Proceedings of the 9th International Conference on J-35Properties and Applications of Dielectric MaterialsJuly 19-23,2009, Harbin, China

The Condition Assessment System of XLPE Cables Using the IsothermalRelaxation Current Technique

Jiandong WU * , Yi YIN, Xuguang LI, Yaqun WANG, Dengming XIAO

Department of Electrical Engineering, School of Electronics and Electrical Engineering

Shanghai Jiao Tong University, Shanghai, 200240, China

* Email: djwu@sjtu.edu.cn

ABSTRACT: The isothermal relaxation current (IRC)

technology is a powerful method for assessing the

condition of power cable insulation. In this paper a

condition assessment system was designed to meet the

diagnosis of XLPE cables insulation. The assessment

system consisted of the IRC measurement system and

IRC analysis system. The processes of IRC

measurement, such as polarizing voltage, discharging

and depolarization current measurement, were

controlled by software. The third order exponential

decay model was utilized in analysis software to

process measured IRC data. The parameters of each

kind of depolarization current component and the

relationship between I*t and log (t) were showed

directly in software for evaluating the aging status.

Based on the empirical aging factor (A-factor), the

aging level of cables insulation is obtained after data

analysis. At last, one part of three-phase AC XLPE

cable was measured used this system. The result

shows that the system is precise and the diagnostic

criteria of A-factor should be corrected to meet the

XLPE cable made in china.

Key words: XLPE cable, isothermal relaxation current;

life-time evaluation, aging factor

INTRODUCTION

Nowadays, the majority of power cables are insulated

with polymeric materials. Cross-linked polyethylene

(XLPE), as the main polymeric insulation, has already

widely used in power cables for its super insulating

and mechanical property since 1960s [1]. However,

the insulation degradation is inevitable during the

service and the failure rate of XLPE cable increases

978-1-4244-4368-0/09/$25.00 ©2009 IEEE 1114

with the service time [2]. So it would be advantageous

to cable users to acquire the insulation condition of

cable during operation. The condition assessment is an

effective method to determine the residual lifetime of

installed cable. Cable users could plan maintenance

more effectively according to degradation diagnosis

and would therefore be able to minimize costs due to

failure outages.

IRC is a non-destructive diagnostic method, which is

able to assess the remaining mechanical strength of

cable insulation [3,4]. According to Simmons and

Tam [5], the isothermal relaxation current decreases

monotonically with the time based on depolarization

of the insulation material. The time characteristic of

each kind of depolarization current component in

isothermal relaxation current varies significantly with

the residual strength of cable insulation material.

Therefore, the insulation aging status of cable could be

obtained by the isothermal relaxation current analysis.

This paper mainly presents a condition assessment

system of XLPE cable using the IRC method. In

addition, the condition assessment of one part of

three-phase AC XLPE cable is showed at last used this

system.

THEORY OF ISOTHERMAL RELAXATION

CURRENT

From the theory of Simons and Tam [5], it is known

that the trap levels are discrete and distributed over the

complete energy band in insulation. Furthermore, the

time constants and magnitude of trap levels are

different during the different insulation status. When

the traps in insulation are energized and the energizing

Measured isothermal relaxation current can be

approximated by three time dependent components

according to (6)

Obviously, the product of current and time is linearly

proportional to the occupied trap density N(E).

Consequently, plotting l(t).t-IogJOt directly shows the

energy distribution of the occupied traps in cable

insulation.

shielded box10M

1M

shows the schematic diagram of the IRC measurement

system. The measurement is mainly composed of high

voltage DC source, electrometer, relay group, control

board, computer and shield box. The output range of

high voltage DC source is 0-5kV, controlled by analog

voltage 0-5V from control board. The electrometer is

designed based on a monolithic electrometer

operational amplifier AD549L. Its maximum input

bias current is 60fA. The current measurement range

and the accuracy of electrometer are ± lnA and ±O.2

pA, respectively. Fig.2 shows the curves of current vs.

voltage about a resistance measured by the

electrometer designed and Keithley-6517A

electrometer, respectively. Obviously, the accuracy of

electrometer designed IS suitable for IRC

measurement.

Fig.l . The schematic diagram of IRe measurement

(2)3

l(t)=10 +~>i .e Ti

i=1

Where:

q: charge

L: Thickness of the insulation

fore): Initial occupancy of the traps by electrons

N(E): Trap density

E: trap energy

source is then removed at constant temperature, the

depolarization current decreases monotonically with

time. This depolarization current represents a

superposition of different relaxation processes

depending on trap levels. The isothermal relaxation

current is given as [5]

I (t) = q . L .k -T .fa (E) .N (E ) (I)2t

600500300 400

Vollage (mV)

200100

I 0 Diagnositc system ./

+ Keithley-6517A I //

//

//aa

'00

500

600

C 300

~U

200

400

~

Where the parameters a; t, are strongly correlated with

the different traps levels in insulation. The time

constant T3 is related to water degradation of the cable

insulation.

The polarizing voltage applied on cable is DC voltage

during the IRC measurement; however, it is less than

10% of the rated voltage of power cable and would not

damage the insulation [7]. Therefore, the IRC method

is the non-destructive diagnostic measurement.

Condition Diagnostic System Fig.2. The compari son of current measurement

A. Measurement system

The processes of IRC measurement, such as polarizing

voltage, discharging and depolarization current

measurement, were controlled by software. Fig.1

The relay group is composed of three high voltage

reed relays, controlled by DC 24V from the control

board. It's the executive unit to change the circuits.

The magnitude of depolarization current after charging

is ultralow small. Hence, the shielded box is useful to

11 15

reduce the electromagnetic influence . In addition, the

control board communicates with computer by USB

ports. To protect the computer, optical couplings

boards are designed between the control board and

each controlled device.

B. IRC analys is System

IRC analysis system IS design based on Labview

software workbench. Fig.3 shows the interface of

analysis system. In the IRC analysis system, the

measured isothermal relaxation current is fitted

according to the third order exponential decay model

given by equation (2). And then an empirical ageing

factor (A-factor) is calculated to classify the ageing

condition of the cable. The A-factor can be expressed

as [4]

EXPERIMENT AND ANALYSIS

To verify the preCISIOn and credibility, a condition

assessment of three-phase XLPE cable made in China

was initially performed in the laboratory. The sample

is the 0.635m AC XLPE cable cut from field, which

went into service in 2005 and broke down in 2009.

The outer protective sheath was removed from the

XLPE insulated cable, exposing the copper screen.

The inner conductor was connected to DC source

while the outer copper screen was connected to earth,

as showed in Fig.l. The isothermal relaxation current

was measured during 1800s following the application

of a DC voltage IkY across the insulation about 5400s

at ambient temperature.

The raw current data obtained is given in FigA. It is

obvious that current measured by IRC is smooth and

the anti-interference feature of system is powerful.

Fig.5 shows the I(t).t vs. loglot after data analysis , the

relaxation currents of each phase cables are similar.

Clearly, three different relaxation processes of XLPE

cable could be determined with IRC method during

1800s. The weights of three relaxation process in each

phase cable are diverse under service aging. Obviously,

the insulation condition of phase C is the severest than

the condition of phase A and B.

o measured data-phase Ao measured data-phase B6. measured data-phase C

400-j-- - -t-- - --i

(3)

The time constant t , and coefficient OJ could be

acquired from data fitting. Tab.1 shows the aging

classes of XLPE cable manufactured in Germany

according to A-factor [8]. The insulation condition of

XLPE cable is divided into four classes.

Where

300+-- - -+-- - - - -1-- - - - -+--1

IRe analysis system Version 1.0

Fig.3. The interface of IRC analysis system

Tab. I . The aging classes of XLPE cable

1000100

time (8)

10

100 -t--~l:f

Tab.2 shows the parameters of each phase cable

evaluated by (3-5). According to the diagnostic criteria

of Tab.1, the insulation of phase C cable is in middle

life status and the phase A and B are both perfect.

~E 200 -hl--~-t------+-----+------j

1!G

Fig.4. The raw data of IRC measurement

> 2.10

Critic al

insulation condition

Old

1.90-2.101.75-1.90

Middle life

< 1.75

Perfect

' E:\IRC\A13-o.txl IliIi'

insulation paramete rs

A-fact

1116

CONCLUSION

REFERENCES

[I] Chen Jiqun, Wang Shoutai, "Method of

Estimating the Remaining Life of the 10kV XLPE

Cable Operated 10 Years". The 6th International

Conference on Properties and Applications of

Dielectric Materials. vo!.l , pp204-208, 21-26 Jun.

2000.

[2] M. Shuvalov, M. mavrin, V. Ovsienko, et aI,

"Analysis of Water Trees in Power Cable Polymeric

Insulation". Journal Applied Polymer Science.

Vo!.88,1543-1549,2003.

[3] Yin Yi, Tu Demin, Li Ming, et aI, "Study on the

action mechanism of the free radical scavenger with

isothermal-current-decay method-an the free radical

scavenger with isothermal-current-decay method-an

experimental verification of trap theory for electrical

aging in polymer". Proceedings of the CSEE. vo1.20,

issue 3, pp.13-15,25, 2000.

[4] B. S. Oyegoke, F. Foottit, D. Birtwhistle et aI,

"Condition Assessment of XLPE Insulated Cables

Using Isothermal Relaxation Current Technique".

IEEE Power Engineering Society General Meeting.

pp.18-22, 2006.

[5] J.G. Simmons, M. C. Tam, "Theory of Isothermal

Current and the Direct Determination of Trap

Parameters In Semiconductors and Insulators

Containing Arbitrary Trap Distributions". Physical

Review B. vo!.7, no.8, pp.3706-37l3, 1973.

[6] R. Hofmann, H.-G. Kranz, D. Steinbrink,

"IRC-analysis: destruction free dielectric diagnosis of

mechanical and service aged polymeric insulation".

Eleventh International Symposium on High Voltage

Engineering. vol. 4, pp.253-256, 23-27 Aug. 1999.

[7] G. Hoff, H.-G. Kranz, "On-site dielectric

diagnostics of power cables using the isothermal

relaxation current measurements". Power Engineering

Society Winter Meeting. vo!.3, pp. 1593 - 1598, 23-27

Jan. 2000.

[8] P. Birkner, "Field Experience With a

Condition-Based" Maintenance Program of 20-kV

XLPE Distribution System Using IRC-Analysis".

IEEE Transactions on Power Delivery. vol.19, issue1,

pp.3-8, 2004.

o fitted data-phase Ao fitted data-phase B'" fitted data-phase C

- peak1-phase A-+--_;'q-------'~-I- - peak2-phase A

• • • eak3- hase A

1800

1600

1400

1200

"' 1000

'".eo 800

'"600

s80

400

"200

. . .10

Fig.S, The IRC curves after analysis

Tab.2. The parameters of each phase

Q(T2) Q(T3) A-fact

Phase A 1522.81 2664.659 1.74983

Phase B 1571.007 2701.256 1.71944

Phase C 2946.94 5303.616 1.7997

The current measurement range of electrometer

designed for IRC system are ± lnA and the accuracy is

satisfactory for lRC measurement. The processes of

measurement, such as polarizing voltage, discharging

and depolarization current measurement, could be

performed automatically.

The analysis system could fit the IRC data quickly.

The three different relaxation processes are determined

in IRC of XLPE cable measured in the range from 0 to

1800 seconds. However, the diagnostic criteria of

A-factor should be corrected to meet the XLPE cable

manufactured in china.

Acknow Iedgments

The authors wish to thank National Natural Science

Foundation of China (NSFC) for financial support

(50677038).

However, this three-phase XLPE cable has been

broken down, and the insulation condition of one

phase cable would be old at least. It mainly caused by

different materials technology and manufacturing

process between two countries. Therefore, the

diagnostic criteria of Tab.1 could no be applied to

cable manufactured in china directly and should be

correct further in later research.

100 1000

time (5)

1117