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Dielectric spectroscopy measurements on very lowloss cross-linked polyethylene power cablesTo cite this article Tong Liu et al 2009 J Phys Conf Ser 183 012002

View the article online for updates and enhancements

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Dielectric spectroscopy measurements on very low loss cross-

linked polyethylene power cables

Tong Liu John Fothergill Steve Dodd Ulf Nilsson

University of Leicester UK LE1 7RH Borealis AB SE-444 86 Stenungsund Sweden

tl57leicesteracuk

Abstract The principles of dielectric spectroscopy are reviewed and the techniques in both

time and frequency domains are explored in search of appropriate methods for measurement on

low loss XLPE cables By combining the techniques of frequency response analyzer

transformer ratio bridge and discharging current measurements some preliminary tests results

on homopolymer XLPE model cables have been presented and analyzed in a wide frequency

range of 10-4

Hz~2times104Hz Dielectric loss mechanisms of XLPE cables are discussed based on

the measurement results

1 Introduction

High voltage cables are widely used to convey electrical power Research and development of the

electrical insulation of power cables are important for improved performance and reliability Oil-paper

insulated power cables were invented by Ferranti in 1891 but modern cables employ polymeric

materials as the primary insulation such as polyethylene (PE) cross-linked polyethylene (XLPE) and

ethylene propylene rubber (EPR) [1] Nowadays the insulation system of power cables needs to

withstand extremely high voltages of up to 1000kV with reliable long-term operation and with

insulation thickness as thin as possible to minimise manufacturing costs Any insulation defects

present in power cables can cause insulation degradation and subsequent electrical breakdown

Because of XLPErsquos intrinsic breakdown strength of up to 800kVmm and enhanced melting

temperature from 75degC to 90degC [2] XLPE insulation is the most widely used insulation in high

voltage cables Therefore in the present study the dielectric properties of XLPE based cable

insulation systems will be used to study the mechanisms of dielectric loss

Dielectric spectroscopy is a technique used to study the interaction of a material and the applied

electric field It is widely used a tool for the detection of material ageing and fault diagnosis for

insulation systems including power cables and hence it has become a popular and powerful research

technique [3] Although various techniques can be used for dielectric spectroscopy including

measurements in the frequency and time domains the dielectric loss measurement of XLPE cables is

usually beyond the abilities of many commercial instruments due to the very low dielectric loss

exhibited by this class of material The low loss of XLPE arises due to its non-polar molecular

structure low levels of impurity and additives Direct measurements on cable systems impose more

difficulty owing to the much larger insulation thickness compared with thin film samples that are often

used in laboratory tests making it difficult to set up the high electrical fields necessary for sensitive

dielectric measurements for a given applied voltage In addition measurements on thin film samples

may not be representative due to differences in the local morphology of the semicrystalline materials

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

ccopy 2009 IOP Publishing Ltd 1

obtained under cable manufacturing conditions and those present for the formation of thin films Also

in typical cables semiconducting shields which may also contribute to the dielectric loss of the cable

are extruded along with the cable insulation and it would be difficult to replicate the concentration and

distribution of the shields and of the crosslinking by-products and impurities using thin film test

specimens For this reason triple extruded model cables having reduced insulation thickness are used

in this study The dielectric loss mechanisms will be studied by means of the complementary use of

frequency and time domain dielectric spectroscopy in order to cover the widest possible frequency

range with the required sensitivity

11 Principle of dielectric spectroscopy

Dielectric spectroscopy is based on the phenomena of electrical polarization and electrical conduction

in materials There are a number of different dielectric polarization mechanisms operating at the

molecular or microscopic level Each polarization mechanism either a relaxation or resonance

processes is centred around its particular characteristic frequency which is the reciprocal of the

characteristic time of the process and therefore separable in frequency

The most common mechanisms can be divided into three main categories as shown in Figure 1 At

the highest frequencies the electric field will cause a slight displacement of the electrons of any atom

with respect to the positive nucleus electronic polarization while at lower frequencies atomic

polarization is due to the distortion of the arrangement of atomic nuclei in a molecule or lattice All

polymeric materials have these two types of high frequencies polarization and which occur above

infra-red frequency Orientational polarization occurs when particular molecular groups exhibiting a

permanent dipole moment initially orientated randomly in space tend to be aligned by the applied

field to give a net polarization in that direction The rate of dipolar orientation is highly dependent on

inter- and intra-molecular interaction Orientation of molecular dipoles can therefore occur over a wide

range of frequency dependent on the ease with which the dipoles can rotate Dipoles due to absorbed

moisture will be much more easily rotated than polar groups associated with the main polymer chains

which may require the co-operative motion of surrounding molecular chains for orientation For this

reason dielectric spectroscopy ideally suited for the identification and differentiation of various polar

groups on main chain or side chains At very low frequencies DC conduction will become significant

and usually manifests as a slope of -1 in the imaginary permittivity whilst the real part remains

constant Thus dielectric spectroscopy is also well suited for the determination of DC conductivity of

materials Other common dielectric responses include the quasi-DC mechanism where the low

frequency permittivity real and imaginary have the same slope resulting from partially mobile charge

carriers gradually moving or hopping within the material Interfacial polarisation (Maxwell-Wagner

polarization) can also manifest at low frequency and in this case the real permittivity has a slope of -2

while the imaginary part has slope of -1

Figure 1 Dielectric permittivity spectrum over a

wide range of frequencies

Figure 2 Summary of dielectric spectroscopy

techniques

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

2

2 Choice of techniques for XLPE cables Various dielectric spectroscopy techniques and their associated frequency ranges are summarized in

Figure 2 [4] As we are interested in the dielectric loss mechanisms in high voltage power cables

operated at 50Hz the low frequency techniques covering 10-4

Hz~106Hz have been explored in search

of those with appropriate sensitivity to measure the extremely low loss model cables In practice most

techniques only have sufficient sensitivity over limited frequency ranges and it is necessary to

combine many techniques to achieve the required sensitivity over a wide range of frequency

The frequency response analyzer (FRA) in conjunction with a dielectric interface has a wide

frequency range As shown in Figure 3 phase sensitive voltmeters are used to compare the voltage

)(1 ωU and )(2 ωU (the latter proportional to Is(ω)) Commercial instruments often employ a reference

impedance ZR within the dielectric interface to facilitate accurate measurements The complex

impedance )(ωSZ of the sample can be calculated from the measured data by the equation

)1)(

)((

)(

)()(

2

1minus==

ω

ω

ω

ωω

U

UR

I

UZ

S

SS

Where ω is the angular frequency The complex relative permittivity can be then calculated from

)(

1

0CZi S ωωε =

lowast

provided that the geometric capacitance C0 of the sample is known [5] However tests have shown

that sensitivity of loss tangent is limited to the range 10-4

to 10-2

dependent on frequency as can be

seen from the background measurements in figure 6 In practice a Solartron FRA 1255 and Dielectric

Interface 1296 were used for the measurement of the model XLPE cables

Transformer ratio bridges (TRB) as shown in Figure 4 are often used to cover the audio frequency

range Using a set of standard impedances and a multi-tap transformer this type of bridge allows for

high precision measurements compared with other measurement techniques However the frequency

range is more restricted than the FRA When the currents 1I and

2I flowing through standard SZ and

unknown UZ are equal in magnitude

US Z

V

Z

V 21 =

they will combine to produce zero core flux in the current transformer Zero current in the neutral line

can be detected with a null indicator because the two currents will be 180deg out of phase to each other

In this study Wayne Kerr universal bridge B221 was used to determine the dielectric loss for XLPE

cable systems over the frequency range of 200Hz~20kHz and loss tangent resolution of below 10-4

Figure 3 Principle of frequency response analyzer Figure 4 Principle of transformer ratio bridge

In order to achieve sufficient sensitivity around 50Hz alternative methods were employed

involving time domain dielectric spectroscopy This involves time domain measurement of the

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

3

chargingdischarging currents The principle of obtaining frequency spectra from current measurement

is shown in Figure 5 In this study an electrometer was used to measure the discharging current of

XLPE cable samples I(t) with the time domain data captured using a digitizing oscilloscope The

complex permittivity was determined using the Fourier transform

)exp()(1

)(0000 C

GjdttjtI

VC ωωεωε minusminus+= int

infin

infin

lowast

where εinfin is the high frequency permittivity V0 is the applied dc voltage and G is the conductance

Figure 5 Principle of time domain dielectric spectroscopy

3 Experiment results The XLPE model cables were produced by Borealis AB and degassed for 5 days at 80C before they

were prepared into 5m long test samples Conductive adhesive copper tape was used to wrap the cable

sample in order to make good electrical contact with the outer semicon layer

31 Results of different techniques

FRA measurement results are shown in Figure 6 as a function of cable temperature DC conduction

behaviour (slope of -1 in the tanδ response) can be observed above 40C in the range of 10-4

Hz~1Hz

At frequencies above 10Hz the results cannot be distinguished from the instrument background

measured using a specially designed empty cell cable This was made of copper with the same

geometry as the model cables Arrhenius behaviour with an activation energy of 102eV was found in

the DC conductivity using the equation

)exp(0kT

Etimes= σσ

10-4

10-2

100

102

104

106

10-8

10-6

10-4

10-2

100

102

Frequency (Hz)

tan

δ

40oC

60oC

80oC

100oC

background

10-3

10-2

10-1

100

101

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

Time (s)

Dis

ch

arg

ing

Curr

ent

(A)

10oC

20oC

30oC

40oC

50oC

Figure 6 FRA results on XLPE cables Figure 7 Discharging current of XLPE cables

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

4

TDDS (time domain dielectric spectroscopy) measurements were obtained from discharging

current measurements taken over the range 1ms~10s (as shown in Figure 7) Excluding unreliable data

due to truncation noise and the DFT algorithm the usable spectroscopy range was about 05~200Hz

The frequency spectra were calculated using equation

NkikN

jix

VNCk

N

i

K2102

exp)(2

)(10

=

minus

+= sum=

infin

lowast πεε

and are shown in Figure 8 The loss tangent of the XLPE cables was found to be around 10

-4 at about

1Hz and increases to 10-3

with a peak at approximately 200Hz

Transformer bridge measurement results with different with and without the outer copper tape are

shown in Figure 9 Additional losses above 3kHz are associated with loss due to the conductance of

the semicon material Hence for frequencies above this value copper conductive tape must be used to

improve electrical contact along the whole length of the cable With copper tape the loss tangent of

XLPE cables in this frequency region decreases from 10-3

to below 10-4

at 6kHz

100

101

102

10-4

10-3

10-2

Frequency (Hz)

tan

δ

10oC

20oC

30oC

40oC

50oC

103

104

10-4

10-3

10-2

Frequency (Hz)

tan

δ

Electrode with semicon surface resistance

Electrode with conductive copper tape

Figure 8 Frequency spectra of TDDS data after

FT

Figure 9 TRB results on XLPE cables with

different measuring electrodes

32 Master curve of XLPE cable spectra

The 40C measurement data using the 3 spectroscopy techniques was merged together to give the

master curve of the XLPE cables in a frequency range from 10-4

Hz to 104Hz as shown in Figure 10

10-4

10-2

100

102

104

10-5

10-4

10-3

10-2

10-1

Frequency (Hz)

tan

δ

Das-Gupta RT

Kuschel 25oC

Peter Werelius 22oC

Tong Liu 40oC

T-bridge T Liu

TDDS T Liu

FRA T Liu

FRA data

Bridge data

TDDS data

Figure 10 Comparison of XLPE cable spectra with Das-Gupta Kuschel and Peter Werelius

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

5

At low frequencies the master curve exhibits DC conduction and the conductivity of the XLPE cable

at 40C was calculated to be 11times10-15

Sm A loss peak was also found at 200Hz at the join of the

TDDS data and the bridge data In Figure 10 published data by Das-Gupta [6] Kuschel [7] and Peter

Werelius [8] [9] are shown for comparison Using high voltage dielectric spectroscopy using a

frequency response analyzer Peter Werelius measured 12kV XLPE cables from 10-3

Hz to 100Hz At

lower frequencies a DC conduction can be identified with the significantly smaller conductance

(compared with the master curve) being due to the lower test temperature Kuschel also measured non-

aged 1220kV XLPE cables of different German manufacturers with an active length of between 05m

and 2m in Technid University of Berlin Very low flat loss was found in the loss tangent spectra at

1kV and 25C The dielectric loss is also increasing at higher frequency which is similar with the

master curve in this study Combining both FDDS and TDDS techniques Das-Gupta investigated a

wide frequency range loss spectra on 15kV XLPE cable samples The lower frequency spectrum was

transformed from discharging current with Hamon Approximation While no DC conduction should

be included in the TDDS results a low frequency peak was assumed by fitting the data with the

Universal Relaxation Law [10] The middle frequency range is flat without any measurement data and

another broad peak was fitted by Das-Gupta The additional loss at higher frequencies was due to the

conductance of tap water used as an outer measuring electrode

4 Conclusion and future work In conclusion the measurement results have identified 3 dielectric loss mechanisms

1 In the lower frequency range below 1Hz DC conduction behaviour was found to be dominant

2 In the middle frequency range of 100Hz~300Hz a relaxation peak was found centred at a

frequency of 200Hz

3 In the higher frequency range of 500Hz~20kHz the electrical conductance of the outer semicon

layer contributes additional dielectric loss This can be eliminated by improved electrical contact

to the outer semicon material using copper tape

Future work includes improving the time domain technique and validating the loss peak at 200Hz

The loss mechanisms of XLPE cables will be further studied with using different types of

insulationsemicon materials as well as non-degassed and aged cables

5 References [1] R M Black The History of Electric Wires and Cables Peter Pergrinus London 1983

[2] Vahdat Vahedy ldquoPolymer Insulated High Voltage Cablesrdquo IEEE Electrical Insulation Magazine Vol 22

No 3 2006

[3] Walter S Zaengl ldquoApplications of Dielectric Spectroscopy in Time and Frequency Domain for HV Power

Equipmentrdquo IEEE Electrical Insulation Magazine Vol 19 No 6 2003

[4] Tony Blythe David Bloor Electrical Properties of Polymers Cambridge University Press 2005

[5] James P Runt John J Fitzgerald Dielectric Spectroscopy of Polymeric Materials Fundamentals and

Applications American Chemical Society 1999

[6] P C N Scarpa A Svatiacutek D K Das-Gupta ldquoDielectric spectroscopy of polyethylene in the frequency

range of 10-5

Hz to 106 Hzrdquo Polymer Engineering and Science Vol 26 No 8 1996

[7] MKuschel and WKalkner ldquoDielectric response measurements in time and frequency domain ofdifferent

XLPE homo- and copolymer insulated medium voltage cablesrdquo IEE Proceedings - Science Measurement

and Technology Vol 146 No 5 1999

[8] Peter Werelius Development and Application of High Voltage Dielectric Spectroscopy for Diagnosis of

Medium Voltage XLPE Cables PhD thesis Stockholm KTH Electrical Engineering 2001

[9] P Werelius P Tharning R Eriksson B Holmgren and U Gafvert ldquoDielectric spectroscopy for diagnosis

of water tree deterioration in XLPE cablesrdquo IEEE Transactions on Dielectrics and Electrical Insulation Vol

8 No 1 pp 27ndash42 2001 [10]A K Jonscher Universal Relaxation Law Chelsea Dielectrics Press London 1996

Acknowledgements The author is grateful to Mr Ulf Nilsson and Borealis AB Sweden for the sponsorship

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

6

obtained under cable manufacturing conditions and those present for the formation of thin films Also

in typical cables semiconducting shields which may also contribute to the dielectric loss of the cable

are extruded along with the cable insulation and it would be difficult to replicate the concentration and

distribution of the shields and of the crosslinking by-products and impurities using thin film test

specimens For this reason triple extruded model cables having reduced insulation thickness are used

in this study The dielectric loss mechanisms will be studied by means of the complementary use of

frequency and time domain dielectric spectroscopy in order to cover the widest possible frequency

range with the required sensitivity

11 Principle of dielectric spectroscopy

Dielectric spectroscopy is based on the phenomena of electrical polarization and electrical conduction

in materials There are a number of different dielectric polarization mechanisms operating at the

molecular or microscopic level Each polarization mechanism either a relaxation or resonance

processes is centred around its particular characteristic frequency which is the reciprocal of the

characteristic time of the process and therefore separable in frequency

The most common mechanisms can be divided into three main categories as shown in Figure 1 At

the highest frequencies the electric field will cause a slight displacement of the electrons of any atom

with respect to the positive nucleus electronic polarization while at lower frequencies atomic

polarization is due to the distortion of the arrangement of atomic nuclei in a molecule or lattice All

polymeric materials have these two types of high frequencies polarization and which occur above

infra-red frequency Orientational polarization occurs when particular molecular groups exhibiting a

permanent dipole moment initially orientated randomly in space tend to be aligned by the applied

field to give a net polarization in that direction The rate of dipolar orientation is highly dependent on

inter- and intra-molecular interaction Orientation of molecular dipoles can therefore occur over a wide

range of frequency dependent on the ease with which the dipoles can rotate Dipoles due to absorbed

moisture will be much more easily rotated than polar groups associated with the main polymer chains

which may require the co-operative motion of surrounding molecular chains for orientation For this

reason dielectric spectroscopy ideally suited for the identification and differentiation of various polar

groups on main chain or side chains At very low frequencies DC conduction will become significant

and usually manifests as a slope of -1 in the imaginary permittivity whilst the real part remains

constant Thus dielectric spectroscopy is also well suited for the determination of DC conductivity of

materials Other common dielectric responses include the quasi-DC mechanism where the low

frequency permittivity real and imaginary have the same slope resulting from partially mobile charge

carriers gradually moving or hopping within the material Interfacial polarisation (Maxwell-Wagner

polarization) can also manifest at low frequency and in this case the real permittivity has a slope of -2

while the imaginary part has slope of -1

Figure 1 Dielectric permittivity spectrum over a

wide range of frequencies

Figure 2 Summary of dielectric spectroscopy

techniques

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

2

2 Choice of techniques for XLPE cables Various dielectric spectroscopy techniques and their associated frequency ranges are summarized in

Figure 2 [4] As we are interested in the dielectric loss mechanisms in high voltage power cables

operated at 50Hz the low frequency techniques covering 10-4

Hz~106Hz have been explored in search

of those with appropriate sensitivity to measure the extremely low loss model cables In practice most

techniques only have sufficient sensitivity over limited frequency ranges and it is necessary to

combine many techniques to achieve the required sensitivity over a wide range of frequency

The frequency response analyzer (FRA) in conjunction with a dielectric interface has a wide

frequency range As shown in Figure 3 phase sensitive voltmeters are used to compare the voltage

)(1 ωU and )(2 ωU (the latter proportional to Is(ω)) Commercial instruments often employ a reference

impedance ZR within the dielectric interface to facilitate accurate measurements The complex

impedance )(ωSZ of the sample can be calculated from the measured data by the equation

)1)(

)((

)(

)()(

2

1minus==

ω

ω

ω

ωω

U

UR

I

UZ

S

SS

Where ω is the angular frequency The complex relative permittivity can be then calculated from

)(

1

0CZi S ωωε =

lowast

provided that the geometric capacitance C0 of the sample is known [5] However tests have shown

that sensitivity of loss tangent is limited to the range 10-4

to 10-2

dependent on frequency as can be

seen from the background measurements in figure 6 In practice a Solartron FRA 1255 and Dielectric

Interface 1296 were used for the measurement of the model XLPE cables

Transformer ratio bridges (TRB) as shown in Figure 4 are often used to cover the audio frequency

range Using a set of standard impedances and a multi-tap transformer this type of bridge allows for

high precision measurements compared with other measurement techniques However the frequency

range is more restricted than the FRA When the currents 1I and

2I flowing through standard SZ and

unknown UZ are equal in magnitude

US Z

V

Z

V 21 =

they will combine to produce zero core flux in the current transformer Zero current in the neutral line

can be detected with a null indicator because the two currents will be 180deg out of phase to each other

In this study Wayne Kerr universal bridge B221 was used to determine the dielectric loss for XLPE

cable systems over the frequency range of 200Hz~20kHz and loss tangent resolution of below 10-4

Figure 3 Principle of frequency response analyzer Figure 4 Principle of transformer ratio bridge

In order to achieve sufficient sensitivity around 50Hz alternative methods were employed

involving time domain dielectric spectroscopy This involves time domain measurement of the

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

3

chargingdischarging currents The principle of obtaining frequency spectra from current measurement

is shown in Figure 5 In this study an electrometer was used to measure the discharging current of

XLPE cable samples I(t) with the time domain data captured using a digitizing oscilloscope The

complex permittivity was determined using the Fourier transform

)exp()(1

)(0000 C

GjdttjtI

VC ωωεωε minusminus+= int

infin

infin

lowast

where εinfin is the high frequency permittivity V0 is the applied dc voltage and G is the conductance

Figure 5 Principle of time domain dielectric spectroscopy

3 Experiment results The XLPE model cables were produced by Borealis AB and degassed for 5 days at 80C before they

were prepared into 5m long test samples Conductive adhesive copper tape was used to wrap the cable

sample in order to make good electrical contact with the outer semicon layer

31 Results of different techniques

FRA measurement results are shown in Figure 6 as a function of cable temperature DC conduction

behaviour (slope of -1 in the tanδ response) can be observed above 40C in the range of 10-4

Hz~1Hz

At frequencies above 10Hz the results cannot be distinguished from the instrument background

measured using a specially designed empty cell cable This was made of copper with the same

geometry as the model cables Arrhenius behaviour with an activation energy of 102eV was found in

the DC conductivity using the equation

)exp(0kT

Etimes= σσ

10-4

10-2

100

102

104

106

10-8

10-6

10-4

10-2

100

102

Frequency (Hz)

tan

δ

40oC

60oC

80oC

100oC

background

10-3

10-2

10-1

100

101

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

Time (s)

Dis

ch

arg

ing

Curr

ent

(A)

10oC

20oC

30oC

40oC

50oC

Figure 6 FRA results on XLPE cables Figure 7 Discharging current of XLPE cables

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

4

TDDS (time domain dielectric spectroscopy) measurements were obtained from discharging

current measurements taken over the range 1ms~10s (as shown in Figure 7) Excluding unreliable data

due to truncation noise and the DFT algorithm the usable spectroscopy range was about 05~200Hz

The frequency spectra were calculated using equation

NkikN

jix

VNCk

N

i

K2102

exp)(2

)(10

=

minus

+= sum=

infin

lowast πεε

and are shown in Figure 8 The loss tangent of the XLPE cables was found to be around 10

-4 at about

1Hz and increases to 10-3

with a peak at approximately 200Hz

Transformer bridge measurement results with different with and without the outer copper tape are

shown in Figure 9 Additional losses above 3kHz are associated with loss due to the conductance of

the semicon material Hence for frequencies above this value copper conductive tape must be used to

improve electrical contact along the whole length of the cable With copper tape the loss tangent of

XLPE cables in this frequency region decreases from 10-3

to below 10-4

at 6kHz

100

101

102

10-4

10-3

10-2

Frequency (Hz)

tan

δ

10oC

20oC

30oC

40oC

50oC

103

104

10-4

10-3

10-2

Frequency (Hz)

tan

δ

Electrode with semicon surface resistance

Electrode with conductive copper tape

Figure 8 Frequency spectra of TDDS data after

FT

Figure 9 TRB results on XLPE cables with

different measuring electrodes

32 Master curve of XLPE cable spectra

The 40C measurement data using the 3 spectroscopy techniques was merged together to give the

master curve of the XLPE cables in a frequency range from 10-4

Hz to 104Hz as shown in Figure 10

10-4

10-2

100

102

104

10-5

10-4

10-3

10-2

10-1

Frequency (Hz)

tan

δ

Das-Gupta RT

Kuschel 25oC

Peter Werelius 22oC

Tong Liu 40oC

T-bridge T Liu

TDDS T Liu

FRA T Liu

FRA data

Bridge data

TDDS data

Figure 10 Comparison of XLPE cable spectra with Das-Gupta Kuschel and Peter Werelius

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

5

At low frequencies the master curve exhibits DC conduction and the conductivity of the XLPE cable

at 40C was calculated to be 11times10-15

Sm A loss peak was also found at 200Hz at the join of the

TDDS data and the bridge data In Figure 10 published data by Das-Gupta [6] Kuschel [7] and Peter

Werelius [8] [9] are shown for comparison Using high voltage dielectric spectroscopy using a

frequency response analyzer Peter Werelius measured 12kV XLPE cables from 10-3

Hz to 100Hz At

lower frequencies a DC conduction can be identified with the significantly smaller conductance

(compared with the master curve) being due to the lower test temperature Kuschel also measured non-

aged 1220kV XLPE cables of different German manufacturers with an active length of between 05m

and 2m in Technid University of Berlin Very low flat loss was found in the loss tangent spectra at

1kV and 25C The dielectric loss is also increasing at higher frequency which is similar with the

master curve in this study Combining both FDDS and TDDS techniques Das-Gupta investigated a

wide frequency range loss spectra on 15kV XLPE cable samples The lower frequency spectrum was

transformed from discharging current with Hamon Approximation While no DC conduction should

be included in the TDDS results a low frequency peak was assumed by fitting the data with the

Universal Relaxation Law [10] The middle frequency range is flat without any measurement data and

another broad peak was fitted by Das-Gupta The additional loss at higher frequencies was due to the

conductance of tap water used as an outer measuring electrode

4 Conclusion and future work In conclusion the measurement results have identified 3 dielectric loss mechanisms

1 In the lower frequency range below 1Hz DC conduction behaviour was found to be dominant

2 In the middle frequency range of 100Hz~300Hz a relaxation peak was found centred at a

frequency of 200Hz

3 In the higher frequency range of 500Hz~20kHz the electrical conductance of the outer semicon

layer contributes additional dielectric loss This can be eliminated by improved electrical contact

to the outer semicon material using copper tape

Future work includes improving the time domain technique and validating the loss peak at 200Hz

The loss mechanisms of XLPE cables will be further studied with using different types of

insulationsemicon materials as well as non-degassed and aged cables

5 References [1] R M Black The History of Electric Wires and Cables Peter Pergrinus London 1983

[2] Vahdat Vahedy ldquoPolymer Insulated High Voltage Cablesrdquo IEEE Electrical Insulation Magazine Vol 22

No 3 2006

[3] Walter S Zaengl ldquoApplications of Dielectric Spectroscopy in Time and Frequency Domain for HV Power

Equipmentrdquo IEEE Electrical Insulation Magazine Vol 19 No 6 2003

[4] Tony Blythe David Bloor Electrical Properties of Polymers Cambridge University Press 2005

[5] James P Runt John J Fitzgerald Dielectric Spectroscopy of Polymeric Materials Fundamentals and

Applications American Chemical Society 1999

[6] P C N Scarpa A Svatiacutek D K Das-Gupta ldquoDielectric spectroscopy of polyethylene in the frequency

range of 10-5

Hz to 106 Hzrdquo Polymer Engineering and Science Vol 26 No 8 1996

[7] MKuschel and WKalkner ldquoDielectric response measurements in time and frequency domain ofdifferent

XLPE homo- and copolymer insulated medium voltage cablesrdquo IEE Proceedings - Science Measurement

and Technology Vol 146 No 5 1999

[8] Peter Werelius Development and Application of High Voltage Dielectric Spectroscopy for Diagnosis of

Medium Voltage XLPE Cables PhD thesis Stockholm KTH Electrical Engineering 2001

[9] P Werelius P Tharning R Eriksson B Holmgren and U Gafvert ldquoDielectric spectroscopy for diagnosis

of water tree deterioration in XLPE cablesrdquo IEEE Transactions on Dielectrics and Electrical Insulation Vol

8 No 1 pp 27ndash42 2001 [10]A K Jonscher Universal Relaxation Law Chelsea Dielectrics Press London 1996

Acknowledgements The author is grateful to Mr Ulf Nilsson and Borealis AB Sweden for the sponsorship

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

6

2 Choice of techniques for XLPE cables Various dielectric spectroscopy techniques and their associated frequency ranges are summarized in

Figure 2 [4] As we are interested in the dielectric loss mechanisms in high voltage power cables

operated at 50Hz the low frequency techniques covering 10-4

Hz~106Hz have been explored in search

of those with appropriate sensitivity to measure the extremely low loss model cables In practice most

techniques only have sufficient sensitivity over limited frequency ranges and it is necessary to

combine many techniques to achieve the required sensitivity over a wide range of frequency

The frequency response analyzer (FRA) in conjunction with a dielectric interface has a wide

frequency range As shown in Figure 3 phase sensitive voltmeters are used to compare the voltage

)(1 ωU and )(2 ωU (the latter proportional to Is(ω)) Commercial instruments often employ a reference

impedance ZR within the dielectric interface to facilitate accurate measurements The complex

impedance )(ωSZ of the sample can be calculated from the measured data by the equation

)1)(

)((

)(

)()(

2

1minus==

ω

ω

ω

ωω

U

UR

I

UZ

S

SS

Where ω is the angular frequency The complex relative permittivity can be then calculated from

)(

1

0CZi S ωωε =

lowast

provided that the geometric capacitance C0 of the sample is known [5] However tests have shown

that sensitivity of loss tangent is limited to the range 10-4

to 10-2

dependent on frequency as can be

seen from the background measurements in figure 6 In practice a Solartron FRA 1255 and Dielectric

Interface 1296 were used for the measurement of the model XLPE cables

Transformer ratio bridges (TRB) as shown in Figure 4 are often used to cover the audio frequency

range Using a set of standard impedances and a multi-tap transformer this type of bridge allows for

high precision measurements compared with other measurement techniques However the frequency

range is more restricted than the FRA When the currents 1I and

2I flowing through standard SZ and

unknown UZ are equal in magnitude

US Z

V

Z

V 21 =

they will combine to produce zero core flux in the current transformer Zero current in the neutral line

can be detected with a null indicator because the two currents will be 180deg out of phase to each other

In this study Wayne Kerr universal bridge B221 was used to determine the dielectric loss for XLPE

cable systems over the frequency range of 200Hz~20kHz and loss tangent resolution of below 10-4

Figure 3 Principle of frequency response analyzer Figure 4 Principle of transformer ratio bridge

In order to achieve sufficient sensitivity around 50Hz alternative methods were employed

involving time domain dielectric spectroscopy This involves time domain measurement of the

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

3

chargingdischarging currents The principle of obtaining frequency spectra from current measurement

is shown in Figure 5 In this study an electrometer was used to measure the discharging current of

XLPE cable samples I(t) with the time domain data captured using a digitizing oscilloscope The

complex permittivity was determined using the Fourier transform

)exp()(1

)(0000 C

GjdttjtI

VC ωωεωε minusminus+= int

infin

infin

lowast

where εinfin is the high frequency permittivity V0 is the applied dc voltage and G is the conductance

Figure 5 Principle of time domain dielectric spectroscopy

3 Experiment results The XLPE model cables were produced by Borealis AB and degassed for 5 days at 80C before they

were prepared into 5m long test samples Conductive adhesive copper tape was used to wrap the cable

sample in order to make good electrical contact with the outer semicon layer

31 Results of different techniques

FRA measurement results are shown in Figure 6 as a function of cable temperature DC conduction

behaviour (slope of -1 in the tanδ response) can be observed above 40C in the range of 10-4

Hz~1Hz

At frequencies above 10Hz the results cannot be distinguished from the instrument background

measured using a specially designed empty cell cable This was made of copper with the same

geometry as the model cables Arrhenius behaviour with an activation energy of 102eV was found in

the DC conductivity using the equation

)exp(0kT

Etimes= σσ

10-4

10-2

100

102

104

106

10-8

10-6

10-4

10-2

100

102

Frequency (Hz)

tan

δ

40oC

60oC

80oC

100oC

background

10-3

10-2

10-1

100

101

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

Time (s)

Dis

ch

arg

ing

Curr

ent

(A)

10oC

20oC

30oC

40oC

50oC

Figure 6 FRA results on XLPE cables Figure 7 Discharging current of XLPE cables

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

4

TDDS (time domain dielectric spectroscopy) measurements were obtained from discharging

current measurements taken over the range 1ms~10s (as shown in Figure 7) Excluding unreliable data

due to truncation noise and the DFT algorithm the usable spectroscopy range was about 05~200Hz

The frequency spectra were calculated using equation

NkikN

jix

VNCk

N

i

K2102

exp)(2

)(10

=

minus

+= sum=

infin

lowast πεε

and are shown in Figure 8 The loss tangent of the XLPE cables was found to be around 10

-4 at about

1Hz and increases to 10-3

with a peak at approximately 200Hz

Transformer bridge measurement results with different with and without the outer copper tape are

shown in Figure 9 Additional losses above 3kHz are associated with loss due to the conductance of

the semicon material Hence for frequencies above this value copper conductive tape must be used to

improve electrical contact along the whole length of the cable With copper tape the loss tangent of

XLPE cables in this frequency region decreases from 10-3

to below 10-4

at 6kHz

100

101

102

10-4

10-3

10-2

Frequency (Hz)

tan

δ

10oC

20oC

30oC

40oC

50oC

103

104

10-4

10-3

10-2

Frequency (Hz)

tan

δ

Electrode with semicon surface resistance

Electrode with conductive copper tape

Figure 8 Frequency spectra of TDDS data after

FT

Figure 9 TRB results on XLPE cables with

different measuring electrodes

32 Master curve of XLPE cable spectra

The 40C measurement data using the 3 spectroscopy techniques was merged together to give the

master curve of the XLPE cables in a frequency range from 10-4

Hz to 104Hz as shown in Figure 10

10-4

10-2

100

102

104

10-5

10-4

10-3

10-2

10-1

Frequency (Hz)

tan

δ

Das-Gupta RT

Kuschel 25oC

Peter Werelius 22oC

Tong Liu 40oC

T-bridge T Liu

TDDS T Liu

FRA T Liu

FRA data

Bridge data

TDDS data

Figure 10 Comparison of XLPE cable spectra with Das-Gupta Kuschel and Peter Werelius

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

5

At low frequencies the master curve exhibits DC conduction and the conductivity of the XLPE cable

at 40C was calculated to be 11times10-15

Sm A loss peak was also found at 200Hz at the join of the

TDDS data and the bridge data In Figure 10 published data by Das-Gupta [6] Kuschel [7] and Peter

Werelius [8] [9] are shown for comparison Using high voltage dielectric spectroscopy using a

frequency response analyzer Peter Werelius measured 12kV XLPE cables from 10-3

Hz to 100Hz At

lower frequencies a DC conduction can be identified with the significantly smaller conductance

(compared with the master curve) being due to the lower test temperature Kuschel also measured non-

aged 1220kV XLPE cables of different German manufacturers with an active length of between 05m

and 2m in Technid University of Berlin Very low flat loss was found in the loss tangent spectra at

1kV and 25C The dielectric loss is also increasing at higher frequency which is similar with the

master curve in this study Combining both FDDS and TDDS techniques Das-Gupta investigated a

wide frequency range loss spectra on 15kV XLPE cable samples The lower frequency spectrum was

transformed from discharging current with Hamon Approximation While no DC conduction should

be included in the TDDS results a low frequency peak was assumed by fitting the data with the

Universal Relaxation Law [10] The middle frequency range is flat without any measurement data and

another broad peak was fitted by Das-Gupta The additional loss at higher frequencies was due to the

conductance of tap water used as an outer measuring electrode

4 Conclusion and future work In conclusion the measurement results have identified 3 dielectric loss mechanisms

1 In the lower frequency range below 1Hz DC conduction behaviour was found to be dominant

2 In the middle frequency range of 100Hz~300Hz a relaxation peak was found centred at a

frequency of 200Hz

3 In the higher frequency range of 500Hz~20kHz the electrical conductance of the outer semicon

layer contributes additional dielectric loss This can be eliminated by improved electrical contact

to the outer semicon material using copper tape

Future work includes improving the time domain technique and validating the loss peak at 200Hz

The loss mechanisms of XLPE cables will be further studied with using different types of

insulationsemicon materials as well as non-degassed and aged cables

5 References [1] R M Black The History of Electric Wires and Cables Peter Pergrinus London 1983

[2] Vahdat Vahedy ldquoPolymer Insulated High Voltage Cablesrdquo IEEE Electrical Insulation Magazine Vol 22

No 3 2006

[3] Walter S Zaengl ldquoApplications of Dielectric Spectroscopy in Time and Frequency Domain for HV Power

Equipmentrdquo IEEE Electrical Insulation Magazine Vol 19 No 6 2003

[4] Tony Blythe David Bloor Electrical Properties of Polymers Cambridge University Press 2005

[5] James P Runt John J Fitzgerald Dielectric Spectroscopy of Polymeric Materials Fundamentals and

Applications American Chemical Society 1999

[6] P C N Scarpa A Svatiacutek D K Das-Gupta ldquoDielectric spectroscopy of polyethylene in the frequency

range of 10-5

Hz to 106 Hzrdquo Polymer Engineering and Science Vol 26 No 8 1996

[7] MKuschel and WKalkner ldquoDielectric response measurements in time and frequency domain ofdifferent

XLPE homo- and copolymer insulated medium voltage cablesrdquo IEE Proceedings - Science Measurement

and Technology Vol 146 No 5 1999

[8] Peter Werelius Development and Application of High Voltage Dielectric Spectroscopy for Diagnosis of

Medium Voltage XLPE Cables PhD thesis Stockholm KTH Electrical Engineering 2001

[9] P Werelius P Tharning R Eriksson B Holmgren and U Gafvert ldquoDielectric spectroscopy for diagnosis

of water tree deterioration in XLPE cablesrdquo IEEE Transactions on Dielectrics and Electrical Insulation Vol

8 No 1 pp 27ndash42 2001 [10]A K Jonscher Universal Relaxation Law Chelsea Dielectrics Press London 1996

Acknowledgements The author is grateful to Mr Ulf Nilsson and Borealis AB Sweden for the sponsorship

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

6

chargingdischarging currents The principle of obtaining frequency spectra from current measurement

is shown in Figure 5 In this study an electrometer was used to measure the discharging current of

XLPE cable samples I(t) with the time domain data captured using a digitizing oscilloscope The

complex permittivity was determined using the Fourier transform

)exp()(1

)(0000 C

GjdttjtI

VC ωωεωε minusminus+= int

infin

infin

lowast

where εinfin is the high frequency permittivity V0 is the applied dc voltage and G is the conductance

Figure 5 Principle of time domain dielectric spectroscopy

3 Experiment results The XLPE model cables were produced by Borealis AB and degassed for 5 days at 80C before they

were prepared into 5m long test samples Conductive adhesive copper tape was used to wrap the cable

sample in order to make good electrical contact with the outer semicon layer

31 Results of different techniques

FRA measurement results are shown in Figure 6 as a function of cable temperature DC conduction

behaviour (slope of -1 in the tanδ response) can be observed above 40C in the range of 10-4

Hz~1Hz

At frequencies above 10Hz the results cannot be distinguished from the instrument background

measured using a specially designed empty cell cable This was made of copper with the same

geometry as the model cables Arrhenius behaviour with an activation energy of 102eV was found in

the DC conductivity using the equation

)exp(0kT

Etimes= σσ

10-4

10-2

100

102

104

106

10-8

10-6

10-4

10-2

100

102

Frequency (Hz)

tan

δ

40oC

60oC

80oC

100oC

background

10-3

10-2

10-1

100

101

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

Time (s)

Dis

ch

arg

ing

Curr

ent

(A)

10oC

20oC

30oC

40oC

50oC

Figure 6 FRA results on XLPE cables Figure 7 Discharging current of XLPE cables

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

4

TDDS (time domain dielectric spectroscopy) measurements were obtained from discharging

current measurements taken over the range 1ms~10s (as shown in Figure 7) Excluding unreliable data

due to truncation noise and the DFT algorithm the usable spectroscopy range was about 05~200Hz

The frequency spectra were calculated using equation

NkikN

jix

VNCk

N

i

K2102

exp)(2

)(10

=

minus

+= sum=

infin

lowast πεε

and are shown in Figure 8 The loss tangent of the XLPE cables was found to be around 10

-4 at about

1Hz and increases to 10-3

with a peak at approximately 200Hz

Transformer bridge measurement results with different with and without the outer copper tape are

shown in Figure 9 Additional losses above 3kHz are associated with loss due to the conductance of

the semicon material Hence for frequencies above this value copper conductive tape must be used to

improve electrical contact along the whole length of the cable With copper tape the loss tangent of

XLPE cables in this frequency region decreases from 10-3

to below 10-4

at 6kHz

100

101

102

10-4

10-3

10-2

Frequency (Hz)

tan

δ

10oC

20oC

30oC

40oC

50oC

103

104

10-4

10-3

10-2

Frequency (Hz)

tan

δ

Electrode with semicon surface resistance

Electrode with conductive copper tape

Figure 8 Frequency spectra of TDDS data after

FT

Figure 9 TRB results on XLPE cables with

different measuring electrodes

32 Master curve of XLPE cable spectra

The 40C measurement data using the 3 spectroscopy techniques was merged together to give the

master curve of the XLPE cables in a frequency range from 10-4

Hz to 104Hz as shown in Figure 10

10-4

10-2

100

102

104

10-5

10-4

10-3

10-2

10-1

Frequency (Hz)

tan

δ

Das-Gupta RT

Kuschel 25oC

Peter Werelius 22oC

Tong Liu 40oC

T-bridge T Liu

TDDS T Liu

FRA T Liu

FRA data

Bridge data

TDDS data

Figure 10 Comparison of XLPE cable spectra with Das-Gupta Kuschel and Peter Werelius

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

5

At low frequencies the master curve exhibits DC conduction and the conductivity of the XLPE cable

at 40C was calculated to be 11times10-15

Sm A loss peak was also found at 200Hz at the join of the

TDDS data and the bridge data In Figure 10 published data by Das-Gupta [6] Kuschel [7] and Peter

Werelius [8] [9] are shown for comparison Using high voltage dielectric spectroscopy using a

frequency response analyzer Peter Werelius measured 12kV XLPE cables from 10-3

Hz to 100Hz At

lower frequencies a DC conduction can be identified with the significantly smaller conductance

(compared with the master curve) being due to the lower test temperature Kuschel also measured non-

aged 1220kV XLPE cables of different German manufacturers with an active length of between 05m

and 2m in Technid University of Berlin Very low flat loss was found in the loss tangent spectra at

1kV and 25C The dielectric loss is also increasing at higher frequency which is similar with the

master curve in this study Combining both FDDS and TDDS techniques Das-Gupta investigated a

wide frequency range loss spectra on 15kV XLPE cable samples The lower frequency spectrum was

transformed from discharging current with Hamon Approximation While no DC conduction should

be included in the TDDS results a low frequency peak was assumed by fitting the data with the

Universal Relaxation Law [10] The middle frequency range is flat without any measurement data and

another broad peak was fitted by Das-Gupta The additional loss at higher frequencies was due to the

conductance of tap water used as an outer measuring electrode

4 Conclusion and future work In conclusion the measurement results have identified 3 dielectric loss mechanisms

1 In the lower frequency range below 1Hz DC conduction behaviour was found to be dominant

2 In the middle frequency range of 100Hz~300Hz a relaxation peak was found centred at a

frequency of 200Hz

3 In the higher frequency range of 500Hz~20kHz the electrical conductance of the outer semicon

layer contributes additional dielectric loss This can be eliminated by improved electrical contact

to the outer semicon material using copper tape

Future work includes improving the time domain technique and validating the loss peak at 200Hz

The loss mechanisms of XLPE cables will be further studied with using different types of

insulationsemicon materials as well as non-degassed and aged cables

5 References [1] R M Black The History of Electric Wires and Cables Peter Pergrinus London 1983

[2] Vahdat Vahedy ldquoPolymer Insulated High Voltage Cablesrdquo IEEE Electrical Insulation Magazine Vol 22

No 3 2006

[3] Walter S Zaengl ldquoApplications of Dielectric Spectroscopy in Time and Frequency Domain for HV Power

Equipmentrdquo IEEE Electrical Insulation Magazine Vol 19 No 6 2003

[4] Tony Blythe David Bloor Electrical Properties of Polymers Cambridge University Press 2005

[5] James P Runt John J Fitzgerald Dielectric Spectroscopy of Polymeric Materials Fundamentals and

Applications American Chemical Society 1999

[6] P C N Scarpa A Svatiacutek D K Das-Gupta ldquoDielectric spectroscopy of polyethylene in the frequency

range of 10-5

Hz to 106 Hzrdquo Polymer Engineering and Science Vol 26 No 8 1996

[7] MKuschel and WKalkner ldquoDielectric response measurements in time and frequency domain ofdifferent

XLPE homo- and copolymer insulated medium voltage cablesrdquo IEE Proceedings - Science Measurement

and Technology Vol 146 No 5 1999

[8] Peter Werelius Development and Application of High Voltage Dielectric Spectroscopy for Diagnosis of

Medium Voltage XLPE Cables PhD thesis Stockholm KTH Electrical Engineering 2001

[9] P Werelius P Tharning R Eriksson B Holmgren and U Gafvert ldquoDielectric spectroscopy for diagnosis

of water tree deterioration in XLPE cablesrdquo IEEE Transactions on Dielectrics and Electrical Insulation Vol

8 No 1 pp 27ndash42 2001 [10]A K Jonscher Universal Relaxation Law Chelsea Dielectrics Press London 1996

Acknowledgements The author is grateful to Mr Ulf Nilsson and Borealis AB Sweden for the sponsorship

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

6

TDDS (time domain dielectric spectroscopy) measurements were obtained from discharging

current measurements taken over the range 1ms~10s (as shown in Figure 7) Excluding unreliable data

due to truncation noise and the DFT algorithm the usable spectroscopy range was about 05~200Hz

The frequency spectra were calculated using equation

NkikN

jix

VNCk

N

i

K2102

exp)(2

)(10

=

minus

+= sum=

infin

lowast πεε

and are shown in Figure 8 The loss tangent of the XLPE cables was found to be around 10

-4 at about

1Hz and increases to 10-3

with a peak at approximately 200Hz

Transformer bridge measurement results with different with and without the outer copper tape are

shown in Figure 9 Additional losses above 3kHz are associated with loss due to the conductance of

the semicon material Hence for frequencies above this value copper conductive tape must be used to

improve electrical contact along the whole length of the cable With copper tape the loss tangent of

XLPE cables in this frequency region decreases from 10-3

to below 10-4

at 6kHz

100

101

102

10-4

10-3

10-2

Frequency (Hz)

tan

δ

10oC

20oC

30oC

40oC

50oC

103

104

10-4

10-3

10-2

Frequency (Hz)

tan

δ

Electrode with semicon surface resistance

Electrode with conductive copper tape

Figure 8 Frequency spectra of TDDS data after

FT

Figure 9 TRB results on XLPE cables with

different measuring electrodes

32 Master curve of XLPE cable spectra

The 40C measurement data using the 3 spectroscopy techniques was merged together to give the

master curve of the XLPE cables in a frequency range from 10-4

Hz to 104Hz as shown in Figure 10

10-4

10-2

100

102

104

10-5

10-4

10-3

10-2

10-1

Frequency (Hz)

tan

δ

Das-Gupta RT

Kuschel 25oC

Peter Werelius 22oC

Tong Liu 40oC

T-bridge T Liu

TDDS T Liu

FRA T Liu

FRA data

Bridge data

TDDS data

Figure 10 Comparison of XLPE cable spectra with Das-Gupta Kuschel and Peter Werelius

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

5

At low frequencies the master curve exhibits DC conduction and the conductivity of the XLPE cable

at 40C was calculated to be 11times10-15

Sm A loss peak was also found at 200Hz at the join of the

TDDS data and the bridge data In Figure 10 published data by Das-Gupta [6] Kuschel [7] and Peter

Werelius [8] [9] are shown for comparison Using high voltage dielectric spectroscopy using a

frequency response analyzer Peter Werelius measured 12kV XLPE cables from 10-3

Hz to 100Hz At

lower frequencies a DC conduction can be identified with the significantly smaller conductance

(compared with the master curve) being due to the lower test temperature Kuschel also measured non-

aged 1220kV XLPE cables of different German manufacturers with an active length of between 05m

and 2m in Technid University of Berlin Very low flat loss was found in the loss tangent spectra at

1kV and 25C The dielectric loss is also increasing at higher frequency which is similar with the

master curve in this study Combining both FDDS and TDDS techniques Das-Gupta investigated a

wide frequency range loss spectra on 15kV XLPE cable samples The lower frequency spectrum was

transformed from discharging current with Hamon Approximation While no DC conduction should

be included in the TDDS results a low frequency peak was assumed by fitting the data with the

Universal Relaxation Law [10] The middle frequency range is flat without any measurement data and

another broad peak was fitted by Das-Gupta The additional loss at higher frequencies was due to the

conductance of tap water used as an outer measuring electrode

4 Conclusion and future work In conclusion the measurement results have identified 3 dielectric loss mechanisms

1 In the lower frequency range below 1Hz DC conduction behaviour was found to be dominant

2 In the middle frequency range of 100Hz~300Hz a relaxation peak was found centred at a

frequency of 200Hz

3 In the higher frequency range of 500Hz~20kHz the electrical conductance of the outer semicon

layer contributes additional dielectric loss This can be eliminated by improved electrical contact

to the outer semicon material using copper tape

Future work includes improving the time domain technique and validating the loss peak at 200Hz

The loss mechanisms of XLPE cables will be further studied with using different types of

insulationsemicon materials as well as non-degassed and aged cables

5 References [1] R M Black The History of Electric Wires and Cables Peter Pergrinus London 1983

[2] Vahdat Vahedy ldquoPolymer Insulated High Voltage Cablesrdquo IEEE Electrical Insulation Magazine Vol 22

No 3 2006

[3] Walter S Zaengl ldquoApplications of Dielectric Spectroscopy in Time and Frequency Domain for HV Power

Equipmentrdquo IEEE Electrical Insulation Magazine Vol 19 No 6 2003

[4] Tony Blythe David Bloor Electrical Properties of Polymers Cambridge University Press 2005

[5] James P Runt John J Fitzgerald Dielectric Spectroscopy of Polymeric Materials Fundamentals and

Applications American Chemical Society 1999

[6] P C N Scarpa A Svatiacutek D K Das-Gupta ldquoDielectric spectroscopy of polyethylene in the frequency

range of 10-5

Hz to 106 Hzrdquo Polymer Engineering and Science Vol 26 No 8 1996

[7] MKuschel and WKalkner ldquoDielectric response measurements in time and frequency domain ofdifferent

XLPE homo- and copolymer insulated medium voltage cablesrdquo IEE Proceedings - Science Measurement

and Technology Vol 146 No 5 1999

[8] Peter Werelius Development and Application of High Voltage Dielectric Spectroscopy for Diagnosis of

Medium Voltage XLPE Cables PhD thesis Stockholm KTH Electrical Engineering 2001

[9] P Werelius P Tharning R Eriksson B Holmgren and U Gafvert ldquoDielectric spectroscopy for diagnosis

of water tree deterioration in XLPE cablesrdquo IEEE Transactions on Dielectrics and Electrical Insulation Vol

8 No 1 pp 27ndash42 2001 [10]A K Jonscher Universal Relaxation Law Chelsea Dielectrics Press London 1996

Acknowledgements The author is grateful to Mr Ulf Nilsson and Borealis AB Sweden for the sponsorship

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

6

At low frequencies the master curve exhibits DC conduction and the conductivity of the XLPE cable

at 40C was calculated to be 11times10-15

Sm A loss peak was also found at 200Hz at the join of the

TDDS data and the bridge data In Figure 10 published data by Das-Gupta [6] Kuschel [7] and Peter

Werelius [8] [9] are shown for comparison Using high voltage dielectric spectroscopy using a

frequency response analyzer Peter Werelius measured 12kV XLPE cables from 10-3

Hz to 100Hz At

lower frequencies a DC conduction can be identified with the significantly smaller conductance

(compared with the master curve) being due to the lower test temperature Kuschel also measured non-

aged 1220kV XLPE cables of different German manufacturers with an active length of between 05m

and 2m in Technid University of Berlin Very low flat loss was found in the loss tangent spectra at

1kV and 25C The dielectric loss is also increasing at higher frequency which is similar with the

master curve in this study Combining both FDDS and TDDS techniques Das-Gupta investigated a

wide frequency range loss spectra on 15kV XLPE cable samples The lower frequency spectrum was

transformed from discharging current with Hamon Approximation While no DC conduction should

be included in the TDDS results a low frequency peak was assumed by fitting the data with the

Universal Relaxation Law [10] The middle frequency range is flat without any measurement data and

another broad peak was fitted by Das-Gupta The additional loss at higher frequencies was due to the

conductance of tap water used as an outer measuring electrode

4 Conclusion and future work In conclusion the measurement results have identified 3 dielectric loss mechanisms

1 In the lower frequency range below 1Hz DC conduction behaviour was found to be dominant

2 In the middle frequency range of 100Hz~300Hz a relaxation peak was found centred at a

frequency of 200Hz

3 In the higher frequency range of 500Hz~20kHz the electrical conductance of the outer semicon

layer contributes additional dielectric loss This can be eliminated by improved electrical contact

to the outer semicon material using copper tape

Future work includes improving the time domain technique and validating the loss peak at 200Hz

The loss mechanisms of XLPE cables will be further studied with using different types of

insulationsemicon materials as well as non-degassed and aged cables

5 References [1] R M Black The History of Electric Wires and Cables Peter Pergrinus London 1983

[2] Vahdat Vahedy ldquoPolymer Insulated High Voltage Cablesrdquo IEEE Electrical Insulation Magazine Vol 22

No 3 2006

[3] Walter S Zaengl ldquoApplications of Dielectric Spectroscopy in Time and Frequency Domain for HV Power

Equipmentrdquo IEEE Electrical Insulation Magazine Vol 19 No 6 2003

[4] Tony Blythe David Bloor Electrical Properties of Polymers Cambridge University Press 2005

[5] James P Runt John J Fitzgerald Dielectric Spectroscopy of Polymeric Materials Fundamentals and

Applications American Chemical Society 1999

[6] P C N Scarpa A Svatiacutek D K Das-Gupta ldquoDielectric spectroscopy of polyethylene in the frequency

range of 10-5

Hz to 106 Hzrdquo Polymer Engineering and Science Vol 26 No 8 1996

[7] MKuschel and WKalkner ldquoDielectric response measurements in time and frequency domain ofdifferent

XLPE homo- and copolymer insulated medium voltage cablesrdquo IEE Proceedings - Science Measurement

and Technology Vol 146 No 5 1999

[8] Peter Werelius Development and Application of High Voltage Dielectric Spectroscopy for Diagnosis of

Medium Voltage XLPE Cables PhD thesis Stockholm KTH Electrical Engineering 2001

[9] P Werelius P Tharning R Eriksson B Holmgren and U Gafvert ldquoDielectric spectroscopy for diagnosis

of water tree deterioration in XLPE cablesrdquo IEEE Transactions on Dielectrics and Electrical Insulation Vol

8 No 1 pp 27ndash42 2001 [10]A K Jonscher Universal Relaxation Law Chelsea Dielectrics Press London 1996

Acknowledgements The author is grateful to Mr Ulf Nilsson and Borealis AB Sweden for the sponsorship

Dielectrics 2009 Measurement Analysis and Applications 40th Anniversary Meeting IOP PublishingJournal of Physics Conference Series 183 (2009) 012002 doi1010881742-65961831012002

6