IEEE Transactions on Power Delivery, Vol. 13, No. 3, July 1998
712
A NEW CONCEPT IN CONSTRUCTION OF CABLE TERMINATIONS FOR
MEDIUM VOLTAGES
S .V. Nikolajevic, Member IEEE
The Electric Power Distribution Co.
Belgrade, Yugoslavia
N.M. Pekaric-Nad
R.M. Dimitrijevic
Faculty of Technical Sciences
University of Novi Sad, Yugoslavia
The Cable Factory
Jagodina, Yugoslavia
Abstract This paper describes a new concept in construction
of cable terminations for medium voltages. Layers with a high
permittivity and embedded electrodes (EEs) were used. Three
groups of configurations were examined. In the first group,
the layer of high permittivity was placed partly over the cable
insulation and partly over the cable screen. In the second
group, the high permittivity layer (HPL) was placed partly
over the cable insulation and partly under the semiconducting
material, connecting with cable insulation screen. In the third
group the cable screen was partly inserted into the H P L whose
other part was placed over the cable insulation. The EEs were
made in a shape of rings around the HPL. The rings were
made either of copper tape or copper wire. Different positions
of the EEs were examined. Numerical models of the cable
terminations were used to monitor how the electric field
changes as a function of the EE distance from the cable screen
end. Finally, the new terminations were tested in a high
voltage laboratory, according to the standards VDE 0278 and
1
L
I
I
1
I
I
Figure 1. The cable termination construction referred as K1. ( 1cable insulation, 2-high permittivity layer (HPL), 3-screen end, 4insulation of shrinking tube, 5-sheat of shrinking tube, 6-embedded
electrode, EE, grounded (G),nongrounded (N)or absent (A))
5
IEEE-404.
I INTRODUCTION
XLPE cables have been widely used for medium voltages
for many years. Their h g h reliability has already been
confirmed. The problem of cable terminations still remains
incompletely solved. The cable terminations are supposed
to have small dimensions and very good service
characteristics. There is a number of cable terminations
developed in last few years [l-61. Cable failures still
happen, causing a great economic loss, mainly because of a
cable termination breakdown. For that reason any
improvement in the cable termination construction is of
interest.
Cable breakdown most often happens because of a
strong electric field in the cable insulation, close to the
cable screen end. Commonly, it is controlled by deflectorsdielectric cones, conventional stress relief cones. SRC.
which are geometric solution to the problem. Cable
termination constructions with layers of high resistivity or
of high dielectric constant are also well known [ 2 ] . Recent
papers [7,8] initiated a study of a new cable termination
construction. The electric field at the cable termination was
controlled by high permittivity material and embedded
electrodes. The results were not completely satisfying,
Figure 2. The cable termination construction referred as 11. ( 1- cable
insulation, 2-high permittivity layer W L ) ,3-screen end, 4-insu1ation
of shrinking tube, 5-sbeat of shrinking tube, 6-embedded electrode,
EE, grounded (G), nongrounded
or
(A))
~
m
Figure 3.a. The cable termination construction referred as ET. ( 1cable insulation, 2-high permittivity layer (HPL), 3-screen end, 4insulation of shrinking tube, 5-sheat of shrinking tube, 6-embedded
electrode, EE, grounded (G), nongrounded
or absent (A))
I
PE-893-PWRD-2-06-1997 A paper recommended and approved by
the IEEE Insulated Conductors Committee of the IEEE Power
Engineering Society for publication in the IEEE Transactions on Power
Delivery.Manuscript submitted December 31, 1996; made available for Figure 3.b. The cable termination construction referred as I2M. ( 1printing June 6, 1997.
cable insulation, 2-high permittivity layer (HPL), 3-screen end, 4insulation of shrinking tube, 5-sheat of shrinking tube, 6-embedded
electrode, EE, grounded (G), nongrounded (N) or absent (A))
1
0885-8977/98/$10.00 0 1997 IEEE
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713
and further investigations were needed. In this paper a
hybrid concept is introduced, involving both geometrical
control of the electric field, the high permittivity material
and embedded electrodes (EEs).
Numerical models of different cable terminations
were used to calculate and compare the influence of
different constructions on electric field mitigation.
were formed. Commercially available software was used
for preprocessing and postprocessing of the data, as well as
for the automated grid generation. The analyzed region was
firstly divided into isoparametric triangular elements. The
unknown variable, electric potential, V, was defined on the
nodes of every element. Two dimensional Laplace's
equation,
div(-E grad V)=O
U THE TERMINATION CONSTRUCTIONS
A good cable termination should have no current
leakage, no surface erosion and no discharges between the
cable screen and the cable conductor. Main problem in the
cable termination construction is the stress field reduction
in the cable insulation, under the cable screen. Possible
methods for the electric field regulation are:
1. geometrical, SRC [ I ]
2. non-linear resistive field grading coatings [6]
3. refractive field grading coatings [2]
4. combination of 2 and 3 [ 3 ]
5. capacitive method and
6. complex method (introduction of ferrite etc.)
Introduction of a new concept in this work, is an
effort to make a good hybrid of already existing methods.
Three types of configurations (See Figs.1-3) were
considered in this work:
1. The high permittivity layer (HPL) placed partly over the
cable insulation and partly over the cable screen (Fig.
1 ). For simplicity, it shall be referred as K1.
2. The HPL placed partly over the cable insulation and
partly under the semiconducting material, connected
with cable insulation screen (Fig. 2). For simplicity, it
shall be referred as 11.
3. The cable screen partly inserted into the HPL, whose
other part was placed over the cable insulation (Fig. 3).
For simplicity, it shall be referred as 12.
was solved in cylindrical coordinates. The boundaries were
defined by the phase conductor potential, V1=10 kV, and
the screen ground potential V2=0. Equipotential map was
calculated for each termination construction. The electric
field was calculated fkom the corresponding potentials. The
layers with different relative permittivity were examined.
The analysis was performed with the EEs on the top of the
HPL. The EEs were isolated or grounded. Different EE
distances (denoted as L in Figs.1-3) from the screen end
were examined too.
Relative permittivities, Er, of some materials used
for modeling the cable termination were as follows:
Polyethylene 2.3, the HST 3.2, Semiconductive material
1000, Al, Cu 10000, the HPL 10.4 to 40.4.
1V. NUMERICAL RESULTS
Numerical models for different termination
constructions, (Kl, 11, I2), different relative permittivities
of the HPL (typically between 20.4 and 40.4), ring
construction and different ring position, L, were examined.
All constructions were compared with a conventional stress
relief cone, SRC. For a model of the SRC, illustrated in Fig.
5 , maximum electric field was calculated to be
Emax=2.12 kV/mm,
with an axial component of
Emax=l.33 kV/mm.
kablrl
600
Figure 4. The cable termination construction.
The EEs were made of either copper tape or
copper wire. Typically, the tape was 10 mm wide, with
variable thickness, 0.2 mm to 1".
The different diameters
( less than 2 mm) of wire were also examined. Different
numbers of the EEs were considered, but one EE was found
to be sufficient. In each configuration, the EEs were placed
on the top of the HPL, under the heat shrinkable tube
(HST), (See Fig. 4), except for the construction I2M, where
the EEs were placed in the middle of the HPL, as illustrated
in Fig. 3.b. The EEs were either grounded (G) or floating
potential-nongrounded (N). The electric field reduction was
monitored as a fbnction of the EE distance, L, from the
cable screen end.
[r
00
-30 0
10 0
50.0
7a.~
z
nl NUMERICAL MODELS
Numerical models, based on finite element method
(FEM) for each of the above mentioned configurations
Figure 5. Equipotential map of the stress relief cone as a cable
termination. Legend in k V a-0.5, b-1, c-1.5, d-2, e-2.5, f-3, 8-3.5, h-4,
i-4.5, j-5, k-5.5,1-6, m-6.5, n-7, 0-7.5, p-8, q-8.5, r-9, s-9.5, t-10.
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714
These two values were taken as the upper limits.
All new constructions were supposed to have field
intensities lower than that.
Numerical calculations had shown that all
examined constructions reduced the maximum field, Emax,
near the cable screen to:
0
1.99 kV/mm, HPL with ~r=20.4and
0
1.60 kV/mm, HPL with ~r=40.4.
The differences between the constructions had a
clear effect on the axial component of the electric field, Ez.
For the different examined constructions and different EE
distances, L, the greatest values of the Ez, E m a x , are
illustrated in Figs. 6-9. The place of the greatest potential
gradient is the location of the maximum total field, Emax,
and it coincides with the location of the E m a x
Figs. 10 and 11 represent the equipotential maps
for the cable construction referred as 12T and I2M. In Fig.
10, the EE was placed on the top (T) of the HPL and in Fig.
11, the EE was placed in the middle (M). Both constructions similarly reduce the maximum field intensity, but the
axial component of the field is better reduced in the case of
I2M, as may be seen from Figs. 8 and 9.
-12NT
I
s
W
700
1"
E
lo"
20"
I
Figure 8. Cable termination E T with a grounded EE on the top (
of the HPL: Intensity of axial component of electric field as a
function of the EE position, L-distance from the screen end. (SRC conventional stress relief cone, 12GT-the EE-grounded, IZNT-the EE
nongrounded, The UA-without the EE.)
I
1400
1200
1000
800
600
400
1"
3"
5"
lo"
20"
EE distance from the cable screen
1800
E
5:
5"
3"
EE distance from the cable screen end
E
4 K 1 A -R-SRC
I
1700
I
E
E 1200
5
-KIM
4 1 2 A +SRC
J!ieure Y. Cable termination U M C ; with grounded KK in the mid( le
of the HPL: Intensity of axial component of electric field as a
function of the EE position, L-distance from the screen end. (SRC conventional stress relief cone, UGM-the EE-grounded, 12NM-the
EE nongrounded, The I2A-without the EE.)
0
1300
*I
w
800
I"
3"
5mm
lo"
20"
EE distance from the cable screen
Figure 6. The cable termination K1 with grounded EE on the top o.
the HPL: Intensity of axial component of electric field as a function
of the EE position, L-distance from the screen end. (SRC conventional stress relief cone, K1G-the EE-grounded. K1N-the EE
nongrounded, The K1A-without the EE.)
400
!
lmm
I
3mm
5miu
lOmm
20mm
EE distance from the cable screen
'igure 7.Cable termination I1 with grounded
EE on the top of the
HPL: Intensity of axial component of electric field as a function of
the EE position-distance from the screen end. (SRC -conventional
stress relief cone, I1G-the EE-grounded, I1N-the EE nongrounded,
The 11A-without the EE.)
z
igure 10. Equipotential map of the new type, I2TG, cable
termination with grounded EE, made of 0.2 mm thick, 10 mm wide
copper tape. Legend in k V a-0.5, b-1, c-1.5, d-2, e-2.5, f-3, 8-3.5, h-4,
i-4.5, j-5, k-5.5,I-6, m-6.5, n-7, 0-7.5, p-8, q-8.5, r-9, s-9.5, t-10.
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715
VI EXPERIMENTAL RESULTS
iznekr
The examined configurations have passed the tests
according to the standards. Some of those results and the
results of some additional nonstandard tests are listed in
Table 1.
35.a
30.0
i
25.Q
+KIG
ITI
--.-KIN
+KIA
20a
25
1
150
10 0
-I
/
;
-
A
50
300
3511
450
u.0
500
-I
z
Figure 11. Equipotential map of the new type, I2MG, cab
termination with grounded EE, made of 0.2 mm thick, 10 mm wide
copper tape. Legend in kV: a-0.5, b-1, c-1.5, d-2, e-2.5, f-3, 8-3.5, h-4,
i-4.5, j-5, k-5.5,1-6,m-6.5, n-7, 0-7.5, p-8, q-8.5, r-9, s-9.5, t-10.
V. TESTING
Based on numerical models, blends of the high
permittivity material were prepared for the HPL. Relative
permittivity was ~ ~ 4 0 . Dimensions
4.
of the HPLs were:
length 10 cm, thickness 1".
The EEs were made in a
shape of 0.2 mm thick copper tape. Samples of standard 10
kV cables were terminated by different types of
terminations. Combination pairs of the tested terminations
may be seen from the first column of the Table 1. The
terminations were tested according to the German standard
VDE 0278 and International standard IEEE-48.
Additional examinations included voltage
withstand tests, up to flashover or break down. The cable
samples were exposed to the increasing voltage, starting
from 40 kV, in 5 kV steps, in 5 min intervals. In the case of
flashovers, the voltage was reduced by 5 kV and the cable
samples were left at reduced voltage until breakdown
occurred.
The measurements, whose results are illustrated in
Figs. 12-15, were performed with the cable open ended,
conductor on potential V1=10 kV, and the screen grounded
on both sides (V2=0). The top of the voltage probe was
centered to the cable axis.. The effect of the cable
termination construction was monitored by measuring the
voltage using a probe. Fixed lcm probe movements were
performed in axial direction, on the top of the HST. The
probe voltages illustrated in Figs. 12-15 were plotted versus
the distance from the cable screen end. Because of the
insufficient input impedance of the instrument (Digital
voltmeter, input impedance 10 MR. ) and considerable
influence of the probe, the results of the measurements can
be taken as a relative indicator only. Higher voltages and
greater slope indicate a worse situation.
Similar measurements were not possible for the
conventional stress relief cone (SRC), because the
conductive cover of the cone short-circuited the probe.
o
I
2
3
4
5
6
a
7
9 1 0
bistance from the screen end (cm)
~
~~
Figure 12. The probe voltage (PV) vs. axial distance from the screen
end for the cable termination construction Kl. (KlG-the EEgrounded, K1N-the EE nongrounded, The K1A-without the EE.) The
distance of the EE from the cable screen, L=12 mm.
-1
0
1
2
3
4
5
6
7
8
9
10
Distance from the screen end (cm)
Figure 13. The probe voltage (PV) vs. axial distance from the screen
end for the cable termination 11. (IlG-the EE-grounded, I1N-the EE
nongrounded, The I1A-without the EE.) The distance of the EE from
the cable screen, L=12 m a
I
20
pv (VI
'
m
--I
I
8
9
15
5
0
-1
0
1
2
3
4
6
6
7
10
Distance from the screen end (cm)
Figure 14. The probe voltage (PV) vs. axial distance from the screen
end for the cable termination E T . (I2GT-the EE-grounded, I2NT-the
EE nongrounded, The I2A-without the EE.) The distance of the EE
from the cable screen, L=12 mm.
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716
Tested
constructions,
pairs
examined
Partial
discharge
Cycling
heating
t,,,=lOO°C 3
cycles at
30 kV
satisfied
Partial
ADDITIONAL TESTS
discharge (10
Voltage withstand UD to breakdown
at 2U0=24kV
(Voltage was increased from 40 kV,
(max 20 pC)
Q
KlA,KlG5
(PC)
at 2U0=24kV
(max 20 pC)
OpC
KlG12,
KlN12
lOpC(G)/
20 PC(N)
satisfied
2 PC(GY
4 PC(N)
llG5,12GT5
2 pC
satisfied
0 PC
I1G12,
12GT12
2 PC
satisfied
0 PC
I2GT5,
12GM5
0 PC
satisfied
0 PC
IlG5, I2GM5
0 pC
satisfied
0 PC
IlA, 12A
0 PC
satisfied
0 PC
11GT12, SRC
0 pC
satisfied
0 PC
SRC, SRC
0 PC
satisfied
0 PC
in steps of SkV,
after each 5 min.)
0 PC
breakdown of K1G5 at 75 kV, after 1 min
(both passed 70 kV for 5 min)
breakdown of KlG12 at 60 kV, after 2 min
( both passed 55 kV for 5 min)
breakdown of I1G5 at 85 kV, 0 min
(both passed 80 kV for 5 min)
flashover of IlG12 at 85 kV after 3 min
(both passed 80 kV for 5 min)
breakdown of 11G12 at 75 kV after 80 min
flashover of I2GM5 at 80 kV after 0 min,
breakdown of I2GM5 at 75 kV after 10 min
flashover of I2GM5 at 75 kV,
breakdown of I2GM5 at 70 kV after 9 hours
flashover of 12A at 85 kV after 3 min, (both passed
80 1 V for 5 min), breakdown of I1A at 75kV after
60 min.
flashover of SRC at 55 kV after 0 min,
breakdown of at SO kV after 150 min.
flashover of SRC at 65 kV after 0 min,
breakdown of at 60 kV after 120 min.
The KlG5 (L=5 mm) had a breakdown at 75 kV,after 1
minute.
The I1 configurations were found to be very good. They
satisfied standard tests. With the EE grounded, as may
be seen ffom Fig. 7, E m a x was always lower than in
SRC. For L=12 mm the I1 configurations were able to
stand 75 kV for 80 minutes. As may be seen from Table
1, one sample I1 G5 in a combination with the I2GM5
termination, was not destroyed after 540 minutes on 70
20
15
PV(V) 10
5
kV.
0
-1
0
1
2
3
4
5
6
7
8
9 1 0
Distance from the screen end (cm)
igure 15. The probe voltage (PV) vs. axial distance from the screen
end for the cable termination construction I2M. (UGM-the EEgrounded, I2NM-the EE nongrounded, The I2A-without the EE.) The
distance of the EE from the cable screen. L=12 mm.
As the value Ezmax=1,33 kVimm for the SRC was
considered a limit, a new cable termination construction
may be accepted O n l y if it has h e r l%"x value. Figures
12-15 may be used to judge which constructions may be
considered better then the SRC.
From all the tests it follows that
* The K1 configuration appeared to be the poorest of the
three examined in this work. From Fig. 6 it appears that
the IClG configuration had Ezinax greater than the SRC.
Although the standard tests were satisfied, the KlG12
(L=12 mm) had a breakdown at 60 kV, after 2 minutes.
The 12T configurations were found to be very good.
From Fig. 8 it may be seen that grounded EE kept
Ezinax lower than in the SRC. The construction 12GT12
(for L=12 mm) was not destroyed after 80 minutes on
75kV. It passed standard tests successfully.
The 12M configurations were found to be satisfactory.
Froin Fig. 9 it can be seen that both grounded and
nongrounded configurations had the E m a x lower than
the SRC. The I2MGM5 configuration was able to staid
70 kV for 540 minutes.
After breaking down, all terminations were
examined, For the new-concept
the signs of
damage were found at the end of the HPL distant from the
cable Screen end. In the case of the SRC, the breakdown
occurred typically between the cable screen and the cable
conductor.
V11 DISCUSSION
b
T h e e configurations were analyzed:
K1 -cable screen end partly covered with the HPL
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717
11- cable screen end 1 cin on the top of the HPL
12 - cable screen end 1 cm on the top of the HPL,
covered with another high permittivity wrap, 1 mm
thick.
Each configuration was analyzed with or without
addition of the embedded electrode (EE). Two types of
electrodes were considered:
-copper wire ring (radius 0.5 mm to 2 mm) and
-copper tape, 1 cm wide, 0.2 mm to 1 inm thick
Greater diameter wire was not found to be suitable,
because it introduced additional field disturbances.
Grounded 0 . 6 m to lmm diameter wire and grounded
tapes either 0.2” or 0.4 mm thick, had similar effects.
Thicker tape produced 10% lower tangential field
components.
The EE were either grounded (G) or floating potential nongrounded (N). Grounded EEs were generally found
to be better than nongrounded. They moved the
strongest electric field away from the cable insulation
and reduced the slope of the field rise near the cable
screen end (See Figs. 12-15).
Typically, relative permittivity, Er, of the HPL was 40.4,
but 20.4 was also considered. The lower permittivity
was acceptable for maximum field reduction, but higher
permittivity was more efficient for suppression of the
tangential field components.
By introduction of grounded EE, the worst electric
stress moved away froin the screen end. Grounded EEs
made the field rise slower.
The breakdown in the new construction terminations
occurred at the end of the HPL, which could be made
longer if needed. In the SRC terminations the
breakdown occurred always at the screen end.
According to the numerical calculations (Figs. 6-9) and
the results of measurements (Figs. 12-15), it was
expected that the constructions I2GM might bc the best
of all. This configuration was found to be satisfactory,
but, judging from the Table 1, the configurations I1G
and I2GT were found to stand more severe conditions.
VI11 CONCLUSIONS
If compared by intensity of maximum electric field and
its axial component, some of considered new
construction cable terminations were found to reduce
the electric stress better (25%) than standard stress
relief cones, SRC.
The new concept cable terminations represent a hybrid
solution for the cable end electric stress reduction. They
involve high permittivity material, but also make use of
geometrical relaxation of the field.
The constructions I1G and I2GT were found to be the
most serious candidates for the new generation cable
terminations. Their advantages over the SRC were in
reducing the axial component of the electric field and in
channeling the worst electric stress away froin the cable
screen end to the HPL.
1X REFERENCES
[ 11. McPartland, J.F. Handbook of practical electrical
design, McGraw-Hill Inc., 1984, pp. 9.25-9.30
[2]. Nelson, P.N., Hervig, H.C., “High dielectric constant
materials for primary voltage cable terminations”, IEEE
Trans. Vol. PES-103, Nov. 1984, pp 3211-3216.
[3]. Andersen, O.W. “Laplacian electrostatic field
calculations by finite elements with automatic grid
generation”, IEEE PES Winter Meeting, New York, 1973.
[4]. Blake, A.E. et al. ”Improvements in stress control
materials”, 7th IEEE/PES Transmition and Distribution
Conference, April, 1979.
[ 5 ] . Weedy, B.M. and Turvey, N.J. “Resistive stress
relieving materials for XLPE cable joints”, Second IEE
Conf. on PCA 1OkV to 180 kV, 1986
[6]. Virsberg, L.G. and Ware, P.H.”A new termination for
underground distribution”, IEEE Trans. PAS, Sept. 1967
[7].Nikolajevic S. et al. “Optimization of cable
terminations” IEEEPES Summer Meeting, Denver, 1996.
SM 369-9-PWRD
[8]. Nikolajevic, S.V. et al. “Modeling of cable
terminations with embedded electrodes, Proc. of the IEEE
Int. Symp. on Electrical Insulation, Montreal 1996, pp
703-706
X. BIOGRAPHY
Stojan V. Nikolajevie was born in village Vucadelci near
Surdulica in Yugoslavia, 1944. He received the B.S., M.S.
and Ph.D. degrees from the Faculty of Electrical
Engineering, University of Belgrade, in 1968, 1974 and
1987 respectively. In 1968 he joined the Cable Factory,
Svetozarevo-Jagodina. In 1994 he joined the Electric
Power Distribution Company, Belgrade. His work has been
in areas of construction, testing and cable service. Currently
he is an Assistant Professor of Faculty of Electrical
Engineering, University of Belgrade.
Neda M. Pekaric-Nadj was born in 1954 in Novi Sad,
Yugoslavia. She received the B.S. degree in 1978 from the
Faculty of Technical Sciences, University of Novi Sad, and
M.S. and Ph. D. from the Faculty of Electrical Engineering,
University of Belgrade, in 1981 and 1984 respectively. She
joined the Faculty of Technical Sciences, University of
Novi Sad, in 1978 as a teaching assistant. Currently she is
an Associated Professor teaching an introductory course in
Elecrotechnics. Her work is in connection with electric and
magnetic fields calculation.
Radisa Dimitrijevic was born in 1961 in Jagodina. He
received the B.C. degree in 1986 from the Faculty of
Electrical Engineering, University of Belgrade. Since then
he has been with the Research Center of the Cable Factory,
Jagodina. His work is in connection with cable accessories
development. He is working towards his M.S. degree at the
Faculty of Electrical Engineering, University of Belgrade.
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