Development of 400kV premolded joint for XLPE cable
Doo-Sung Shin, Eung-Seo Ji, Eui-Gon Ko, Seung-Ik Jeon and In-Ho Lee
EHV Technical Solution Team, LS Cable
190 Gong-Dan Dong, Gu-Mi city, Kyung-Buk Korea
Abstract
Unlike tape-molded joint, factory-made premolded joint (PMJ) is pre-tested and requires relatively simple and easy
installation with a low degree of skill from the jointers.
Therefore, as the cost of power transmission facilities
reduces and easy control of quality is required, the premolded joint is preferred for EHV cable system.
With the numerous test experience of premolded joint and cumulated knowledge of silicone materials, the 400kV
premolded joint has been successfully developed. In order to verify the design and the reliability of the system, the
type test was performed at the witness of internationally independent laboratory. According to IEC62067, the type test
including the outer protection test was carried out. This paper describes the design and development tests of 400kV
premolded joint and reviews the result of the type test.
Keywords: 400kV, EHV, silicone, premolded joint, type test, power cable
1. Introduction
2. Development of 400kV Premolded Joint
Cross-linked polyethylene (XLPE) insulated cables are
now widely used all over the world for extra-high
voltage underground transmission system. Also in Korea,
the portion of XLPE cables has rapidly increased since
they were introduced for 154kV cable line in 1983. For
XLPE cables, tape molded joint (TMJ) has been mainly
applied due to its high reliability using the same
insulation material with XLPE cables. Unlike tape
molded joint, factory-made pre-molded joint is pre-tested
and requires relatively simple and easy installation with a
low degree of skill from the jointers. Therefore, as the
cost of power transmission facilities reduces and easy
control of quality is required, the pre-molded joint is
preferred for EHV cable system [1], [2]. With the
numerous test experience of 132kV to 230kV
pre-molded joints and cumulated knowledge of silicone
materials, the 400kV pre-molded joint has been
successfully developed.
In order to verify the design
and the reliability of the system, the type test was
performed at the witness of internationally independent
laboratory. According to IEC62067 [3], the type test,
including outer protection test, was carried out. This
paper describes the design and development tests of the
joint and introduces the result of the type test.
2.1 Material selection for premolded joint
Premolded joint requires an elastomeric material that
is mechanically, thermally and electrically compatible to
the XLPE cables. Among the elastomeric materials,
EPDM and silicone rubber have been used for EHV
cable accessories.
We chose silicone rubber for EHV premolded sleeve
in consideration of dielectric strength, tensile strength,
permanent set, heat resistance and compatibility of
manufacturing equipment. The summary of material
properties of silicone rubber is given in table 1.
Table 1. Properties of silicone rubber
Item
Unit
Silicone
Elongation at break
%
594
Hardness
Shore A
30
2
Tensile strength
N/mm
7.1
Tear strength
N/mm
40
Dielectric strength
kV/mm
23
Thermal conductivity
W/mK
0.17
Permanent set (250%)
%
2.1
2.2
Premolded joint design
Probability Plot
99.00
2.2.1 Electrical design
Weibull
RTV 600
P=3, A=RR3-S
F=23 | S=0
50.00
Unreliability, F(t)
Premolded joint is made of silicone rubber, which
incorporates an embedded high voltage electrode and
two stress relief cones that are made of semiconductive
silicone rubber.
10.00
5.00
Before designing the premolded joint, we have to know
the electrical strength of the silicone rubber material
itself. Therefore, we have tested the AC and Impulse
breakdown voltage for 1 mm thick specimen which was
specially designed to have embedded electrodes. The
electrical field distribution in the test setup is shown in
fig. 1.
1.00
10.00
100.00
1000.00
Electrical field [kV/mm]
β=3.61, η=58.14, γ=73.98, ρ=0.95
Figure 3. Weibull distribution of Impulse
breakdown strength
The weibull distributions for AC and Impulse voltage
are shown in fig. 2 and fig. 3, respectively. From these
data, we could find that the minimum possibility
breakdown stress, EL for AC and Impulse application are
higher than 8.22 and 73.98kV/mm, as can be seen in
equation (1), (2) respectively.
 E − 8.22 3.20  ----------------- (1)
) 
PF ( Eac ) = 1 − exp− ( ac
50.08


 Eimp − 73.98 3.61  -------------- (2)
PF ( EIm p ) = 1 − exp− (
) 
58.14


Figure 1. Electrical field distribution in the test setup
with embedded semiconductive electrodes
We have made AC and Impulse breakdown tests over
20 specimens respectively, and analyzed the data using
the 3-parameter weibull distribution [4].
Probability Plot
99.00
With the help of these results and considering the
thickness effect, we could determine the electrical stress
design criteria for the 400kV premolded joint.
The profile of electrical field for embedded HV electrode,
as can be seen in fig. 4, depends on the shape of HV
electrode and stress relief cones. The electric field in the
longitudinal direction at the interface between rubber and
cable insulation is also important factor to influence the
design. The shape and dimension of premolded rubber
sleeve was optimized by the aid of computer simulation.
Weibull
RTV 600
P=3, A=RR3-S
F=23 | S=0
E
Unreliability, F(t)
50.00
τ
10.00
L
5.00
1.00
10.00
Electrical field [kV/mm]
100.00
β=3.20, η=50.08, γ=8.22, ρ=0.99
Figure 2. Weibull distribution of AC breakdown
strength
E : the maximum electric field in the rubber sleeve
τ : the maximum tangential electric field
along the cable/rubber sleeve
L : interfacial distance between the electrodes
Figure. 4 Electrical design parameters
2.2.2 Mechanical design
To assure the electrical stability at the interface, the
interfacial pressure is the key consideration. In this case,
the interfacial pressure is ensured by the elastic retention
of material itself without any mechanical device. Thus,
considering 30 years of safe operation, the determination
of appropriate range of initial pressure has become
essential. We determined the proper range as
0.5~2kgf/cm2 with the rubber sleeve expansion ratio of
10% ~ 40%. The simulation result of interfacial pressure
was represented in fig. 5.
In order to investigate the temperature effect on the
interfacial pressure during the heating cycle, the cable
was heated up to the conductor temperature of 90℃.
Both the simulation and experimental results show that it
is almost temperature-independent.
composed of filling, curing and post curing processes.
During filling phase, it is most important to get the
uniform flow of liquid silicone compound. Not only
process condition such as filling speed, filling
temperature of liquid silicone rubber and the mold, but
also the structure of the mold such as gate, runner and so
on, are very important to prevent the turbulences of the
liquid that might make the unwanted micro air bubbles
inside the rubber sleeve. For this purpose, we conducted
3D injection simulation of filling and curing phases of
premolded joint rubber. Figure 6 shows the filling phase
of 15%, 20%, 35% and 40% respectively.
4.0
2
Interfacial Pressure [kgf/cm ]
3.5
(a) 15% filling
(b) 20% filling
(c) 35% filling
(d) 40% filling
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
10 0
2 00
30 0
40 0
5 00
60 0
700
Po sition [m m ]
Figure 5.
Profile of interfacial pressure distribution
Figure 6. Simulation results of filling phase of liquid
silicone rubber in the mold
2.2.3 Thermal design
For the thermal design of the pre-molded joint,
preventing the local hot spot is essential, especially
where the conductor is connected. Thus, for the easy
emission of the heat and avoiding local heat
concentration, the low thermal resistivity material is
usually used to fill the gap between the ferrule and
corona shield. With the special design of heat emission,
we can control the temperature increase at the ferrule
without low resistivity material lower than that of cable
conductor.
Table 2 shows the results of temperature difference
between ferrule and corona shield for with and without
low resistivity material during 4 heat cycles without
voltage application.
Table 2. Temperature difference
between ferrule and corona shield
With material
Without material
Ferrule
80.7OC
88.1 OC
O
Corona shield
75.2 C
56.4 OC
The experiments of filling liquid silicone rubber in the
real mold were conducted as shown in fig 7 and
compared with the simulation results.
(a) 15% filling
(b) 20% filling
(c) 35% filling
(d) 40% filling
O
Note : when the conductor temperature 95 C
3.
Manufacturing Process Design
Manufacturing process of premolded rubber sleeve is
Figure 7. Experiment of filling phase of liquid silicone
rubber in the mold
From these experiments, we could find that the patterns
of filling phase of liquid silicone rubber in the mold
between simulation and experimental results coincide
very well. With the aid of simulation results, we could
obtain the optimal filling conditions, such as filling speed
and counter pressure after filling the mold.
For the curing phase, the most important parameters are
curing temperature and time. Fig. 8 and 9 show the
internal temperature distribution and corresponding
curing rate at the certain time during curing phase,
respectively. With these results, we could determine the
optimal curing process.
4.
Performance tests
As the verification of manufacturing process and the
base materials, lots of the electrical tests including long
term tests were carried out successfully. Fig. 10 shows
the construction of the developed premolded joint.
Figure 10.
The construction of premolded joint
To evaluate electrical performance and reliability, the
breakdown voltage tests were carried out numerous times.
Fig. 11 and 12 show the results of AC and Impulse
breakdown voltage tests.
Probability - Weibull
99.00
Weibull
Impulse breakdown data
W2 RRX - SRM MED
90.00
Internal temperature distribution of silicone
compound during curing phase
50.00
Unreliability, F(t)
Figure 8.
F=13 / S=7
10.00
5.00
1.00
1000.00
10000.00
Breakdown voltage [kV]
β=18.3288, η=1891.1244, ρ=0.9356
Figure 11. Weibull distribution of Impulse breakdown
voltage for 400kV premolded joint
Probability - Weibull
99.00
Curing rate distribution during curing phase.
From our earlier experiences of the development, the
adhesion between the semiconductive and insulation
parts affects the electrical performances. Thus the post
curing process control became important. By the design
of experiments between the post curing condition and
adhesion forces, the optimal condition of post curing
process could be found. For every rubber sleeves,
visual inspection and X-ray analysis are performed in
order to detect possible defects during the course of
manufacturing process. Electrical tests such as AC
withstand test and partial discharge test with the same
cables and installation conditions are followed on each
premolded sleeve as a routine test.
Weibull
AC breakdown data
W2 RRX - SRM MED
90.00
F=16 / S=4
50.00
Unreliability, F(t)
Figure 9.
10.00
5.00
1.00
100.00
1000.00
Breakdown voltage [kV]
10000.00
β=14.6253, η=962.2767, ρ=0.9830
Figure 12. Weibull distribution of AC breakdown voltage
for 400kV premolded joint
As can be seen in fig. 11, 12, the newly developed
premolded joint is proven to have excellent electrical
performance for 400kV grade, having breakdown voltage
higher than 900kV for AC and 1800kV for Impulse.
4.
Type test
The type test including outer protection test was
performed at the witness of internationally independent
laboratory in accordance with IEC 62067. All tests were
successfully completed and fully verified the design and
reliability of the system. Fig. 13 shows the test loop
and Table 3 shows the result of type test.
at the witness of internationally independent laboratory
in accordance with IEC62067. As a conclusion, we
could find that the newly developed premolded joint has
excellent electrical performance and long term reliability.
LS cable provided over seas customers with 400kV
premolded joints for 2 projects, the one was successfully
commissioned and the other is now under construction.
References
[1] Willens H., Geene H., et. al., "A new generation of
HV and EHV extruded cable systems”, Jicable 1995,
A.1.6.
[2] Vasseur E., Chatterjee S., “Development of HV
and EHV single piece premoulded joint” Jicable 1995,
A.4.4.
[3] IEC 62067, "Power cables with extruded insulation
and their accessories for rated voltages above
150kV (Um=170kV) up to 500kV(Um=550kV)
-Test methods and requirements", 2001
[4] L. A. Dissado, J. C. Fothergill “Electrical
Figure 13.
degradation
Overview of type test
Table 3.
Type test results
Requirements
Result
1. Partial discharge
2. Tanδ measurement
330kV, ≤5pC
≤10-3
No PD
0.03 %
3. Heating cycle
440kV/20 Cycle
95~100℃
No
failure
4. Partial discharge
5. Switching Impulse
330kV, ≤5pC
(room and High temp)
±1050kV/10shot
95~100℃
No PD
Withstood
6. Lightning Impulse
±1425kV/10shot
95~100℃
Withstood
7. AC voltage
440kV/15min
Withstood
5.
breakdown
in
polymers”
p323-332.
After the completion of all tests required, the whole
system were dismantled and examined at the witness of
LS Cable and internationally independent laboratory. No
sign of deterioration has been observed.
Test Item
and
Conclusion
With the numerous test experience of 132kV to 230kV
pre-molded joints and cumulated knowledge of silicone
materials, the single piece 400kV pre-molded joint has
been successfully developed. To verify the design and
the reliability of the system, the type test was performed
Biographies
D.S. Shin was born in 1971. He received the master
degree in electrical engineering from Seoul National
University, Korea in 1996. Now he is a senior research
engineer of EHV technical solution team in LS Cable.
He research interests include design for EHV power
cable accessories.
E.S. Ji was born in 1968. He received the master degree
in chemical engineering from Kyongbuk University,
Korea in 1995. Now he is senior research engineer of
Power Cable Accessory Production Team in LS Cable.
His research interests include polymeric joint and
insulator for EHV power cable.
Y.G. Kwon was born in 1971. He received the bachelor
degree in mechanical engineering from State University
of New York at Stony Brook, USA in 1998. Now he is a
manager of overseas power cable project team in LS
Cable. His main interests include EHV power cable and
accessories.
S.I. Jeon was born in 1963. He received the master
degree in electrical engineering from Seoul national
university, Korea in 1995. Now he is a principal research
engineer of Electric Power R&D Center in LS Cable. His
research interests include insulation materials and
insulation design for EHV power system
I.H. Lee was born in 1964. He received the bachelor
degree in electrical engineering from Seoul national
university, Korea in 1986. Now he is head of EHV
technical solution team in LS Cable. His research
interests include insulation materials for EHV power
cable