2016 International Conference on Advances in Electrical, Electronic and System Engineering, 14-16 Nov 2016, Putrajaya,
Malaysia
Tangent Delta Extraction of Cable Joints for Aged
11kV Underground Cable System
1
Navitharshaani Permal, 2Chandan Kumar
Chakrabarty,
3
Avinash A.R, 4Tashia Marie, 5 Huzainie Shafi Abd
Halim
Universiti Tenaga Nasional,
Selangor, Malaysia
TNB Research Sdn. Bhd,
Selangor, Malaysia
Abstract-Tangent Delta, also called loss angle or dissipation
factor (į) is a measurement technique to determine the quality
of cable insulation. It utilizes the phase shifts caused by the
displacement current induced due to the existence of impurities
in the insulation or stress on the cable. More than 50% of the
underground cable breakdown is due to cable joint failures and
also aged bulk cable failures. Current Tangent Delta
measurement used is an overall or bulk assessment technique
where it is not able to differentiate the responses of each cable
system component (i.e. joints, terminations and cables).
However, there is no current method is available to extract the
tangent delta of joints from bulk defective cable using HFAC or
other commercial techniques. It is important to distinguish the
cable and its accessories response within the cable system for the
ease of maintenance action. Therefore this experimental
approach is an attempt to distinguish the responses of each
component made using High frequency Alternating Current
(HFAC) Tangent Delta measurement technique. This paper
presents the extraction method for cable joint of 11 kV defect
conditioned cable sample induced with carbon black (CB)
impurities in its insulation. The technique will implement on
defective cable with various combination of two joints of 20
meter length. The tangent delta of cable joint from bulk tangent
delta measurement will be determined using higher frequencies
(>100Hz) at voltages of 1 kV. The current (I), voltage (V) and
phase (ș) were obtained in this experiment using HFAC
measurement setup. The phasor diagram is then constructed
where the actual joint phase is extracted using the basic
trigonometric functions. Hence, the tan delta values of extracted
joints were obtained through the calculations.
One of diagnostic test is tan delta measurement where it is
an analytical method of testing cables to determine the quality
of the cable insulation. The standard method for tan delta
measurement utilized VLF (0.1Hz) and power frequency
(50/60Hz) testing techniques. These existing tangent delta
diagnostic test methods are bulk assessment techniques that
cannot distinguish conductive defect of cable components
from bulk defective cable. It gives an overall condition of the
complete cable system that includes cable, joints and
terminations. It is important to develop a method to identify
and distinguish the defective components from the defective
bulk cable system so that maintenance action can be taken for
the medium voltage underground cables that are in service for
many years.
Hence, this paper presents a method of High Frequency
Alternating Current (HFAC) tangent delta extraction of joints
from defective bulk cable system which can be used to
differentiate the condition of joints in the laboratory.
Artificially defective cable using carbon black in cable
insulation were created to give variation in the dielectric loss
response.
II.
Keywords- tangent delta, cable joints, dissipation factor, high
frequency, extraction method
I.
A set of cable with defective condition were connected
using artificially created defective jointing condition. The
conductors are connected using a connector. The cold shrink
joint is used with semi-conducting conductor shield,
insulation and semi-conducting insulation shield. The surface
of cable insulation between connector and semi conducting
insulation shield were induced with carbon black impurities to
create the defective condition at joint. Combination of
defective cable without joint and with joints are prepared to
differentiate the tangent delta responses and for extraction
purposes.
INTRODUCTION
XLPE cable was introduced decades ago and now it is
widely used for underground cable system especially in urban
areas. Underground XLPE cable has its own electrical
characteristics with good dielectric strength, solvent
resistance, low dielectric permittivity and low loss factor. In
underground system, two or more lengths of cable have to be
joined to produce longer cables. More than 50% of the
underground cable breakdown is due to cable joint failures
and also aged bulk cable failures.[1] Diagnostic tests based on
dielectric response measurement in either time or frequency
domain are used to determine whether the bulk cable has aged
or not[2].
978-1-5090-2889-4/16/$31.00 ©2016 IEEE
TYPE OF TEST SAMPLE
Cable joint is used to connect different sections of cable
because a cable section is limited to certain length. The design
of a cable joint mainly depends on cable type, applied voltage
and cable cores. These factors contribute to the way how
tangent delta are increased or decreased at the cable joint. If
there are defect exist within the cable joint insulation material,
the tangent delta value increases.
265
exceed 104Ÿcm [7, 8]. Table 1 shows the type of cables that
prepared for HFAC testing.
REQUIREMENT OF CABLES FOR HFAC TESTING
(a)
Type of
sample
No. of
sample
No. of
Joint
Condition
of Joint
Length of cable per
sample(meter)
Defect
condition
cable
1
0
n/a
20
Cable with
cold shrink
joint
1
2
2 good
joints
20
Cable with
cold shrink
joint
1
2
1 good joint
and 1 defect
joint
20
Cable with
cold shrink
joint
1
2
2 defect
joints
20
In this experiment, one set of cable without joint with
artificially induced carbon black impurities in cable insulation
with 20 meter is prepared. Also, three sets of 20 meter defect
cable samples with combination of two joints with good and
defective jointing were prepared and tested. The impurities
were added randomly in each cable’s semiconducting layer.
(b)
IV. HFAC MEASUREMENT SETUP
Fig. 1. (a) Good condition cable insulation (b) Defect condition cable
insulation
7
In Fig. 1(a) and Fig. 1(b) shows the examples of condition
of cable insulation before and after the carbon black impurities
is added. The good condition cable shows clearer insulation in
Fig. 1(a) where no impurities is added to the cable insulation
while the defect condition cable are black in color as shown in
Fig. 1(b) when carbon black impurity is added to the cable
insulation.
III.
CABLE PREPARATION
The defective cables were manufactured by cable
manufacturer using extrusion process where carbon black
impurities were added to each sample of cable insulation. In
extruded power cable process, it is necessary to have a very
smooth interface between the insulation and the conductor
since conductor asperities protruding into the insulation would
cause high field regions that may lead to premature
breakdown.
Fig. 2. Experiment setup in cable laboratory
At higher frequencies, the capacitance of the cable will
cause a significant current to be drawn. The displacement
current drawn by the capacitive cable, may cause a potential
to be dropped across these semiconducting layers, which form
a series resistance, RS with the capacitance of the cable. In
power cables, conducting carbon black are commonly used as
a semiconducting layer. Increasing carbon black loading and
process temperature can decrease the volume resistivity,
which usually vary between 10 and 100 Ÿ cm and should not
The experiment is conducted in laboratory as shown in
Fig. 2 above. The equipment for HF Tangent Delta Test as
shown in Fig. 2 above consists of:
1.
2.
3.
4.
266
High Voltage Power Amplifier
Pearson Current Monitor
High Voltage Probe
Digital Oscilloscope
5.
6.
7.
V.
Signal Generator
Screen Cage
11kV Al XLPE Triplex 240mm2 cable
I=
HF AC MEASUREMENT TECHNIQUE
HFAC tangent delta measurement technique was
developed to extract tangent delta results of tested samples.
The equivalent circuit of tangent delta measurement method
is shown as Fig. 3 below.
IR= 0
Fig. 4. Characteristics of a Ideal Insulation(Perfect Capacitor)
IR
= IR+ IC
IC
Fig. 3. Equivalent circuit of tan delta measurement method of a cable
Fig. 5. Characteristics of an Insulation with Impurities
RS is the series resistance of semi-conductive and metallic
shield of cable which is considered to be very small in values.
So, the equivalent circuit is consist only, RC is the resistance
of the cable insulation and CC is the capacitance of cable
insulation. Ideally the equivalent circuit of cable insulation is
represented by RC and CC in Fig. 3. If impurities exist in the
cable insulation, the resistance of the cable insulation
decreases thus increasing the resistive current through the
insulation.
An increasing loss angle indicates an increase in resistive
current through insulation which eventually lead to high
dielectric loss. The degree of change in loss angle (į)
represents the tan delta values of ageing level in cable
insulation which is derived from equations below [4,5]:
‫ ߠ݊ܽݐ‬ൌ െ
Tangent delta is calculated based on ratio of resistive
current flow to the capacitive current flow in cable insulation.
Higher ratio shows high dielectric loss in the insulation. In an
ideal condition, a pure insulator behaves as a perfect capacitor.
So, there will be no resistive current flow through ideal
insulating material which results in zero percent impurity. The
capacitive current (IC) leads the applied voltage (V) by 90° as
illustrated in Fig. 4. However, there will be no pure insulator
exist in real due to ageing of insulator and impurities like dirt
and moisture enter into the cable. The added impurities will
result in decrease of resistance of insulation. This causes the
increase in resistive current (IR) through the insulation making
it no longer a perfect capacitor as illustrated in Fig. 5. A phase
shift between the leading capacitive current (IC) and the
voltage is utilized to determine the tangent delta (tan į) [3].
ଵ
௧௔௡ఋ
Ʌ
= Phase between current and voltage
ߜ
= Dissipation angle
(1)
Fig. 6. Equivalent circuit of cable and joint
With addition of joints, two new components are
introduced which are the resistance of joint Rj and capacitance
of joint Cj in Fig. 6. If impurities exist in the cable insulation
267
or joint, then the resistance of the cable insulation or joint will
decrease. This increases the resistive current through the
insulation or joint [2].
A parallelogram has to be used to isolate the Displacement
current of the joint from the total Displacement current of the
cable and joint. Fig. 7 and Fig. 8 shows the trigonometric
representation of the displacement current and phase for cable
joint.
Fig. 8. Trigonometric representation of the displacement current and phase
(a)
IC
= Capacitive Current
IR
= Resistive Current
IJ
= Joint Current
ICAB = Cable Current
IC+J = Total Current
ș
= Phase
șJ
= Joint Phase
șCAB = Cable Phase
șC+J = Total Phase
(b)
Fig. 7. a) Displacement current of cable and joint if joint is more capacitive
b) Displacement current of cable and joint if joint is more resistive
ߜ
= Total Loss Angle
ߜJ
= Joint Loss Angle
Basic trigonometric equations as in Fig. 8 are used to
extract out the current (IJ) and phase (șJ) of the joint using a
benchmark defect condition cable and comparing it with a
defect conditioned cable with joint.
Fig. 8 below shows trigonometric representation of
displacement current and phase if added joint increases the
resistive current of whole cable system. There are also
methods used to extract the tangent delta of the joint from the
bulk cable system.
The cosine formula:
ܽଶ ൅ ܾ ଶ ൌ ܿ ଶ െ ʹܾܿ ‘• ‫ܣ‬
(2)
Equation (3) and (6) is used to extract the Displacement
current and phase at the joint from Fig. 7.
268
‫ܫ‬௃ ଶ ൌ ሺ‫ܫ‬஼஺஻ ሻ; ൅ ሺ‫ܫ‬஼ା௃ ሻ; െ ሺʹ ‫ܫ כ‬஼஺஻ ‫ܫ כ‬஼ା௃ ܿ‫ߠݏ݋‬ሻ
(3)
ߠ= ߠ஼஺஻ െ ߠ஼ା௃
(4)
‘• ߠ‫ ܤܣܥ‬ൌ
‫ܣ‬
‫ܤܣܥܫ‬
Ǣ ‘• ߠ‫ ܬ‬ൌ
‫ܤ‬
‫ܬܫ‬
Ǣ ‘• ߠ‫ܥ‬൅‫ ܬ‬ൌ
PHASE VALUE FOR DEFECTIVE CABLES WITH TWO JOINTS
FROM HFAC TECHNIQUE
‫ܥ‬
Freq
(Hz)
Since, A+ B = C
‫ ܤܣܥߠ •‘ ܤܣܥܫ‬൅ ‫ ܬߠ •‘ ܬܫ‬ൌ ‫ܥܫ‬൅‫ܥߠ •‘ ܬ‬൅‫ܬ‬
(5)
ߠ௃ = χοσ{≠ሺ൫‫ܫ‬஼ା௃ ܿ‫ߠݏ݋‬஼ା௃ െ ‫ܫ‬஼஺஻ ܿ‫ߠݏ݋‬஼஺஻ ൯/‫ܫ‬௃ )
(6)
ߜ௃ ൌ ͳͺͲι െ ͻͲι െ ߠ௃
Phase(ș)
Phase(ș)
Phase(ș)
Phase(ș)
997.8
121.44
121.52
121.55
121.60
200
1002.0
107.51
107.57
107.65
107.72
300
1001.0
101.41
101.60
101.75
101.95
400
1000.0
98.40
98.51
98.64
98.73
500
999.5
97.48
97.55
97.59
97.64
TAN DELTA VS. FREQUENCY FOR BULK DEFECTIVE
CABLES OF 20METER
2.600E+00 6.152E-01
Bulk defect
cable-2
defect joints
2.100E+00
2 good
joints
2 defect
joints
Current
(A)
Current (A)
Current (A)
TANGENT DELTA
1.600E+00
Bulk defect
cable_2
good joints
6.114E-01 3.195E-01
1.100E+00
3.182E-01
Defect cable
2.116E-01
6.000E-01
6.133E-01 3.166E-01 2.080E-01 1.536E-01
3.155E-01
1 good 1
defect
joints
Current (A)
Bulk defect
cable_1
good and 1
defect joints
6.140E-01
CURRENT VALUE FOR DEFECTIVE CABLES WITH TWO
JOINTS FROM HFAC TECHNIQUE
Voltage
(V)
2 defect
joints
Voltage
(V)
Referring to the tables above, the current and phase values
are obtained from the HFAC technique. By using Equation
(1), the tangent delta values of bulk defective cables for both
with two joints and without joint of 20meter length are
calculated.
A. Measurement Of Defect Cable with Two Joints of 20
Meter with Defect Cable As Reference
Extraction of cable joints from the bulk defective cable is
determined by HFAC technique. The Table 2 and Table 3
shows the current and phase values that obtained by HFAC
testing method which will be used to obtain the tangent delta
values.
Freq
(Hz)
1 good 1
defect
joints
100
HFAC MEASUREMENT RESULTS
Defect
cable
2 good
joints
(7)
HFAC technique is used to extract the dielectric losses of
the cable accessories from the bulk of cable insulation system
for medium voltage power cable. A method to extract
dielectric losses of the cable joint in the tested underground
cable system is also determined using this technique. The
dielectric response of accessories can be extracted out from
the global condition assessment using this method.
VI.
Defect
cable
‫ܥܫ‬൅‫ܬ‬
1.000E-01
100
200
1.341E-01
2.018E-01 1.519E-01 1.329E-01
1.477E-01 1.313E-01
2.053E-01 1.491E-01
1.325E-01
300
400
500
FREQUENCY(Hz)
100
997.8
4.813E-03
4.896E-03
4.924E-03
4.939E-03
Fig. 9. Graph of tan delta vs. frequency of bulk defect cables of 20meter
200
1002.0
1.103E-02
1.109E-02
1.114E-02
1.118E-02
300
1001.0
1.718E-02
1.727E-02
1.793E-02
1.796E-02
400
1000.0
2.335E-02
2.350E-02
2.378E-02
2.394E-02
500
999.5
2.648E-02
2.799E-02
2.972E-02
3.009E-02
The experiment was conducted with 1 set of 20m bulk
defective cable without joint and 3 sets of defective cables
with combination of two joints. Defect cables of 2 good joints,
1 good and 1 defect joints and 2 defects joints are prepared
and tested using HFAC technique. The tangent delta
experiment was performed at 100Hz to 500 Hz with carbon
black impurities induced in cable insulation. In Fig.9, the
graph shows the results of tangent delta of bulk defect cable
without joints and with two joints. The tangent delta results of
20 meter long cables with joints and without joint are well
distinguished at 100Hz, 200Hz and 300Hz. The graph of bulk
defect cable with two defect joints shows the highest tangent
delta value and the defect cable without joints having the
lowest tangent delta value. The difference in the tangent delta
values between bulk defective cable with and without joints
indicates that components of cable (i.e. joints, terminations
269
HFAC method. The tangent delta results of 20 meter long
cables with joints and without joint are clearly well
distinguished at first three frequency. The bulk defective cable
with two defect joints shows the highest tangent delta value
and the defect cable without joints having the lowest tangent
delta value. The difference between the tangent delta values
between bulk defective cable with and without joints indicates
that accessories of cable also important to determine the
quality of the cable system.
and cables) are also included to determine the quality of the
overall cable system.
By using the equation (3) and (6) above, the tangent delta
of cable joints are extracted out from the bulk defective cable.
Fig.10 shows the extracted tangent delta results for 20 meter
defective test samples.
TAN DELTA VS. FREQUENCY FOR DEFECT
CABLE WITH 2 JOINTS OF 20METER
1.200E+00
Defect
cable
1.000E+00
TANGENT DELTA
Apart from that, by using basic trigonometric equation, the
current and phase of joint are extracted from the measurement
results that obtained from HFAC method. The extracted
tangent delta results with combinations of two joints are
clearly discriminated at frequency 300Hz and 400 Hz. The
defective cable with two defect joints shows higher tangent
delta than the defect condition cable without joint as reference
cable. The extracted joints tan delta values from bulk defective
cables are important in order to identify the cable condition
and helps in maintenance action to avoid unnecessary cost
consumption.
2 good
joints
8.000E-01
6.000E-01
1 good
and 1
defect
joints
4.000E-01
2 defect
joints
REFERENCES
2.000E-01
[1]
A P Smith,”AEI Cables, IEE/PG/PS CO LLOQUIUM - Design of
medium voltage polymeric cables - water treeing tests”, Copyright
1994 The Institution of Electrical Engineers. Printed and published by
the IEE. Savoy Place, London WCPR OBL. UK.
[2] Bolarin Oyegoke, Petri Hyvonen, Martti Aro and Ning Gao”
Application of dielectric response measurement on power cable
systems”, IEEE Transactions on Dielectrics and Electrical Insulation
Vol. 10, No. 5; October 2003.
[3] Ramanujam Sarathi, Arya Nandini, Michael G. Danikas,
“Understanding electrical treeing phenomena in xlpe cable insulation
adopting UHF technique’, Journal of Electrical Engineering, Vol 62, 2
(2011) 73-79
[4] A.R. Avinash, Chandan Kumar Chakrabarty and Navinesshani Permal,
“High frequency tan delta measurement method for 132kv transmission
underground cables”, Research Journal of Applied Sciences,
Engineering and Technology 10(8): 903-913, 2015
[5] A.R. Avinash, Chandan Kumar Chakrabarty, Agileswari and
Navitharshaani Permal," Tan delta measurement method for 132kv
XLPE power cable using low voltage and high frequency AC",
Multidisciplinary Wulfenia, vol. 22, no. 9, pp. 2-15, Sept 2015.
[6] Ponniran, A. and M.S. Kamarudin, 2008. “Study on the performance
of underground XLPE cables in service based on tan delta and
capacitance measurements”, Proceeding of IEEE 2nd International
Power and Energy Conference (PECon, 2008), pp: 39-43.
[7] H. R. Gnerlich, “Field testing of HV power cables: Understanding
VLF testing,” IEEE Electrical Insulation Magazine, Vol. 11, Issue 5,
Oct, 1995, pp. 13–16.
[8] Y.Miyashita, Y.Makishi and H.Kato, “New approach to elucidate the
properties of carbon black-filled semiconducting materials for high
voltage power cables”, IEEE Annual Report Conference on Electrical
Insulation and Dielectric Phenomena, CEIDP 1993.
[9] E. J Kim, D. H. Park, H. S. Kim, and G.J. Lee, “An investigation of the
influence of semi conductive electrode materials in the ac breakdown
and the charge accumulation in XLPE”, IEEE Annual Report
Conference on Electrical Insulation and Dielectric Phenomena, CEIDP
1998.
[10] Chunchuan Xu and S. A. Boggs, “High frequency properties of
shielded power cable part 2: Sources of error in measuring shield
dielectric properties,” Electrical Insulation Research Center,
University of Connecticut. DEIS Feature Article IEEE Electrical
Insulation Magazine January/February 2006 — Vol. 22, No. 1.
0.000E+00
0
100
200
300
400
FREQUENCY(HZ)
500
600
Fig. 10. Graph of tan delta vs. frequency of defect cable with combination of
2 joints compared with defect cable reference
The tangent delta experiment was performed at 100Hz to
500 Hz with carbon black impurities induced in cable
insulation. The experiment was conducted with 20m bulk
defective cable without joint and with variation of two joints.
In Fig.10, the graph shows the results of tangent delta of defect
cable with two joints is analysed and compared with defect
condition of bulk cable without joint. The extracted tangent
delta results of 20 meter long cables are clearly well
discriminated at frequency 300 Hz and 400Hz. Trend is
decreasing exponentially with two defect joints having a
highest tangent delta value compared to the other cables. The
difference between the tangent deltas at a particular frequency
resembles the change in resistivity in the joint that might be
caused by the damage the insulation of the cable or improper
installation of the joint.
VII. CONCLUSION
The present tangent delta measurement technique of VLF
and power frequency gives the results of overall condition of
the complete cable system that includes cable, joints and
terminations. However there is no method is available to
extract the tangent delta of joints from bulk defective cable. In
this paper, High Frequency AC (HFAC) method was
introduced to determine tangent delta at higher frequencies
and able to extract the tan delta values of cable joints from
bulk defective cable.
A few sets of artificially induced defect condition cable
with combination of two joints are prepared and tested using
270