, Ltd.

[2] Alstom Grid, “HVDC: Connecting to the Future”, First Edition, 2010.

[3] CEN-CENELEC-ETSI ``Smart Grid Coordination Group Smart Grid Reference Architecture’’, November 2012

[Online].

41


A Review of Existing HVDC Circuit Breaker Technologies

J. H. Carter1, S. B. Tennakoon2, C. A. Gould3.

1, 2 , 3 Staffordshire University

E-mail: [email protected], [email protected], [email protected].

The DC grid purposed for integrating the North Sea wind farms and the solar farms of North Africa is not

possible without A HVDC circuit breaker. There are many designs proposed in literature, but none are

technically or economically viable. The recent hybrid design that incorporate power electronics with fast

disconnectors proposed by ABB (Callavik, 2012) and Alstom in conjunction with Nottingham University

(Effah, 2016) are promising but the application remains to be seen. This paper reviews existing technology,

discusses the parameters needed for a viable HVDC circuit breaker (Frank, 2011) and comments on promising

area’s research that could lead to a viable HVDC circuit breaker (Van Hertem, 2016).

Figure 1: A simple illustration of an HVDC grid with circuit

breaker positions. http://cepri.com.cn/products/grid.html

Callavik, M., Blomberg, A., Häfner, J. and Jacobson, B., 2012. The hybrid HVDC breaker. ABB Grid Systems

Technical Paper.

Effah, F.B., Watson, A.J., Ji, C., Amankwah, E., Johnson, C.M., Davidson, C. and Clare, J., 2016. Hybrid HVDC

circuit breaker with self-powered gate drives. IET Power Electronics, 9(2), pp.228-236.

Franck, C.M., 2011. HVDC circuit breakers: A review identifying future research needs. IEEE Transactions on Power

Delivery, 26(2), pp.998-1007.

Van Hertem, D., Gomis-Bellmunt, O. and Liang, J., 2016. HVDC grids: for offshore and supergrid of the future (Vol.

51). John Wiley & Sons.

42


Conduction characteristics of MIDEL eN 1204 insulating liquid under

DC nonuniform field conditions

T. Wong*, I. Timoshkin, M. Given, M. Wilson, S. MacGregor

Department of Electronic and Electrical Engineering

University of Strathclyde, 204 George Street, Glasgow, G1 1XW, UK


In the continued search for environmentally friendly mineral oil alternatives for industrial applications, the

present study expands on previous studies as in [1,2], with the addition of rapeseed based dielectric fluid

MIDEL eN-1204.

The conduction characteristics of MIDEL eN-1204, both fresh and thermally aged, have been measured and

compared with well-known synthetic ester liquid MIDEL 7131 and mineral oil Diala D. The mobility of charge

carriers under high voltage in a point-plane electrode topology, and the temperature dependence of

conductivity under low-field conditions for each fluid sample have been obtained. Charge carrier mobility in

the tested liquids was found through I1/2-V curves in the method described in [3]. I-V curves were obtained in

the standard cylindrical test cell with electrode separation of 1 mm, energised with voltages up to 70 V at room

temperature (20 Co), 45 Co and 75 Co to analyse the conductivity increase with temperature. The above was

also repeated with thermally aged (with presence of copper) fluid samples.

Obtained results agree well with theoretical conduction models and existing studies. It is therefore found that

MIDEL eN 1204 natural ester fluid possesses a similar carrier mobility in the 0-10 kV range when compared

to MIDEL 7131 synthetic ester liquid, but exhibits a much higher mobility with aging. In the low-field region,

it is found that MIDEL eN possesses a lower conductivity across the temperature ranges, and a lower

conductivity increase with aging compared with the other two fluids.

The conduction data for Diala D are found to demonstrate a transient character under low-field conditions,

with potential viscous and electrohydrodynamic processes being dominant, rendering the obtained data

uncertain and difficult to analyse as a means of comparison, and opening a definite opportunity for further

study.

Figure 1: I1/2-V curves for HV testing of both fresh and aged

samples of liquids under study.

[1] Y. Jing, I. V. Timoshkin, M. P. Wilson, M. J. Given, S. J. MacGregor, T. Wang, J. M. Lehr, “Dielectric Properties

of Natural Ester Midel 7131 and Mineral Oil Diala D”, IEEE Transactions on Dielectrics and Electrical Insulation,

2014, v.21, n.2, pp. 644-652.

[2] T. G. Aakre, T. A. Ve, Ø. L. Hestad, “Conductivity and Permittivity of Midel 7131: Effect of Temperature, Moisture

Content, Hydrostatic Pressure and Electric Field”, IEEE Transactions on Dielectrics and Electrical Insulation”,

2016, v.23, n.5.

[3] R.S. Sigmond, “Simple approximate treatment of unipolar space-charge dominated coronas: The Warburg law and

the saturation current”, J. Appl. Phys., v.53, n.2, 1982, pp.891-898.

43


Investigation of Creeping Discharge at Inter-phase Barrier

Pressboard

W. Thansiphraserth , P. L. Lewin

The Tony Davies High Voltage Laboratory, University of Southampton


Partial Discharge (PD) measurements are an important tool for assessment of the condition of the insulation

systems of power equipment such as high voltage transformers [1]. One of the most dangerous failure modes

in a large autotransformer is due to surface discharge along the interphase barrier board [2]. Identification of

PD patterns due to surface discharging on the barrier board surface is of great importance in terms of

diagnosing potential problems within a large autotransformer. An experiment has been designed at the Tony

Davies High Voltage Laboratory, University of Southampton, with the aim of recreating this particular failure

mode whilst capturing condition monitoring data (e.g. PD measurements). Experiments have been undertaken

with a PD source and barrier board within a two phase field. The results assist in determining the characteristic

features of this form of degradation and will help to improve the diagnostics of field data captured in real-time.

The experiment have been undertaken at the sealed test cell as shown in Fig 1, and one the PRPD result has

been shown in Fig 2. The result shows the combination of surface discharge and high magnitude corona-like

discharge. This result has been collected at the final stage called full discharge stage, the discharge that

temporarily bridge two electrodes without flashover [3]. The shortest time from the discharge occur until the

final stage show the effect of charge transport by the cyclical electric field between two adjacent voltage coils,

and make a sweeping motion on the charged particles [4].

Figure 1: Inter-phase pressboard in the test cell

Figure 2 The PRPD pattern result

[1] P. M. Mitchinson, P. L. Lewin, B. D. Strawbridge and P. Jarman, “Tracking and surface discharge at the oil-

pressboard interface,” Electrical Insulation Magazine, IEEE, vol. 26, no. 2, pp. 34-41, 2010.

[2] V. Sokolov, “Understanding failure modes of transformers,” Proceedings of the Euro TechCon, pp. 43-65, 2005..

[3] H. Zainuddin, P. Mitchinson and P. M. Mitchinson, “Partial discharge characteristics of surface tracking on oil-

impregnated pressboard under AC voltages,” in Solid Dielectrics (ICSD), 2013 IEEE International Conference on,

2013.

[4] W. Thansiphraserth and P. L. Lewin, “Surface tracking along the interphase barrier of a large transformer,”

Conference on Electrical Insulation and Dielectric Phenomenon (CEIDP), 2017 IEEE International Conference on,

2017.

44

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