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Development Process of extruded HVDC cable systems
Dominik HÄRING, Gero SCHRÖDER, Andreas WEINLEIN, Axel BOSSMANN Südkabel GmbH, (Germany)
dominik.haering@suedkabel.com, gero.schroeder@suedkabel.com, andreas.weinlein@suedkabel.com,
axel.bossmann@suedkabel.com
ABSTRACT
Extruded HVAC cable systems up to 500 kV have been
developed successfully in the past decades and several
years of operating experience are available. Because of
increasing demand in power and the ability to transmit
electrical energy over long distances, HVDC cable
systems become more important. However, due to DC
specific influences on the insulating systems of the cable
and accessories, a detailed consideration and evaluation
of these effects must be taken into account during the
development process of extruded HVDC cables and their
accessories.
This paper addresses the main influences of DC stress on
the components of HVDC cable systems. Fundamental
aspects regarding the interface between cable and
accessory will be discussed. The paper describes the
empiric consideration of an extruded XLPE HVDC cable
test system beginning with a model system to an HVDC
cable test system with a voltage level of 150 kV from the
perspective of a cable system manufacturer.
KEYWORDS
HVDC, cable system, XLPE, joint, testing
INTRODUCTION
The increasing power demand worldwide requires the
transmission of electrical energy over long distances.
Extruded HVAC cable systems have been developed
successfully in the past decades and therefore many
years of operating experience are available as mentioned
in references [1] and [2]. However, based on the
capacitive load and the eddy current losses, especially in
large conductor sizes, the operation range to longer
system lengths of HVAC cable systems is limited.
For this reason HVDC cable systems become more
important. While oil filled HVDC cables are a well-known
technology for more than 50 years, extruded HVDC cable
systems have been developed and commercially
introduced in the last decade [3, 5]. Actually the work on a
new standard for extruded HVDC cable systems is in
progress [9]. Based on the low level of experience in this
area and in order to deliver a reliable HVDC cable system,
detailed development actions are necessary to
understand the DC specific effects on extruded cables
and their accessories.
The main DC specific effects to extruded HVDC cable
systems can be addressed as follows:
•
Space charge accumulation in the insulation and
interfaces
•
Resistive field distribution
•
Thermal dependence of insulation materials
According to this, it can be concluded, that the following
aspects must be taken into account during the
development process of an HVDC cable system:
•
Space charge behaviour
materials and interfaces
•
Conductivities of the insulation materials and
their temperature and field strength dependence
•
DC and impulse breakdown strength of the cable
and accessories
in
the
insulation
With these aspects in mind, basic considerations on an
extruded HVDC cable test system, including an extruded
cable and a prefabricated joint, has been done and tested
for VSC (voltage source converter) applications. This
paper provides an overview of the systematic approach of
such a HVDC cable test system, while the main aspects
will be discussed from the perspective of a cable system
manufacturer.
GENERAL DEVELOPMENT ASPECTS OF
HVDC CABLE SYSTEMS
Since the successful development of extruded HVAC
cable systems up to 500 kV, a high level of experience
regarding operation behaviour of the components is
available. In the case of extruded HVDC cable systems,
essential differences in the operation behaviour compared
to HVAC systems must be considered. The following
section addresses those main effects on an HVDC cable
system under the applied DC conditions.
Electric field
conditions
distribution
under
DC
The electric field distribution in a conventional AC cable
can be easily expressed in the following equation [1]:
E (r ) =
U
r 
r ⋅ ln o 
 ri 
[1]
E(r) stands for the electric field strength, U is the voltage,
ro the radius of outer insulation screen, and ri the radius of
the inner insulation screen.
This behaviour is totally different and more complicated
under DC conditions. As opposed to the AC field
distribution, the field distribution in a DC cable is
controlled by the resistivity of the insulation material.
Furthermore, the resistivity of an insulation material is
strongly dependent on the temperature and the applied
electric field. The link between resistivity σ0 and
temperature respective to the electric field has been
published in various mathematical expressions. One of
the main mathematical expressions from [4] can be stated
in equation [2] as:
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σ (r ) = σ 0 e[αT ( r ) + βE ( r )]
[2]
where α and β are the coefficients representing the
dependence of the temperature and the electric field, σ0 is
the insulation resistivity at a reference temperature, T is
the temperature, and E is the electric field.
With consideration of equation [2], the electric field
distribution in a DC cable is highly dependent on the
current load of the cable system. An analytic
approximation of the electric field distribution in a DC
cable is given according to [5] in equation [3]:
r

 ro 
δ −1
δ ⋅U 0 ⋅ 
E (r ) =
[3]
  r δ 
ro 1 −  i  
  ro  
with
bU 0
a∆T
+
ln(ro / ri ) ro − ri
δ=
bU 0
1+
(ro − ri )
[4]
where a and b describe the material constants and ∆T the
temperature drop in the insulation.
Figure 1 shows an example of an electric field distribution
of a cable under AC and DC stress with consideration of
an operating cable (Tconductor > Tambient). The electric field
distribution under DC conditions is similar to the AC
conditions at the moment the system is energized
according to equation [1]. At this moment, no thermal
influence is present and only the effect of the electric field
has to be taken into account. Thereby, the highest stress
occurs on the conductor screen (ISL). However, when the
cable is loaded, the current in the conductor leads to a
temperature gradient over the cable insulation. As a
result, the resistivity of the insulation material changes
according to equation [3]. This leads to an electric field
inversion in the cable insulation where the electric stress
decreases near to the conductor screen and increases
near to the outer insulation screen (OSL).
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The above mentioned electric field distribution under DC
conditions leads to a totally different consideration
compared to AC systems during the development process
of extruded HVDC cable systems. The strong
dependence of the insulating materials on the field
strength and the temperature, especially, requires detailed
knowledge of the insulation materials of the cable system
components. More complicated, yet, is the behaviour
addressed in the case of HVDC cable accessories.
Therefore, several materials with different specific
conductivities and their dependencies on temperature and
electric field strength must be taken into account.
Influence of space charge accumulations
One of the main challenges within the development of
HVDC materials and components is the understanding of
the accumulation of charges in the insulation materials.
Even insulating material consists of free charge carriers.
Under an applied AC field, the direction of the electric field
inverts periodically. In this case the flow of free charge
carriers also inverts its direction. Finally free charges
remain at their position under AC conditions.
Under a DC field the free carriers accumulate especially
near to the inner and outer insulation screen of the cable,
respective of the accessory or between interfaces of cable
and accessory. As a result, the electric field distribution in
a cable insulation system under DC conditions will be
strongly influenced by the accumulation of space charges.
In addition to the above mentioned external field in
equation [3], the electrical field distribution in a DC
insulation system will be strongly influenced further by
accumulations of space charges as stated in equation [5].
Eres = E0 + E s
[5]
Eres shows the resulting field in the insulation, E0 is the
external field as described above and ES is the space
charge field.
This link leads to the conclusion that strong field
enhancements with a critical stress on the insulation
system might occur during the the system’s operation.
Because of these correlations, the materials for DC
applications must be chosen with consideration of the
effect of space charge accumulations. This leads to the
requirement that the investigation of the used DC
materials is necessary to get a detailed understanding of
the process in the insulation system. For a deeper
understanding of the space charge accumulation, figure 2
has been taken from [6] and demonstrates the result of
measuring a space charge accumulation by using the
PEA method (pulsed electro acoustic).
EAC ≠ EDC
Fig. 1: Schematic distribution of resisitive field
(Tconductor > Tambient)
Fig. 2: Result of a space charge measurement
(Courtesy of material manufacturer)
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Two different materials have been investigated. It can be
shown that the standard XLPE obtains a much higher
charge density in the area of the electrodes compared to
the advanced HVDC XLPE. This result shows the
necessity of an optimised material for DC applications.
Consideration of space charge effects is difficult because
of very long time constants, which depend on the type of
insulation, material, field strength, and temperature.
However, for reliable operation of the HVDC cable
system, detailed considerations of the previously
mentioned space charge effects must be taken into
account during the development process.
BASIC CONSIDERATIONS ON EXTRUDED
HVDC CABLE SYSTEMS
The basis for a reliable consideration of the HVDC test
system, essentially, is defined by its technical
requirements.
The technical requirements of a HVDC cable system are
defined by the electrical, thermal, and mechanical
boundary conditions given by the HVDC system
configuration (e.g. nominal voltage, load, laying
conditions). The thermal boundary conditions are defined
by cable construction and system engineering. As already
mentioned above, the thermal behaviour of a cable
system under DC applications is directly linked to the
electrical properties of the insulation system. This means
that the requirements on the electrical design of an HVDC
cable system are an interaction of thermal conditions and
space charge effects as well as nominal and impulse
voltages. Transient impulse voltages (e.g. switching and
lightning impulse voltages) lead to temporary overstresses
on the cable insulation system. Thus, the electrical stress
distribution in the insulation system is then the
superposition of the nominal applied DC voltage (see
equation [5]) and the capacitive field of the impulse (see
equation [1]) according to equation [6]:
E sup erimposed = E res + E impulse
[6]
Esuperimposed shows the resulting field in the insulation, Eres
is the resulting field of the external DC field and the space
charge field, Eimpluse is the capacitive field of the applied
impulse.
The level of the impulse voltage Eimpluse depends on the
configuration of the entire HVDC system. As there are no
impulse levels given in the standards, the insulation
system has to be designed with consideration of the
specific project requirements of the HVDC system as
mentioned in CD IEC 62895 [9]. Finally, the suitablity of
the system components for AC applications in terms of
factory acceptance tests and PD measurements must be
considered as well.
An extruded HVDC test system in a scaled model has
been designed, manufactured and tested by addressing
the above mentioned influences and requirements. The
following section addresses the main aspects of this
process of the pre-stage cable system.
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requirements, basic theoretic considerations in terms of
electrical and thermal design of cable system components
have been done. The components have been designed in
a scaled model for a nominal voltage level of U0 = ±80 kV.
Basic studies on the cable insulation have been done with
consideration of the electrical field strength under different
thermal and electrical operation conditions. A major focus
was on the design of the prefabricated joint. The
advantage of this type of joint, compared to other joint
types (such as tape molded joints), is the fast and easy
installation and the ability to pre-test the insulation body in
the factory. However, the challenge of a prefabricated
joint design is dealing with different materials and the
interface between the cable core and the joint body,
especially keeping in mind the DC specific influences
mentioned earlier. One of the fundamental details in the
design process of the prefabricated joint will be explained
in the following section. The calculations of the electric
field and potential distributions presented are done using
a finite element calculation tool. Figure 3 shows
calculations over a section of a conventional HVAC
prefabricated joint. The simulation shows the potential
distribution of the joint under DC voltage in three different
cases. Figure 3a represents the joint under a zero currentload condition. In this case, similar conductivities of the
XLPE and the silicone rubber can be assumed and lead to
an even potential distribution in the interface between
XLPE cable core and joint body made of silicone rubber.
According to equation [2], the conductivity of the materials
strongly depends on the temperature and the strength of
the electric field. Considering the temperature distribution
in a joint during the operation, different conductivities must
be taken into account. Figure 3b shows the potential
distribution for the simple case of σXLPE < σSiR. The
different conductivities of the materials lead to an uneven
potential distribution in the joint. The result is a strong and
critical field enhancement in the area of the inner field
control element of the conductor connection. As shown in
figure 3c, in the case of σXLPE > σSiR, the potential
distribution leads to a strong electric field enhancement in
the area of the geometric field control element.
a - σXLPE = σSiR
b - σXLPE < σSiR
c - σXLPE > σSiR
Consideration of a XLPE-HVDC pre-stage
cable system in scaled model
The pre-stage cable system in a scaled model consists of
an extruded HVDC cable and a prefabricated joint made
of silicone rubber. Based on the above mentioned system
Fig. 3: Equipotential lines of a joint in the case of
different conductivities of silicone rubber
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Figure 4 is a diagram of the potential distribution with the
interface between the XLPE of the cable and the joint
made of silicone rubber. The graphed lines refer to the
results determined as addressed in figure 3. Thereby, the
even potential distribution in the case of zero current-load
can be shown in with the green line. The blue line
represents the case of σXLPE > σSiR and the red line
represents the case of σXLPE < σSiR.
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It can be shown that the HVDC joint realizes an even
potential distribution in the interface between cable core
and joint body. The simulation shows a less sensitive
behaviour of the joint regarding different operation
temperatures. This distribution enables a reliable thermal
operation range without the occurence of critical local field
enhancements.
After completing the theoretical consideration of the prestage system components of an HVDC cable in a scaled
model with different variations of HVDC joint construction
was manufactured and tested, the produced components
of the pre-stage cable system were factory tested with an
AC high voltage test with a PD measurement in order to
check production quality and to avoid false interpretation
of the test results.
2
Fig. 4: Potential distribution in the interface XLPE and
joint body
The simple field calculation of the joint shows the highly
sensitive behaviour regarding different temperatures resp.
conductivities. The main aim of the joint constructions
must be a decoupled and less sensitive behaviour
between the conductivites of XLPE and the silicone
rubber of the prefabricated joint body. Furthermore, the
electrical stress on the insulation system in the case of a
superimposed voltage must be considered under different
conditions of thermal operation. To overcome the stated
challenges, different variations of prefabricated HVDC test
joints were designed and tested. The respective joints use
advanced materials made of silicone rubber. The
materials have been chosen with consideration of space
charge effects in the joint and its interfaces, dielectric
properties and conductivities. The joint body construction
includes different types of field control systems (geometric
/ resistive) and design field strengths.
The cable in model scale consists of a 300 mm
aluminium conductor. The conductor screen, insulation,
and insulation screen are made by a triple extrusion
process. The materials used have advanced space
charge properties and enable a maximum conductor
temperature of 70°C. The insulation system is designed
for an operation voltage of U0 = ±80 kV. The cable screen
2
consists of a 25 mm copper screen, bedding tapes and
an outer laminated aluminium foil. An extruded outer PE
sheath enables the outer mechanical protection of the
cable. The cable was produced with consideration of the
degassing behaviour of the crosslinking byproducts.
To evaluate the technical performance of the cable and
the different joint constructions, detailed tests have been
carried out with consideration of the above mentioned DC
specific influences on the insulation system. The design of
the joints differ in material, field control system and design
field strengths. Several tests have been carried out to
evaluate an appropriate DC insulation system under all
applied thermal and electrical conditions. Figure 6 shows,
as an example, the results of a long term DC test carried
out with consideration of three different joint constructions.
The investigation was conducted using several test
objects so that a statistical statement can be reached.
Figure 5 is the graph representation of the potential
distribution in the interface between cable core and joint.
All parameters are equal to the ones discussed in figure 3
and 4. However, an advanced design of an HVDC joint
has been calculated.
Fig. 6: Schematic overview of long term test results
on HVDC pre-stage joints
Fig. 5: Potential distribution in the interface XLPE and
joint body
The long term tests have been carried out under
increased voltages at ±160 kV and temperature heating
cycle conditions at a maximum conductor temperature
>75°C. The long term test reveals the suitability of joint
design no. 3 in terms of the described long term stress.
The results obtained were supported by additional tests
e.g. impulse voltage tests and limit field strength tests.
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After evaluation of the addressed test results,
fundamental design rules were defined and transformed in
a common design of a HVDC pre-stage joint.
The mentioned consideration of the HVDC pre-stage
cable system was concluded by an adapted type test
according to CIGRÉ recommendation 219 respectively
496 in [7] and [8]. The components have successfully
passed the adapted type test.
Finally, the result of the described development process
of an HVDC pre-stage cable system indicates the
suitability of the components for HVDC applications.
Testing of a 150 kV extruded HVDC test
cable system
The results obtained from the process developed for the
pre-stage cable system in a scaled model were
implemented in general design rules (field strengths,
material properties, production parameters). Based on
those parameters, the obtained results were implemented
and transformed in an HVDC test system for a nominal
system voltage of U0 = ±150 kV. Figure 7 shows a sketch
of the HVDC cable. The ±150 kV HVDC cable consists of
2
a 630 mm aluminium conductor (1). The conductor
screen, XLPE insulation, and insulation screen are made
by a triple extrusion process (2). The materials used have
advanced space charge properties and enable a
maximum conductor temperature of 70°C. The cable
2
screen consists of a 25 mm copper screen, bedding
tapes and outer laminated aluminium foil (3). An extruded
outer PE sheath enables external mechanical protection
of the cable (4).
Fig. 8: Schematic picture of the HVDC test-setup
The cable terminations have been realized by special test
terminations made of silicone rubber including a
geometric field control element. The cable loop has been
closed by a copper connection between the terminations.
A current transformer induced a heating current in the test
loop. The conductor temperature and temperature drop of
the test cable have been determined by comparing sheath
temperatures with an additional dummy loop. An
aluminium compression sleeve establishes the conductor
connection in the joints. The main data of the conducted
type test are given in Table 1.
Table 1: Adapted HVDC type test procedure for VSC
applications
Description
Parameter
24 cycles at UT = ±290 kV
Load Cycle Test
•
8/16 hours heating / cooling
•
Tconductor = 75°C -80°C
3 cycles at UT = 290 kV
•
24/24 hours heating / cooling
•
Tconductor = 75°C-80°C
UT = ±150 kV DC voltage with
imposed impulse voltages
Superimposed Voltage
Test
•
Switching impulse withstand
tests
•
Lightning impulse withstand
tests
•
Tconductor = 75°C-80°C
Fig. 7: Sketch of the 150 kV extruded HVDC cable
The cable joints have been manufactured with
consideration of the defined design rules obtained during
experimentation of the scaled model pre-stage system.
Based on the electrical, thermal and mechanical design of
the components, an HDVC cable and joint for a nominal
operating voltage of U0 = ±150 kV has been produced. To
evaluate the reliability of the system, a type test adapted
to Cigré recommendation 219 respectively 496 in [7] and
[8] has been conducted. The test has been performed for
VSC applications. For a detailed understanding of the test
procedure, figure 8 shows the schematic test-setup of the
adapted type test.
UT = -150 kV
Subsequent DC Test
•
2 hours
•
ambient temperature
All tests have been passed successfully. No breakdown or
flashover occurred during the tests. Hence, the ±150 kV
test cable system shows the suitability for HVDC
applications. Additional tests, especially those with long
term considerations, are ongoing to support these results.
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The process described from a pre-stage cable system as
a scaled model to the ±150 kV test system shows the
successful approach of cable system components under
DC conditions. However, based on the mentioned DC
specific
influences,
further
investigations
and
considerations are necessary to obtain a full and reliable
understanding of those effects. Because of this, further
scientific considerations are planned and ongoing to get a
deep understanding of DC fields on extruded cables and
accessories made of silicone rubber. A key focus is on the
space charge accumulations under increased field
strengths in the components and interfaces, consideration
of the behaviours of material and the simulation of the
insulation systems. Based on those results, general
design rules can be implemented in the development of
cable system components for higher system voltage
levels.
CONCLUSION
This paper describes the main differences between an AC
and DC field in extruded cables and their accessories.
Under consideration of the DC specific effects on
extruded cable systems, a pre-stage cable system in a
scaled model has been designed. Based on the electric,
thermal, and mechanical design of the components, the
pre-stage system has been manufactured and tested. As
a result of the obtained results, an HVDC cable system
with a nominal voltage level of U0 = ±150 kV has been
manufactured and tested. The system design was
evaluated by a type test adapted to Cigré
recommendation 219 respectively 496. The test results
show the suitability of the ±150 kV cable system regarding
the DC specific influences. Further testing and scientific
activities are planned and ongoing for the consideration of
higher voltage levels - for the purpose of a reliable and
more renewable energy supply in the future.
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Cable Systems for Power Transmission at a Rated
Voltage up to 250 kV, Cigré WG 21-01
[8] 2012, Recommendations for Testing DC Extruded
Cable Systems for Power Transmission at a Rated
Voltage up to 500kV, Cigré WG B1.32
[9] 2014, High Voltage Direct Current (HVDC) power
transmission cables with extruded insulation and their
accessories for rated voltages up to 320 kV for land
applications - Test methods and requirements,
Comitte Draft, CD IEC 62895
GLOSSARY
AC:
DC:
HVAC:
HVDC:
ISL:
OSL:
PEA:
PE:
SiR:
VSC:
XLPE:
Alternating Current
Direct Current
High Voltage Alternating Current
High Voltage Direct Current
Inner Semiconductive Layer
Outer Semiconductive Layer
Pulsed Electro Acoustic Technique
Polyethylene
Silicone Rubber
Voltage Source Converter
Cross-Linked Polyethylene
REFERENCES
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Worldwide Experiences and Challenges with EHV
th
XLPE Cable Projects 330 kV to 500 kV, 9
International Conference on Insulated Power Cables Jicable 2015
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2011, Development, Qualification and Experience
th
with 500kV XLPE Cable Systems, 8 International
Conference on Insulated Power Cables - Jicable
2011
[3] T. Worzyk, 2009, Submarine Power Cables, Springer
Verlag, Heidelberg, Germany, 974
[4] M. Jeronese, 1997, Charges and Discharges in
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[5] G. Mazzanati, M. Marzinotto, 2013, Extruded Cables
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