[IEEE 2013 IEEE 7th International Power Engineering and Optimization Conference (PEOCO) - Langkawi Island, Malaysia (2013.06.3-2013.06.4)] 2013 IEEE 7th International Power Engineering

Development of Treeing Detection Method for Cable

with Three Insulation Regions Using TDR Technique

Tze Mei Kuan* and Azrul Mohd Ariffin Universiti Tenaga Nasional

Jalan IKRAM-UNITEN, 43000 Kajang, Selangor

*[email protected]

Abstract- Treeing is a typical factor that contributes to aging of

polymeric insulated cables. The occurrence and proliferation of

these water trees cause electricity disruptions which interfere

with the proper operation of the lines. This has resulted in a

strong need for techniques to identify treeing degradation in

polymeric insulated cables. In previous studies, a method that

uses time domain reflectometry (TDR) technique to detect and

locate water tree within two insulation regions has been shown to

be feasible. Thus, this paper attempts to investigate on the

potential of using the same method on cable with three insulation

regions. Cable is modelled using MATLAB simulation tool to

obtain the cable impedance for PSpice set-up simulation where

the TDR technique is applied. The pulse reflections from the

PSpice simulations are recorded and analysed to investigate

whether this technique is capable of detecting water tree within

three insulation regions.

Keywords- Insulation; reflection; trees

I. INTRODUCTION

Water treeing is one of the main forms of polymeric

insulation degradation in medium voltage cables. Water trees

gradually reduce the electrical breakdown strength of cables

and eventually lead to cable failures that cause electricity

disruptions which interfere with the proper operation of the

lines. This has resulted in a strong need for techniques to

identify treeing degradation in polymeric insulated cables so

that appropriate service time can be scheduled to prioritize

cable replacement. However, such degradation is difficult to

detect in the cables using non-destructive techniques as partial

discharges are not involved in the growth of water tree.

Nevertheless, studies in [1] have shown that insulation

containing water trees has lower insulation resistance and

higher dielectric losses. Thus, the measurement of these

properties can be used to assess the condition of the insulation.

Diagnostic tests have been identified which can detect and

evaluate the degradation phenomena that causes failure in

cables. These tests include destructive and non-destructive

techniques which can be carried out on-site or in the

laboratory. However, most of the techniques could only give

average information on the status of the whole cable and

cannot detect which part along the cable that is degraded the

most. In previous studies [2,3], a method that uses time

domain reflectometry (TDR) technique to detect and locate

water tree within two insulation regions has been shown to be

feasible. Thus, this paper attempts to investigate on the

potential of using the same method to detect and locate water

tree within three cable insulation regions. Cable is modelled

using MATLAB simulation tool to obtain the cable impedance

for PSpice set-up simulation where the TDR technique is

applied. Fig. 1 shows the TDR system circuit model that has

been proposed. The basic principal on how this TDR

technique works has been discussed in [3]. Using the time

delay equation in [3], water tree along the cable can be located

from the PSpice results.

Fig. 1. Cable Model for Un-degraded Condition [4]

II. METHODOLOGY

The cable type and size used in this study is cross linked

polyethylene (XLPE), 9.0mm, 630mm2

(33kV) with a length

of 30m. Before performing simulations using PSpice, the

XLPE cable has to be modelled by the MATLAB simulation

tool to obtain the cable characteristic impedance. In order to

investigate for the potential of the TDR technique to detect

and locate water tree within the cable insulation, the cable has

to be modelled in two conditions where the first is assuming

the cable with a clean or un-degraded insulation and the

second is assuming a portion in the cable insulation has been

degraded due to the growth of water tree.

In [4], the cable model for the un-degraded condition as

illustrated in Fig. 2 has been proposed from the application of

circuit and transmission line theories.

Fig. 2. Cable Model for Un-degraded Condition [4]

978-1-4673-5074-7/13/$31.00 ©2013 IEEE

2013 IEEE 7th International Power Engineering and Optimization Conference (PEOCO2013), Langkawi, Malaysia. 3-4 June2013

172

Another cable model is proposed in [2] by assuming part of

the cable insulation has been degraded due to the growth of

water tree in the XLPE cable insulation. Fig. 3(a) shows part

of the XLPE cable insulation has been degraded due to water

tree while Fig. 3(b) shows the model for this cable insulation.

Fig. 3. (a) XLPE Insulation Partially Degraded due to Water Tree

(b) XLPE Insulation Model with Water Tree

Fig. 4 below shows the full cable model incorporating the

water tree from Fig. 3(b) into Fig. 2.

Fig. 4. Cable Model for Degraded Condition

From the MATLAB simulations on these two cable

conditions, the characteristic impedance of the cable can be

obtained using equation (1) in [2] for PSpice simulation. As

mentioned earlier, studies in [2,3] have investigated on the

potential use of TDR technique to detect and locate water tree

within XLPE insulated cable that has two insulation regions as

shown in Fig. 5.

Fig. 5. Cross Section of Partially Degraded XLPE Cable

(2 Insulation Regions)

As this paper attempts to test the capability of this method in

detecting and locating water tree within the cable insulation, a

third region is introduced to the cable in Fig. 5 which is

illustrated in Fig. 6 below.

Fig. 6. Cross Section of Partially Degraded XLPE Cable

(3 Insulation Regions)

Thickness of

XLPE insulation XLPE insulation

still in good

condition

Part of XLPE insulation

that has been infected by

water tree

Length of Ttest

(Region 1)

Length of Ttest2

(Region 2)

Conductor

screen

Thickness of

XLPE insulation XLPE insulation

still in good

condition

Part of XLPE insulation that has

been infected by

water tree

Length of Ttest

(Region 1)

Length of

Ttest3

(Region 3)

Conductor

screen

Length of

Ttest2

(Region 2)

XLPE

insulation still in good

condition

(a)

(b)


173

Studies in [2,3] performed the PSpice simulations using

three different length combinations for Ttest and Ttest2 from

Fig. 5. The cable lengths are 10/20m, 15/15m and 20/10m

where the first figure represents the length for Ttest while the

second length is referring to Ttest2. Since the analysis of those

results show that water tree can still be detected and located

even though the water tree grows at different length along the

cable insulation, thus the lengths used in this paper in the first

attempt on three insulation regions cable in Fig. 6 are taken at

10m for Ttest, 5m for Ttest2 and 15m for Ttest3.

Although the study in [2] has carried out the simulations on

three levels of degradation, this paper only concentrates on

one which is 25% of the XLPE insulation thickness is

assumed to be degraded by water tree. Therefore, all results

are analyzed based on this specific level of degradation. The

pulse reflected from the PSpice simulations due to impedance

difference as the signal travels along the set-up are recorded

and analysed to investigate whether this technique is capable

of detecting water tree within three insulation regions.

III. RESULTS AND DISCUSSIONS

The study in [3] shows the simulation of PSpice set-up

illustrated in Fig. 7 below for testing 30m long cable with un-

degraded insulation outputs the result as shown in Fig. 8. As

discussed in [3], there are four pulses detected but only the

first three pulses are valid which is due to impedance

difference of two cables in the set-up. The fourth pulse is

observed to be multiple reflections due to Ropen. The signal

routes as it travels through the set-up in Fig. 7 can be seen in

Fig. 9.

Fig. 7. PSpice Set-up for Testing Un-degraded Cable

Fig. 8. Reflected Pulses for Un-degraded Cable

Fig. 9. Signal Routes for Results in Fig. 8

In the same study, the simulation is repeated on partially

degraded cable which now has two insulation regions using

the set-up in Fig. 10 and the result is shown in Fig. 11. The

cable is now represented by two components; Ttest and Ttest2

with 15m long each. Since Ttest represents the un-degraded

portion, the pulse A1 is observed to occur at an earlier time

between P2 and P3 with a negative magnitude. Pulse A1

occurs at an earlier time because the length of Ttest is now

15m and therefore the shorter time is required for the signal to

be reflected. Since the impedance of Ttest2 which is degraded

has smaller impedance than Ttest, the subtraction term in the

reflection coefficient, ρ from equation (1) causes the pulse A1

to have a negative magnitude.

� � ��2 � �1�1 � �2�1�

where

Z1 = Impedance of current component that the signal

travels through

Z2 = Impedance of the next component that the signal

detects

As Ropen has much higher impedance compared to Ttest2

which is degraded due to water tree, the subtraction term of

the reflection coefficient, ρ gives a positive value. This

explains the positive magnitude of pulse P3. Since the

impedance of a degraded cable due to water tree is usually

lower than other cables, this makes the subtraction term in the

reflection coefficient to be in positive value, therefore, it can

be assumed that an additional pulse with positive magnitude

detected between P2 and P3 shows a possibility of water tree

degraded cable. Then, using the time delay equation, water

tree can be located as discussed in [3]. Similarly to the un-

degraded cable simulation, pulses A2, P4 and A3 are detected

due to multiple reflections at Ropen which can be neglected

when the time delay equation is used to locate the water tree.

The signal routes for the first four pulses detected in Fig. 11

can be seen in Fig. 12.

P1 = 16.955ns P3 = 417.940ns

P2 = 117.836ns P4 = 717.015ns

Time (µs)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

9.0mm, 630mm2 (33kV) (30m)

-1.0V

0V

1.0V

2.0V

3.0V

P1

P2

P3

P4

P1

P2

P3


174

Fig. 10. PSpice Set-up for Testing Degraded Cable (2 Insulation Regions)

Fig. 11. Reflected Pulses for Degraded Cable (2 Insulation Regions)


It is known that the same set-up is used for testing the un-

degraded and degraded cable (2 insulation regions) besides the

additional component introduced to the degraded set-up to

incorporate the water tree to the cable. Thus, comparing the

results in Fig. 11 to Fig. 8, it can be observed that the

additional pulse, A1 detected in Fig. 11 indicates the existence

of water tree region in the cable insulation which is recorded

by the pulse P3. This shows that this system is capable of

detecting water tree along the cable due to impedance

difference between the un-degraded insulation and the

degraded insulation caused by the growth of water tree. Then,

using the time delay equation, water tree can be located as

discussed in [3].

As the result in Fig. 11 shows the capability of this method

to detect and locate water tree for the two insulation regions

cable, the study in this paper extends the simulation to test on

three insulation regions cable as illustrated earlier in Fig. 6.

The PSpice set-up for this simulation is shown in Fig. 13.

The simulation of set-up in Fig. 13 gives the result in Fig.

14. Observing the result, it can be noticed that there are now

two additional pulses recorded between P2 and P3, named B1

and B2. Similarly to results in Fig. 8 and Fig. 11, the pulses

detected after the time where P3 is detected are all classified

as repeated pulses due to multiple reflections from Ropen

which are neglected. The signal routes for the first five pulses

recorded in Fig. 14 are shown in Fig. 15.

Fig. 13. PSpice Set-up for Testing Degraded Cable (3 Insulation Regions)

Fig. 14. Reflected Pulses for Degraded Cable (3 Insulation Regions)


P1

P2

A1

P3

Time (µs)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

9.0mm, 630mm2 (33kV)

(10/5/15m)

0V

-1.0V

1.0V

2.0V

3.0V

P1 = 16.955ns P3 = 416.667ns B6 = 616.667ns B8 = 816.667ns

P2 = 117.836ns B3 = 466.667ns P4 = 716.667ns B9 = 866.667ns

B1 = 216.667ns B4 = 516.667ns B7 = 766.667ns B10 = 916.667ns

B2 = 266.667ns B5 = 566.667ns

P1

P2

P3

P4

B1 B2 B3 B4 B5 B6 B7 B8 B9 B10

P1

P2

B1

B2

P3

Time (µs)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

9.0mm, 630mm2 (33kV) (15/15m)

-1.0V

0V

1.0V

2.0V

3.0V P1

P2 A1

P3

A2

P4

A3

P1 = 16.955ns A2 = 567.090ns

P2 = 117.836ns P4 = 717.015ns

A1= 267.240ns A3 = 867.097ns

P3 = 417.940ns

1.0


175

Referring to the time delay table from Table III in [2], the

first pulse, P1 is detected by the oscilloscope after the incident

wave travels through coaxial cable T1 and T3 as shown in Fig.

15 which is the shortest path for the wave to reach the

oscilloscope. The pulse reaches node 8 at time 16.955ns as

indicated in Fig 14. The time occurrence for P1 can be

obtained by adding the theoretical delay time or time of

propagation down the line of length T1 and T3 which is

described as:-

Total delay time = TD in T1 + TD in T3

= 10.11ns + 7.08ns

= 17.19ns

The calculated value is close to the measured value. The fall

time for the pulse is at 56.955ns, since the pulse width, PW is

set to 40ns.

From Fig. 14, the second pulse, P2 occurs at 117.836ns,

which is obtained by adding the delay time or time of

propagation down the line of length T1, 2 × T2 and T3. The

fall time for the pulse is at 157.836ns, since the pulse width,

PW is set to 40ns. P2 in Fig. 14 has a negative value which is

again due to the reflection coefficient, ρ since the impedance

of Ttest is smaller than the impedance of T2, then the

subtraction between these two impedances will give a negative

value.

Since the water tree is assumed to grow within the cable

length, the test cable which is now represented by three Test

cables causes more reflections to the oscilloscope. The

additional pulse, B1 reached the oscilloscope at time

216.667ns with negative magnitude. This negative magnitude

is also due to the reflection coefficient, ρ since the impedance

of Ttest2 is smaller than the impedance of Ttest. Therefore,

the subtraction between these two impedances will give a

negative value. The time is obtained by adding the delay time

or time of propagation down the line of length T1, 2 × T2, 2 ×

Ttest and T3. The fall time for the pulse is at 256.667ns, since

the pulse width, PW is set to 40ns.

From the simulation of the earlier PSpice set-up of degraded

cable with the two insulation regions, an assumption has been

made where if there is any additional pulse with positive

magnitude detected between P2 and P3, it shows a possibility

of water tree degraded cable. Referring to Fig. 14, the second

additional pulse, B2 reached the oscilloscope at 266.667ns

with a positive magnitude. This pulse, B2 indicates the

existence of water tree degradation in the test cable. This pulse

can also be obtained by adding the delay time or time of

propagation down the line of length T1, 2 × T2, 2 × Ttest, 2 ×

Ttest2 and T3. The fall time for the pulse is at 306.667ns,

since the pulse width, PW is set to 40ns.

Although the signal is reflected due to impedance mismatch

between Ttest2 and Ttest3, only small part of the signal is

reflected. This can be seen by evaluating the magnitude of the

pulse voltage. Due to high frequency signal, the wave still

travels through Ttests3 and meets the open end. This causes

another reflection and therefore pulse, P3 is detected by the

oscilloscope at time 417.940ns which is equivalent to the

pulse P3 recorded in Fig. 14. The time is obtained by adding

the delay time or time of propagation down the line of length

T1, 2 × T2, 2 × Ttest, 2 × Ttest2, 2 × Ttest3 and T3. The fall

time for the pulse is at 457.940ns, since the pulse width, PW is

set to 40ns.

From the discussions on the PSpice simulation result in Fig.

14, it shows that this method that implements the TDR

technique is capable of detecting the existence of water tree in

the cable insulation. To further investigate on the potential of

this technique to locate the water tree along the 30m long

cable, the time delay equation from study in [3] is used. The

time delays for all coaxial cables in the set-up from Fig. 13 are

again obtained from time delay table in [2]. In earlier

discussion, it has been explained that pulse B1 is resulted from

the signal travelling through coaxial cables T1, T2, Ttest and

reflected due to impedance mismatch between Ttest and

Ttest2, and back to Ttest, T2 and finally T3 (illustrated in Fig.

15). Therefore, time delay of the Ttest can be found by taking

the total time occurrence of B1 minus the total time delay of

the other coaxial cables. Hence, the length of the Ttest can be

calculated from B1 as follows:-

TD for (2 × Ttest) = Time occurrence for B1 from

Fig. 14 – [TD in T1 +

(2 × TD in T2) + TD in T3]

= 216.667ns – [10.11ns +

(2 × 50.55ns) + 7.08ns]

= 98.377ns

∴ TDforTtest � 98.377��2

= 49.1885��

Substituting the TD for Ttest into time delay equation:-

ℓ !"#! =$% !"#! × '(

√*+,+=

$% !"#!5.0553��

= 49.1885��

5.0553��

= 9.73m ≈ 10m

The calculated length of 9.73m is quite closed to the actual

length of 10m. Since the magnitude of B1 is negative due to

the reflection coefficient, therefore, it can be concluded that

the first 10m of this test cable is still in good condition as the

impedance of Ttest2 is lower than Ttest (indicating Ttest2 has

deteriorated).

B2 from Fig. 14 is resulted from the signal travelling

through coaxial cables T1, T2, Ttest, Ttest2 and reflected due

to Ttest3 and back to Ttest2, Ttest, T2 and finally T3

(illustrated in Fig. 15). Therefore, time delay of the Ttest2 can

be found by taking the total time occurrence of B2 minus the

total time delay of Ttest and the other coaxial cables. Take

note that the time delay for Ttest used in this calculation is


176

from the calculated value not from the time delay table.

Hence, the length of the Ttest2 can be calculated from B2 as

follows:-

TD for (2 × Ttest2) = Time occurrence for B2 from

Fig. 14 – [TD in T1 +

(2 × TD in T2) + (2 × TD in Ttest) +

TD in T3]

= 266.667ns – [10.11ns + (2 × 50.55ns) +

(2 × 50.55ns) + 7.08ns]

= 47.277ns

∴ TDforTtest2 = 47.277��

2= 23.6385��

Substituting the TD for Ttest2 into time delay equation:-

ℓ !"#!/ =$% !"#!/ × '(

√*+,+=

$% !"#!/5.0553��

= 23.6385��

5.0553��

= 4.676m ≈ 5m

From the calculation, it was found that the length of the

Ttest2 is 4.676m which is quite close to 5m. Since this pulse

gives a positive magnitude, it can be concluded that the test

cable is degraded about 5m long. This is because of the

positive magnitude from the reflection coefficient which

indicates that the Ttest3 has higher impedance than Ttest2.

Now, it is known that 5m of the total cable length is degraded

due to water tree and is located from the 10th meter until the

15th

meter of the cable.

P3 in Fig. 14 is resulted from the signal travelling through

coaxial cables T1, T2, Ttest, Ttest2, Ttest3 and reflected back

to Ttest3, Ttest2, Ttest, T2 and finally T3 due to the open end.

Therefore, time delay of the Ttest3 can be found by taking the

total time occurrence of P3 minus the total time delay of Ttest,

Ttest2 and the other coaxial cables. Take note again that the

time delay for Ttest used in this calculation is from the

calculated value not from time delay table. Hence, the length

of the Ttest3 can be calculated from P3 as follows:-

TD for (2 × Ttest3) = Time occurrence for P3 from

Figure 14 – [TD in T1 +

(2 × TD in T2) + (2 × TD in Ttest) +

(2 × TD in Ttest2) + TD in T3]

= 417.940ns – [10.11ns + (2 × 50.55ns) +

(2 × 50.55ns) + (2 × 25.28ns) + 7.08ns]

= 147.99ns

∴ TDforTtest3 = 147.99��

2= 73.995��

Substituting the TD for Ttest3 into time delay equation:-

ℓ !"#!/ =$% !"#!0 × '(

√*+,+=

$% !"#!05.0553��

= 73.995��

5.0553��

= 14.637m ≈ 15m

From the calculation, it was found that the length of the

Ttest3 is 14.637m which is close to 15m. Although this pulse

gives a positive magnitude, it is not degraded because Ttest3

has higher impedance than Ttest2. The pulse has a positive

value because the impedance of the open end is very much

larger than the impedance of Ttest3. Thus, it is now known

that another 15m of the total cable length is un-degraded

which is located from the 15th meter until the 30

th meter of the

cable.

The newly found cable conditions with their respective

lengths are quite closed to the cable conditions and lengths

used earlier in the set-up for simulation. This again shows that

this technique from analysing the pulse to calculating the

lengths of both un-degraded and degraded cable conditions

and finally locating the water tree along the cable length is

able to produce quite accurate results. Thus, this technique is

applicable even on cable degraded with three insulation

regions.

IV. CONCLUSION

This study investigated the potential of using time domain reflectometry (TDR) technique to detect and locate water tree in cable degraded with three insulation regions as previous study have shown that this technique is capable of detecting and locating the water tree in cable degraded with two insulation regions. From the analysis of the result, it was found that this technique is still capable of detecting the existence of water tree even with three insulation regions. The additional pulse detected with positive magnitude between pulses P2 and P3 are crucial in determining the existence of the water tree. Upon detecting the existence of water tree, its location can be found through calculations using the time delay equation. The potential use of this technique can be further investigated to test on cable with higher number of insulation regions; more than one region is degraded due to water treeing.

REFERENCES

[1] S. D. Grigorescu, M. Plopeanu, P. V. Notingher and C. Stancu,

“Equipment for Fast Water Trees Resistance Measurement of Power

Cable Insulations,” 8th International Conference on Insulated Power

Cables, E.5.2.14, June 2011.

[2] A. Mohd. Ariffin, T. M. Kuan, S. Sulaiman and H. A. Illias,

“Application of Time Domain Reflectometry Technique in Detecting

Water Tree Degradation within Polymeric-Insulated Cable,” 2012

International Conference on Condition Monitoring and Diagnosis, pp.

1163-1166, 23-27 September 2012, Bali, Indonesia.

[3] Tze Mei Kuan, Azrul Mohd Ariffin and Suhaila Sulaiman, “Signal Analysis to Detect Water Tree Location in Polymeric Underground Cables,” The 9th IEEE Student Conference on Research and Development, pp. 359-365, 19-20 December 2011, Cyberjaya, Malaysia.

[4] T. M. Kuan, A. Mohd. Ariffin, S. Sulaiman and Y. H. Md Thayoob,

“Wave Propagation Characteristics of Polymeric Underground Cables,”

The 5th International Power Engineering and Optimization Conference,

pp. 410-415, 6-7 June 2011, Shah Alam, Selangor, Malaysia.


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[IEEE 2013 IEEE 7th International Power Engineering and Optimization Conference (PEOCO) - Langkawi Island, Malaysia (2013.06.3-2013.06.4)] 2013 IEEE 7th International Power Engineering